Discontinuing Ventilatory Support

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Discontinuing Ventilatory Support

ROBERT M. KACMAREK

CHAPTER OBJECTIVES

After reading this chapter you will be able to:

◆ Discuss the relationship between ventilatory demand and ventilatory capacity as well as their relationship with ventilator discontinuance.

◆ List the factors associated with ventilator dependence.

◆ Explain how to evaluate a patient before attempting ventilator discontinuation or weaning.

◆ List acceptable values for specific weaning indices used to predict a patient’s readiness for discontinuation of ventilatory support.

◆ Describe factors that should be optimized before an attempt is made at ventilator discontinuation or weaning.

◆ Describe techniques used in ventilator weaning, including daily spontaneous breathing trials, synchronized intermittent mandatory ventilation, pressure support ventilation, and other newer methods.

◆ Contrast the advantages and disadvantages associated with various weaning methods and techniques.

◆ Describe how to assess a patient for extubation.

◆ List the primary reasons why patients fail a ventilator discontinuance trial.

◆ Explain why some patients cannot be successfully weaned from ventilatory support.

CHAPTER OUTLINE

Reasons for Ventilator Dependence Ventilatory Workload and Demand Ventilatory Capacity

Global Criteria for Discontinuing Ventilatory Support Patient Evaluation

The Most Important Criterion Weaning Indices

Ventilation Oxygenation Acid-Base Balance Metabolic Factors

Renal Function and Electrolytes Cardiovascular Function

Psychologic Factors and Central Nervous System Assessment

Integrated Indices Evaluation of the Airway Preparing the Patient

Optimizing the Patient’s Medical Condition Patients’ Psychologic and Communication

Needs

Caregiver Preparation Methods

Rapid Ventilator Discontinuation

Patients Who Need Progressive Weaning of Ventilatory Support

Spontaneous Breathing Trials Continuous Positive Airway Pressure

Synchronized Intermittent Mandatory Ventilation Pressure Support Ventilation

Synchronized Intermittent Mandatory Ventilation With Pressure Support Ventilation

Spontaneous Awaking Trials

Newer Techniques for Facilitating Ventilator Discontinuance

Mandatory Minute Volume Ventilation Adaptive Support Ventilation/Intellivent Computer-Based Weaning

Automatic Tube Compensation Volume Support

Proportional Assist Ventilation and Neurally Adjusted Ventilatory Assist

2. Synchronized intermittent mandatory ventilation (SIMV) 3. Pressure support ventilation (PSV)

Other techniques that may facilitate ventilator discontinua- tion include the use of volume-support ventilation (VSV);

adaptive support ventilation (ASV)/Intellivent; automatic tube compensation (ATC); proportional assist ventilation (PAV), which is also known as proportional pressure support (PPS); neurally adjusted ventilatory assist (NAVA); and con- tinuous positive airway pressure (CPAP). However, little data exist that support the use of any of these techniques except for CPAP, which seems potentially beneficial during the ventilator discontinuation process.

Techniques for predicting when patients are ready for venti- lator discontinuation and weaning have been studied exten- sively.⁴ Many weaning indices designed to predict successful ventilatory discontinuation have been proposed. Despite this, there are no universally applicable indices for predicting success.

Of all of the methods studied, SBTs and PSV have been shown to be the most effective methods for ventilator discontinuation and weaning. Evidence-based reviews recommend the use of at least daily SBTs.¹ Protocols for ventilator discontinuation administered by an interdisciplinary team of respiratory thera- pists, nurses, and physicians can be highly effective, and have been recommended.^1,4,5-9 Regardless of the method used, success is unlikely unless the precipitating problems that caused the ventilator dependency have been resolved.^1,4,9 After these prob- lems are resolved, an organized plan or protocol should be followed, and variations should be based on each patient’s response.^1,4,5

Some patients cannot be successfully removed from mechan- ical ventilatory support. This group of ventilator-dependent patients poses clinical, economic, and ethical concerns.^10,11

The term weaning has been used as a general term to refer to the process of discontinuing ventilatory support, regardless of the time frame or method involved. The term has also been used to refer to reductions in fractional inspired oxygen con- centration (FiO2), PEEP, and CPAP. Alternatively, the term

T he purpose of mechanical ventilation is to support the patient until the disease state or condition that caused the need for support is alleviated or resolved. Ventila- tory support can sustain life, but it cannot cure disease. Further, many complications and hazards are associated with mechani- cal ventilation. Consequently, ventilatory support should be withdrawn as soon as the patient is able to adequately resume spontaneous breathing.¹ All patients who are mechanically ven- tilated should be evaluated on a daily basis, beginning with the day of intubation, for their ability to wean from ventilatory support.^2,3 Frequently this evaluation is very quick, but it should be performed daily regardless of the patient’s status.

