High-frequency oscillatory

High-frequency oscillatory ventilation in adults: Respiratory

therapy issues

Jason Higgins, BS, RRT; Bob Estetter, RRT; Dean Holland, RRT; Brian Smith, RRT; Stephen Derdak, DO

H

igh-frequency oscillatory ventilation (HFOV) is an al-ternative mode of ventila-tion that may be considered for acute respiratory distress syndrome (ARDS) in adult patients who are failing conventional ventilation (CV). Observa-tional studies have demonstrated that HFOV may improve oxygenation when used as a rescue modality in adult pa-tients with severe ARDS failing CV (1, 2). A recent multicentered, randomized, controlled trial of HFOV (3), compared with pressure control ventilation, dem-onstrated that HFOV was safe and effec-tive for adult ARDS. Of particular impor-tance to the respiratory therapist (RT), HFOV does not offer traditional monitor-ing capabilities (e.g., tidal volume, flow-time graphs, flow-volume loops, and so on) used to identify and optimize conven-tional ventilator strategies for changes in pulmonary mechanics. A multidisci-plinary approach is required to optimize the management of patients on HFOV. RTs caring for adult patients with ARDS should develop increased knowledge of mechanical properties intrinsic to HFOV,

an understanding of the underlying pathophysiology, and advanced patient assessment skills unique to this mode of ventilation.

Clinical expertise coupled with ad-vanced patient assessment skills place RTs in a key position in the management of patients on HFOV. The purpose of this chapter is to summarize clinical informa-tion pertinent to RTs caring for adult patients on HFOV. Active involvement of a critical care respiratory therapy team is essential to successful implementation of an adult HFOV program.

IDENTIFYING PATIENTS FOR HIGH-FREQUENCY

OSCILLATORY VENTILATION

Observational rescue trials suggest that early initiation of HFOV in patients with severe ARDS may be important to successful outcomes. Patients transi-tioned to HFOV within 72 hrs may have a better chance of survival than those pa-tients on CV for⬎7 days (1, 2). RTs, as an integral member of the critical care team, are in a frontline position to actively identify patients with ARDS who may be potential candidates for a trial of HFOV. Although the exact severity threshold at which to initiate a trial of HFOV remains unclear, an emerging approach in centers experienced in treating adults with HFOV may serve as guidelines (4, 5). HFOV may be considered for patients with ARDS

when they meet the following severity criteria:

● FIO₂⬎.60 and/or SpO₂⬍88% on CV with

positive end-expiratory pressure⬎15 cm H2O, or

● Plateau pressures (Pplat)⬎30 cm H2O,

or

● Mean airway pressure (MAP)ⱖ24 cm H2O, or

● Airway pressure release ventilation Phighⱖ35 cm H2O.

Once severity criteria are met, and a decision is made to initiate HFOV, em-phasis should be placed on transitioning to HFOV as soon as feasible (within 12–24 hrs).

HIGH-FREQUENCY

OSCILLATORY VENTILATION ‘TEAM APPROACH’

At Parkland Hospital, the HFOV man-agement team consists of the attending intensivist, respiratory care team leader, respiratory care area manager, intensive care unit respiratory therapist, critical care nurse, and an identified consult team member who is on-call 24 hr a day/7 days a wk for troubleshooting (Table 1). Once a potential candidate is identified, the HFOV team will discuss possible rea-sons the patient is failing CV, review other available options (e.g., prone posi-tioning), timing of transition from CV to HFOV (e.g., will bronchoscopy be per-From Parkland Health and Hospital System (JH,

BE, DH, BS), Respiratory Care Department, Dallas, TX; and Pulmonary/Critical Care Medicine (SD), Wilford Hall Medical Center, Lackland AFB, TX.

Copyright © 2005 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins

DOI: 10.1097/01.CCM.0000155922.78943.2D

Objective:To summarize clinical information and assessment techniques relevant to respiratory therapists caring for adult patients on high-frequency oscillatory ventilation (