After the problem or condition that caused the need for mechanical ventilation is resolved, most patients can be quickly and easily removed from ventilatory support. For example, for most patients who need mechanical ventilation as a result of drug overdose or severe asthma, for those who are recovering from postoperative anesthesia, and for those who have received ventilation for 72 hours or less, one may simply discontinue ventilation when the precipitating condition has resolved.^1,4 However, some patients require mechanical ventilation for longer periods. The term ventilator dependent is usually reserved for patients who need ventilatory support for lengthy periods (i.e., 2 weeks or more) or who have not responded to attempts at ventilator discontinuation. For these patients, a more pro- longed ventilator discontinuation process is required.¹

Ventilator discontinuation should be carefully timed. Prema- ture removal from the ventilator may severely stress the cardio- pulmonary system and delay the patient’s recovery.⁴ Premature discontinuation also exposes the patient to the hazards of rein- tubation. However, delays in discontinuing ventilation expose the patient to an increased risk of complications, including nosocomial pneumonia, myocardial infarction, and death.⁴

There are three basic methods for discontinuing ventilatory support¹:

1. Spontaneous breathing trials (SBTs) alternating with mechanical ventilation

KEY TERMS

adaptive support ventilation (ASV) airway occlusion pressure automatic tube compensation

(ATC)

continuous positive airway pressure (CPAP)

Intellivent

mandatory minute volume ventilation (MMV)

pressure support ventilation (PSV) prolonged mechanical ventilation

(PMV)

rapid, shallow breathing index (f/VT) spontaneous awaking trial (SAT) spontaneous breathing trial (SBT) synchronized intermittent

mandatory ventilation (SIMV) Noninvasive Ventilation

Role of Mobility

Respiratory-Therapist–Driven Protocols Selecting an Approach

Monitoring the Patient During Weaning Ventilatory Status

Oxygenation

Cardiovascular Status

Extubation

Artificial Airways and Weaning Ventilator Discontinuance Failure Prolonged Mechanical Ventilation

Chronically Ventilator-Dependent Patients Terminal Weaning

Compliance is determined by the elastic nature of the lung–

thorax system. Resistance is largely related to the nature of the conducting airways. Common causes of decreased lung compli- ance include atelectasis, pneumonia, pulmonary edema, acute lung injury, and acute respiratory distress syndrome. Thoracic compliance may be reduced because of obesity, ascites, or abdominal distention. Airway resistance increases with bron- chospasm, excessive secretions, and mucosal edema.

Mechanical factors that can increase the work of breathing include artificial airways (i.e., endotracheal and tracheotomy tubes), partial obstruction of the airway, ventilator circuits, demand flow systems, auto-PEEP, and inappropriate ventilator flow and sensitivity settings. Factors that may increase ventila- tory workload are summarized in Box 52-1.

Ventilatory Capacity

Ventilatory capacity is determined by CNS drive, ventilatory muscle strength, and ventilatory muscle endurance. Most patients who are being withdrawn from ventilatory support have a normal or an increased drive to breathe. Patients with neuromuscular disorders and those who are receiving sedatives, narcotics, or neuromuscular blocking agents may have a reduced drive to breathe or impaired neuromuscular transmission.

Patients with metabolic alkalosis, hypothyroidism, and sleep deprivation also may have reduced ventilatory drive. Box 52-2 summarizes the factors that may reduce ventilatory drive.

Muscle strength is influenced by age, sex, muscle bulk, and overall health. Malnutrition, starvation, and electrolyte imbal- ances (especially involving calcium, magnesium, potassium, and phosphate) can lead to ventilatory muscle weakness. Criti- cal illness myopathy, critical illness polyneuropathy, and the ventilator discontinuation has been used to refer to the process

of disconnecting a patient from mechanical ventilatory support.

For the purposes of this chapter, the term weaning is defined as a gradual reduction in the level of ventilatory support, whereas discontinuing ventilatory support refers to the overall process of removing the patient from the ventilator, regardless of the method used. In general, patients who are being considered for removal from ventilatory support fall into one of five categories:

1. Those for whom removal is quick and routine, which is normally the vast majority of ventilated patients

2. Those who need a more systematic approach to discontinu- ing ventilatory support, which is normally about 15% to 20% of ventilated patients

3. Those who require days to weeks to wean from ventila- tory support, which is usually less than 5% of ventilated patients

4. Those ventilator-dependent or “unweanable” patients, who compose less than 1% of patients who require ventilatory support

5. Those who have no chance for survival in whom the ventila- tor is discontinued while comfort measures are provided, normally referred to as terminal weaning.¹²

REASONS FOR VENTILATOR DEPENDENCE

Patients may require mechanical ventilation because of apnea, acute or impending ventilatory failure, or severe oxygenation problems that necessitate high levels of PEEP or CPAP. Regard- less of the reason for initiating mechanical ventilation, patients remain dependent on the ventilator because of respiratory, car- diovascular, neurologic, or psychologic factors.¹

Ventilatory Workload and Demand Patients who need mechanical ventilation often have a ventila- tory workload and demand that exceeds their ventilatory capac- ity. This is the most common cause of ventilator dependence.^1,4 The term ventilatory workload refers to the amount of work that the respiratory muscles are asked to perform to provide an appropriate level of ventilation. A patient’s total ventilatory workload is primarily determined by the following: (1) the level of ventilation needed, (2) the compliance of the lungs and thorax, (3) the resistance to gas flow through the airways, and (4) any imposed work of breathing (WOBI) due to ventilatory system mechanical factors.^1,4

The level of ventilation required is determined by the follow- ing: (1) the metabolic rate; (2) the central nervous system (CNS) drive; and (3) the ventilatory dead space. Common causes of an increased demand for ventilation include increased carbon dioxide production (i.e., fever, shivering, agitation, trauma, or sepsis) and increased dead space (i.e., pulmonary emboli or chronic obstructive pulmonary disease [COPD]).

Other common causes of increased ventilatory demand include metabolic acidosis, severe hypoxemia, pain, and anxiety.

Box 52-1 Factors That May Increase Ventilatory Workload

INCREASED VENTILATORY DEMAND: INCREASED LEVEL OF VENTILATION REQUIRED

• Increased CNS drive: hypoxia, acidosis, pain, fear, anxiety, and stimulation of J receptors (e.g., pulmonary edema)

• Increased metabolic rate: increased carbon dioxide production, fever, shivering, agitation, trauma, infection, and sepsis

• Increased dead space: COPD and pulmonary embolus DECREASED COMPLIANCE

• Decreased lung compliance: atelectasis, pneumonia, fibrosis, pulmonary edema, and acute respiratory distress syndrome

• Decreased thoracic compliance: obesity, ascites, abdominal distention, and pregnancy

INCREASED RESISTANCE

• Increased airway resistance: bronchospasm, mucosal edema, and secretions

• Artificial airways: endotracheal tubes, tracheostomy tubes, and partial obstruction of the artificial airway or the patients airway

• Other mechanical factors: ventilator circuits, demand flow systems, and inappropriate ventilator flow or sensitivity settings

1. Ventilatory workload versus ventilatory capacity 2. Oxygenation status

3. Cardiovascular function 4. Psychologic factors

Simply put, when ventilatory workload or demand exceeds ventilatory capacity, successful ventilator discontinuation is unlikely. Excessive ventilatory workload may lead to ventilatory muscle fatigue. When the ventilatory muscles fatigue, they must be rested for at least 24 hours to recover.¹⁴ Ventilatory workload increases with decreased compliance, increased airway resis- tance, or an increased requirement for ventilation. Ventilatory capacity can be reduced by ventilatory muscle fatigue and by a loss of muscle strength and endurance.

Other factors that may contribute to ventilator dependence include inadequate arterial oxygenation, poor tissue oxygen delivery, myocardial ischemia, arrhythmias, low cardiac output, and cardiovascular instability. Neurologic problems that may contribute to ventilator dependence include decreased central drive to breathe and impaired peripheral nerve transmission.

Psychologic issues that may contribute to ventilatory depen- dence include the fear of removal of the life-support system, anxiety, stress, depression, and sleep deprivation. Box 52-3 summarizes the major factors that contribute to ventilator dependence.

PATIENT EVALUATION

Careful patient assessment is required to determine which patients are ready to be removed from ventilatory support quickly, which patients may need a prolonged ventilator discon- tinuation phase, and which patients are not yet ready for the discontinuation of ventilatory support.

An important factor to consider as part of this assessment is the length of time that the patient has been receiving mechani- cal ventilation. In general, those who receive support for 72 hours or less often can be removed quickly from the ventila-

tor.^15,16 Those who need a longer period of support may need a

more prolonged approach. Current guidelines recommend that patients who need mechanical ventilation for more than 48 to 72 hours be carefully assessed to determine all of the possible causes of ventilator dependence.^1,4 These include the respira- tory, cardiovascular, neurologic, and psychologic causes of ventilator dependence that are listed in Box 52-3. This recom- mendation is especially important for the care of patients who have had unsuccessful attempts at the discontinuation of ven- tilation.^1,4 Factors associated with readiness for the discontinu- ation of ventilatory support are summarized in Box 52-4.

The Most Important Criterion

The single most important criterion to consider when evaluat- ing a patient for ventilator discontinuation or weaning is whether there has been significant alleviation or reversal of the disease state or condition that necessitated use of the ventilator in the first place.^1,4,9 The clinician should determine whether the patient’s condition is improving, whether the initial reason for providing ventilatory support is improved or resolved, and prolonged use of neuromuscular blocking agents are major

causes of the development of ventilatory muscle weakness in the intensive care unit (ICU).¹³ Controlled ventilation for pro- longed periods can result in ventilatory muscle discoordination and atrophy. Ventilatory muscle endurance is a function of energy supply versus demand. Energy supply is related to nutri- tion, perfusion, and cell use, whereas demand is related to the amount of work performed and is a function of minute ventila- tion, compliance, and resistance. Figure 52-1 summarizes the relationship between ventilatory demands and capabilities.

Global Criteria for Discontinuing  Ventilatory Support

Success with the discontinuation of ventilatory support is related to the patient’s condition in four main areas^1,2,3,4:

FIGURE 52-1 Ventilatory failure and the need for ventilatory support depend on the balance between ventilatory muscle demands (i.e., loads) and ventilatory muscle capabilities.

CLT, Lung–thorax compliance; RAW, airway resistance, V, minute alveolar ventilation; V, minute dead space ventilation. (Modified from MacIntyre NR: Respiratory factors in weaning from mechanical ventilatory support. Respir Care 40:244–259, 1995.)

Partial support

Mechanical ventilation

needed (intolerable

loads)

Spontaneous ventilation

possible (tolerable

loads)

Increasing reserve

Total support

Demands Capabilities

• Pressure loads CLT

RAW

• Ventilation loads VA ( VCO2, VO2) VD

• Imposed loads

• Neural drive

• Muscle function strength endurance

Normal reserve

•

•

• •

Box 52-2 Factors That May Reduce Ventilatory Drive

• Decreased PaCO2 (respiratory alkalosis)

• Metabolic alkalosis

• Pain (visceral)

• Electrolyte imbalance

• Pharmacologic depressants (narcotics, sedatives)

• Fatigue

• Decreased metabolic rate

• Increased PaCO2 associated with chronic carbon dioxide retention

• Neurologic or neuromuscular disease

whether the patient’s clinical condition is stable. The following specific questions for patient evaluation have been suggested¹: 1. Is there evidence of improvement or reversal of the disease

state or condition that caused the need for mechanical ventilation?

2. Is the patient’s oxygenation status adequate? Specific criteria may include the following: PaO2 of 60 mm Hg or more, FiO2 of less than 0.40 to 0.50, PEEP of less than 5 to 8 cm H2O; PaO2/FiO2 of 150 to 200 or more; and pH of 7.25 or more.

3. Is the patient medically and hemodynamically stable? Spe- cific criteria may include the absence of acute myocardial ischemia or marked hypotension. Patients should have ade- quate blood pressure without vasopressor therapy or with only low-dose vasopressor therapy (i.e., less than 5 µg/kg/

min of dopamine or dobutamine).

4. Can the patient breathe spontaneously? The patient must be able to breathe spontaneously and have a sufficient drive to breathe if ventilator discontinuation is being considered.

If the patient’s condition is improving, if the alleviation or reversal of the precipitating disease state or condition has occurred, if the patient is capable of spontaneous breathing, and if the oxygenation status and hemodynamic values are stable, then ventilator discontinuation should be attempted.¹

Weaning Indices

Mechanical ventilation is hazardous, and unnecessary delays in ventilator discontinuation increase the associated complica- tion rate. Unfortunately, premature ventilator discontinuation may also cause serious problems, including difficulty with reestablishing the artificial airway and serious compromise of the patient’s clinical status. Clinical judgment has been found to be a poor guide to determining whether a patient is ready for ventilator discontinuation, and more specific indicators have been sought. Specific indicators or weaning indices that clearly show whether a patient is ready to have the ventilator removed and help to avoid inappropriate ven- tilator discontinuation have been sought. Unfortunately, none of the current weaning indices are capable of predicting readiness for ventilator discontinuance with a high level of accuracy.^1,4

Traditional discontinuation indices include the PaO2/FiO2

ratio, the alveolar-to-arterial partial pressure of oxygen differ- ence [P(A − a)O2], the maximum inspiratory pressure (MIP), the vital capacity (VC), the spontaneous minute ventilation (VEsp), and the maximum voluntary ventilation (MVV).^3,17 Newer indices include the rapid, shallow breathing index (f/VT), airway occlusion pressure (P0.1), and measures of WOB.^1,4 Although all of these values can be useful, there are enormous discrepancies in the literature regarding their accu- racy with regard to the prediction of “weanability.”^1,4 With respect to the more traditional discontinuation indices, vital capacity and MIP can be highly variable, whereas minute ven- tilation, respiratory rate (f), and f/VT tend to be more reliable.^1,4 However, these measures may not correlate well with Box 52-3 Factors That Contribute to

Ventilator Dependence RESPIRATORY FACTORS

• Ventilatory workload exceeds ventilatory capacity

• Decreased compliance: lung or chest wall

• Increased resistance: artificial airways, bronchospasm, mucosal edema, secretions, and mechanical demand flow systems

• Increased dead space: pulmonary embolus and COPD

• Ventilatory muscle weakness or fatigue

• Oxygenation problems

• ↓ V Q /

• Increased shunt

• ↓ DO2

• ↓ Oxygen extraction ratio NONRESPIRATORY FACTORS

• Cardiovascular factors

• Myocardial ischemia

• Heart failure

• Hemodynamic instability, hypotension, and arrhythmias

• Neurologic factors

• Decreased or increased central drive to breathe

• Decreased peripheral nerve transmission

• Psychologic factors

• Fear and anxiety

• Stress

• Confusion or altered mental status

• Depression

• Poor nutrition

• Multiple-system organ failure

• Equipment shortcomings

Data from MacIntyre N: Respiratory factors in weaning from mechanical ventilatory support. Respir Care 40:244, 1995; Slutsky AS: Mechanical ventilation. American College of Chest Physicians’ Consensus Conference. Chest 104:1833–1859, 1993; Pierson DJ: Nonrespiratory aspects of weaning from mechanical ventilation. Respir Care 40:263–

270, 1995; MacIntyre NR, Cook DJ, Ely EW Jr, et al; American College of Chest Physicians; American Association for Respiratory Care;

American College of Critical Care Medicine: Evidence-based guidelines for weaning and discontinuing ventilatory support: a collective task force facilitated by the American College of Chest Physicians; the American Association for Respiratory Care; and the American College of Critical Care Medicine. Chest 120(6 Suppl):375S–395S, 2001.

Box 52-4 Factors Associated With

Readiness for the Discontinuation of Ventilatory Support

• Reversal or partial reversal of reason for instituting mechanical ventilation

• Good baseline functional status

• Ventilatory capacity that is capable of meeting ventilatory workload

• Good oxygenation status

• Good cardiovascular performance

• Good functional status of other organs and systems

• Short duration of the critical illness

• Short duration of mechanical ventilation

• No psychologic factors affecting current status

Modified from Pierson DJ: Nonrespiratory aspects of weaning from mechanical ventilation. Respir Care 40:263–270, 1995.

that the patient’s condition is unstable and that the patient has a poor prognosis for ventilator removal.²¹

P0.1 is the inspiratory pressure that is measured 100 millisec- onds after airway occlusion.^1,4 The P0.1 is effort independent, and it correlates well with central respiratory drive. Ventilator- dependent patients with COPD who have a P0.1 of more than 6 cm H2O tend to be difficult to wean.^1,4

The f/VT is the ratio of spontaneous breathing frequency (breaths/min) to tidal volume (liters), and it has been found to discontinuation success among all patients and especially

among those receiving long-term ventilatory support, the elderly, and those with major pulmonary abnormalities.^1,4,18

A comprehensive evidence-based review identified a possible role for 66 specific measurements as predictors of weaning success.⁴ Of these, eight values were found to be the most useful for the prediction of successful ventilator discontinuation.^1,4 Useful predictive measures included spontaneous respiratory rate, spontaneous tidal volume, f/VT, minute ventilation, MIP, P0.1, P0.1/MIP, and a combined index called the CROP score that included compliance, respiratory rate, oxygenation, and MIP.^1,4 Unfortunately, these measures all have limitations and relatively high false-positive predictions in specific settings.

It is doubtful that a single index will be found that can be used for consistent discrimination between discontinuation success and failure. Moreover, none of these traditional indica- tors alone has proved useful for the prediction of improvements in patient outcome or in the selection of a particular discon- tinuation method.⁵ The likely explanation for this failure to identify any consistently powerful discontinuation predictor is that patients’ conditions vary greatly and, for research purposes, clinicians already fully consider information from predictors when choosing patients for trials of the reduction or discon- tinuation of ventilatory support.^5,18

Notwithstanding these limitations, the measurement of discontinuation indices in the difficult-to-wean patient may provide guidance with regard to the reasons that patients fail discontinuation trials. Many find it useful to trend these data on a daily basis for those patients who require lengthy weaning times.^10,15 Specific values for respiratory indices that are used to predict the successful discontinuation of ventilatory support are found in Table 52-1.

Ventilation

Increased thoracic cage movement during spontaneous breath- ing and asynchronous chest-wall-to-diaphragm movement are related to an increased workload that may lead to ventilatory muscle fatigue and failure. Tachypnea (i.e., more than 30 to 35 breaths/min in adults) is a sensitive marker of respiratory dis- tress, but it can prolong intubation if it is used as an exclusive criterion. Irregular spontaneous breathing or periods of apnea indicate that the patient is at risk for weaning failure. Asynchro- nous and rapid, shallow breathing patterns—although not definitive—suggest respiratory decompensation.¹⁸ However, decreased ventilatory variability over time (rate, VT, minute ven- tilation) has been clearly shown to identify patients who will failure an SBT.^19,20

The evaluation of patients for the presence of palpable scalene muscle use during inspiration, an irregular ventilatory pattern, palpable abdominal muscle tensing during expiration, and the inability to alter the ventilatory pattern on command can be helpful for the assessment of the potential for prolonged spontaneous ventilation. Patients with none of these signs have a very high probability of successful ventilator discontinuance.

Patients with one or two of these signs usually need continued support. The presence of three or more of these signs can mean

TABLE 52-1

Indices That Are Used to Predict the Success of Weaning and Ventilator Discontinuation

Measurement Criterion

Oxygenation

FiO2 ≤0.40 to 0.50

PEEP (cm H2O) ≤5 to 8

PaO2 (mm Hg) ≥60

SaO2 (%) ≥90

SvO2 (%) ≥60

PaO2/PAO2 ratio ≥0.35

PaO2/FiO2 ratio >150 to 200

P(A-a)O2 (mm Hg) <350

Q Q s T (% shunt) <15% to 20%

No lactic acidosis, adequate Q Q s T, blood pressure

Ventilation

PaCO2 (mm Hg) <50

pH ≥7.35

Ventilatory Mechanics

Respiratory rate (f) (breaths/min) 12 to 30

Tidal volume (VT) (ml/kg) >5

Vital capacity (VC) (ml/kg) >10 to 15 Static compliance (ml/cm H2O) >25

f/VT <105

Respiratory Muscle Strength

Maximum inspiratory force (MIF) (cm H2O) <−20 to −30 Ventilatory Drive (Demand)

Minute ventilation (VE) for

Normal PCO2 (L/min) <10

VDS/VT <0.55 to 0.60

P0.1 (cm H2O) <6

P0.1/MIP <0.30

Work of Breathing

Spontaneous work of breathing <1.6 kg⋅m/min (<0.14 kg⋅m/L)

Pressure-time index <0.15 to 0.18

Ventilatory Reserve

Maximum voluntary ventilation (MVV) (L/min) >20; more than twice the VE

Data from MacIntyre NR, Cook DJ, Ely EW, et al: Evidence-based guidelines for weaning and discontinuing ventilator support: a collective task force facilitated by the American College of Chest Physicians, the American Association for Respiratory Care, and the American College of Critical Care Medicine. Chest 120:375S-395S, 2001; AHRQ publication no. 01-E010, Rockville, MD, 2000, Agency for Healthcare Research and Quality; American College of Chest Physicians: Chest 104:1833, 1993; Burns SM et al: Am J Crit Care 4:4, 1995; Sharar S: Resp Care 40:239, 1995; Bassili HR, Deitel M: JPEN J Parenter Enteral Nutr 5:161, 1981.

WOB would seem to be an excellent way to gauge spontane- ous ventilatory workload. Successful weaning has been found to be less likely among patients with spontaneous WOB levels of more than 1.6 kg/m/min (16 J/min) or 0.14 kg/m/L (1.4 J/L).^1,4 However, WOB may not be predictive of weaning success for specific patients.^1,4 This may be because WOB does not take into account ventilatory muscle capacity or fatigue. Conse- quently, WOB may be less accurate than other conventional discontinuation indices, and it is very difficult to measure at the bedside.

Oxygen cost of breathing (OCB) is the difference between oxygen consumption during spontaneous breathing and oxy- gen consumption during apnea (i.e., during full ventilatory support), which is determined as follows:

OCB VO= 2_sp−VO controlled ventilation2( )

After the OCB has been estimated, the relative proportion of oxygen consumed by the respiratory muscles as compared with the body as a whole can be calculated as follows:

%VO Resp₂( )=(OCB VO_2sp)×100

Both OCB and %VO2 have been correlated with the number of days required to wean patients. Patients with an OCB of 15%

or less of the total VO2 may be more likely to achieve discon- tinuation success.^1,4

Pressure–time product (i.e., the area under the inspiratory pressure–time curve) and pressure–time index (PTI) may be the best measures of ventilatory workload of patients who are receiving mechanical support.¹ The PTI can be calculated as follows:

PTI=(Mean inspiratory pressure MIP)×T Ti tot

where MIP is maximum inspiratory pressure, Ti is the inspira- tory time in seconds, and Ttot is the total respiratory cycle. The Ttot can be calculated by dividing 60 by the respiratory rate (f) (i.e., 60/f). A PTI of more than 0.15 to 0.18 has been associated with diaphragmatic fatigue, and a PTI of more than 0.15 cannot be sustained indefinitely.^1,4 There is currently no well accepted and reliable way to measure ventilatory muscle fatigue in patients who are receiving mechanical ventilation.

Oxygenation

Poor oxygenation status is associated with weaning failure.

Arterial blood gas (ABG) analysis, pulse oximetry, and continu- ous mixed venous oximetry have been used to monitor and assess the oxygenation status of patients before and during a discontinuation trial. In general, a PaO2 of more than 60 mm Hg (or of more than 55 mm Hg for patients with COPD with carbon dioxide retention) with an FiO2 of less than 0.40 to 0.50 and a PEEP of 5 to 8 cm H2O or less should be adequate for ventilator discontinuation. The PaO2/FiO2 ratio should be 150 to 200 mm Hg or more. With these values, a normal hemoglobin level, a normal oxygen saturation (SaO2), and adequate cardiac output and tissue perfusion are assumed.

Specific indices used to assess oxygenation status are found in Table 52-1.

be a good predictor of discontinuation success for many patients who need mechanical ventilation.^1,4,18 The f/VT has less predic- tive ability for patients who need ventilatory support for longer than 8 days, and it may be less useful for predicting discontinu- ation success among elderly patients.^1,4 Despite these limita- tions, an f/VT of less than 105 can be an accurate and early predictor of weaning outcome, and an f/VT of 80 is associated with an almost 95% probability of successful discontinuation.¹⁸ The ratio must be calculated during 1 minute of unsupported spontaneous breathing, and the addition of pressure support during measurement significantly reduces the predictive value of the ratio.¹⁸

MINI CLINI

Calculating and Interpreting the Rapid, Shallow Breathing Index

PROBLEM: You measure the following spontaneous breath- ing values for two patients who are being considered for weaning from mechanical ventilation:

Patient Rate (f) (breaths/min) VT (L)

A 32 0.28

B 28 0.42

For which patient is successful weaning least likely?

Solution: First, compute the rapid, shallow breathing index for each patient as follows:

Patient Rate (f) (breaths/min) VT (L) f/VT

A 32 0.28 114

B 28 0.42 67

Patient A clearly exceeds the threshold criterion of 105 breaths/min per liter, whereas patient B falls well below this criterion. All else being equal, patient A is least likely to be suc- cessfully weaned.

RULE OF THUMB

Adult patients with spontaneous respiratory rates in excess of 35 breaths/min and tidal volumes of less than 5 ml/kg PBW are difficult to wean.

The P0.1/MIP ratio has been found to be a good early predic- tor of discontinuation success,^1,4 and it may be more useful than the MIP by itself. The f/VT also has been found to be a better predictor of discontinuation success than the MIP alone.^1,4 However, even with f/VT of less than 105, as many as 20% of patients have false-positive results (i.e., they cannot be discon- tinued from ventilation despite a favorable index) as a result of unpredictable factors such as congestive heart failure, aspiration, other comorbidities or the development of a new pulmonary lesion. In addition, some patients can be success- fully discontinued from ventilatory support despite poor f/VT

values (>105).

Cardiac output and index measurements as well as central venous pressure measurements may be helpful for the evalua- tion of cardiovascular function. Left ventricular dysfunction, myocardial ischemia, and cardiovascular instability are associ- ated with decreased discontinuation success.^1,4 Table 52-2 pro- vides criteria for confirming cardiovascular stability.

Psychologic Factors and Central  Nervous System Assessment

Adequate CNS function is needed to ensure stable ventilatory drive, adequate secretion clearance (i.e., coughing and deep breathing), and the protection of the airway (i.e., gag reflex and swallow). In addition, the level of consciousness, dyspnea, anxiety, depression, and motivation can affect discontinuation success.^1,4

The patient ideally is awake and alert, free of seizures, and able to follow instructions. Patients should have an intact central drive to breathe and peripheral nerve function. Brain- stem strokes, electrolyte disturbances, sedation, neuromuscular blocking agents, and narcotic drugs can impair the central neu- rologic control of ventilation. Mental status is a good predictor of discontinuation success, and patients who are not alert are at risk for upper airway obstruction, aspiration, and secretion retention. Obtunded patients should, at a minimum, have an adequate gag reflex and cough. Decreased levels of conscious- ness are associated with aspiration after extubation. The level of consciousness is affected by the use of narcotic, sedative, and analgesic drugs. Drugs with CNS depressant effects should be discontinued, if possible, before the withdrawal of ventilatory support and extubation. Protocols to reduce sedation and the daily cessation of sedative drugs may reduce weaning time^7,22,23 (see the section on Spontaneous Awaking Trials). Neuromuscu- lar blocking agents to allow for controlled ventilation should only be administered when absolutely necessary to insure Acid-Base Balance

Ideally the patient should have a normal acid-base balance (i.e., a pH of 7.35 to 7.45), and abnormalities in acid-base status have been corrected, if possible, before weaning. Patients with meta- bolic acidosis often have an increased ventilatory drive that can make weaning difficult. Patients who have metabolic alkalosis or those who have been mechanically hyperventilated for several days may have a reduced ventilatory drive. In these cases, a gradual method of discontinuing ventilatory support may be necessary.

Metabolic Factors

Metabolic factors primarily affect discontinuation in those patients who require long-term ventilatory support. Although nutritional factors are important for all patients, they are unlikely to affect discontinuation in those who only require short-term ventilatory support. Nutrition should be adequate to maintain respiratory muscle mass and contractile force.

Feeding should be adjusted according to individual patient needs; most patients need 1.5 to 2.0 times their resting energy expenditure. In addition, protein intake should be between 1 and 1.5 g/kg per day. Excessive carbohydrate feeding can increase carbon dioxide production and may precipitate acute hypercapnic respiratory failure. Parenteral nutrition solutions that contain amino acid formulations (e.g., arginine/lysine) can cause metabolic acidosis and thus increase ventilatory demand. Metabolic rate can increase as a result of fever or sepsis. Increased WOB, shivering, seizures, and agitation can also increase oxygen demand and should be evaluated (see Chapter 23).

Renal Function and Electrolytes

Adequate renal function is required to maintain acid-base homeostasis, electrolyte concentrations, and fluid balance. The patient ideally should have an adequate urine output (i.e., more than 1000 ml/day), and there should be no inappropriate weight gain or edema.

Renal insufficiency can lead to metabolic acidosis, which increases respiratory drive. Electrolyte disorders can impair ventilatory muscle function. Key electrolytes should be normal (see Chapter 17 for details). Fluid overload can lead to conges- tive heart failure and pulmonary edema, which may impair pulmonary gas exchange.

Cardiovascular Function

Adequate cardiovascular function is needed to provide suffi- cient oxygen delivery to the tissues. Cardiac rate and rhythm and blood pressure should be evaluated. Tachycardia (i.e., a heart rate of more than 100 beats/min) and bradycardia (i.e., a heart rate of less than 60 beats/min) should be controlled. The presence of arrhythmias, hypotension (i.e., a blood pressure of less than 90/60 mm Hg), and severe hypertension (i.e., a blood pressure of more than 180/110 mm Hg) should be evaluated carefully before the discontinuation of ventilatory support is considered.

TABLE 52-2

Criteria for Confirming Cardiovascular Stability Criterion Normal Value Values That May Be

Inconsistent With Weaning Heart rate (beats/min) 60 to 100 <60, >120 Blood pressure

(mm Hg) 90/60 to 150/90 <90/60, >180/110

Qt (L/min) 4 to 8 <4, >8 Cardiac index (L/

min⁻¹⋅m²) 2.5 to 4 <2.1

Cardiac rhythm No major arrhythmias present

Tachycardia,

bradycardia, multiple premature ventricular contractions, heart block

Hemoglobin (g/dl) 12 to 15 Anemia, <8

Hematocrit 40% to 50% <35%

No angina present No lactic acidosis

Program is a 26-item assessment that combines 12 general and 14 respiratory factors into a single score.^1,4 Although integrated indices appear promising, none of these indices has emerged as superior for use in diverse patient populations. Despite the success of these integrated indices in very specific settings, the best approach to determining if a patient can be successfully discontinued from ventilatory support is the patient’s perfor- mance on a spontaneous breathing trial. All patients should be assessed daily, and their ability to breathe spontaneously should be the primary variable to determine if the ventilator can be discontinued.

MINI CLINI

Assessment of Readiness for a Spontaneous Breathing Trial

PROBLEM: A 64-year-old man who underwent a lung resec- tion and who has a long history of COPD is now 24 hours postoperative, and he is being evaluated for readiness for an SBT. The data currently available for this patient include the following:

• Ventilator settings:

• Mode pressure support 10 cm H2O

• Average VT 450 ml (6.5 ml/kg ideal body weight)

• Respiratory rate of 28 breaths/min

• FiO2 of 0.40

• PEEP of 5 cm H2O

The patient is alert and cooperative.

The patient is not receiving any vasoactive drugs, and he is only receiving intermittent sedatives and narcotics.

Should this patient be placed on an SBT?

Solution: By 24 hours, the patient should have initially recov- ered from the effects of the surgical procedure. The patient’s ventilator settings are offering minimal support. Because the patient is alert and able to breathe spontaneously and because he requires only intermittent sedatives or narcotics, it is very appropriate to perform an SBT on this patient at this time.

patient-ventilator interaction and for the shortest period possible.²⁴

Psychologic factors may be among the most important non- respiratory contributing factors that lead to ventilator depen- dence.^1,4 Fear, anxiety, pain, and stress should be minimized, and frequent communication among the staff, the patient, and the patient’s family can be helpful. Box 52-5 summarizes non- respiratory factors that affect discontinuation success.

Integrated Indices

Many factors are associated with discontinuation success. Inte- grated indices improve prediction by combining several mea- sures of ability to breathe without ventilatory support. Current examples of integrated indices include the CROP score, the Adverse Factor/Ventilator Score, the weaning index, and the Burns Weaning Assessment Program.^1,4 The CROP score com- bines measures of ventilatory load, respiratory muscle strength, and gas exchange.

The Adverse Factor/Ventilator Score combines ratings of 15 adverse factors, including hemodynamic values, infection, nutrition, and neurologic/psychiatric state, with ratings of six ventilator factors, including FiO2, compliance, minute ventila- tion, and rate.^1,4 The weaning index combines measures of ven- tilatory strength, endurance, and efficiency of gas exchange. A weaning index of less than 4 suggests successful discontinuation from mechanical ventilation. The Burns Weaning Assessment

Box 52-5 Nonrespiratory Factors That Affect Weaning

• Acid-base status

• Metabolic alkalosis: decreased ventilatory drive

• Metabolic acidosis: increased ventilatory demand

• Mineral and electrolyte balance

• Hypophosphatemia: ventilatory muscle weakness

• Hypomagnesemia: ventilatory muscle weakness

• Hypokalemia: ventilatory muscle weakness

• Hypothyroidism: decreased ventilatory drive and impaired muscle function

• Stability of other organs and systems

• Cardiac: excessive preload (e.g., overall volume overload, increased preload on discontinuation of positive pressure ventilation) and impaired contractility

• Renal: renal insufficiency and metabolic acidosis

• Hepatic: encephalopathy and protein synthesis

• Gastrointestinal: stress-related hemorrhage and ability to take enteral nutrition

• Neurologic: level of consciousness, ability to protect the airway, and clear secretions

• Effects of drugs: narcotics, benzodiazepines, other sedatives and hypnotics, muscle relaxants, and aminoglycosides

• Nutritional status

• Ventilatory muscle function

• Ventilatory drive

• Immune defense system

• Psychologic and motivational factors

Modified from Pierson DJ: Nonrespiratory aspects of weaning from mechanical ventilation. Respir Care 40:289, 1995.

Evaluation of the Airway

The ability to maintain a patent natural airway and the likeli- hood of aspiration should be evaluated as a part of the process of discontinuing ventilatory support. It is important for the clinician to separate the decision to discontinue ventilatory support from the decision to extubate. The clinician must also be aware that most weaning indices do not evaluate airway patency or protection (see the section on Extubation). The inability to protect or maintain the natural airway is a clear contraindication to extubation. Some patients who can be suc- cessfully removed from a ventilator should not be extubated.

Although controversial, evaluating if gas moves freely around the ETT with the cuff deflated may identify an increased likeli- hood of postextubation airway obstruction. If auscultation of the lateral neck does not identify gas flow around the EET, extubation should be delayed until airway edema is properly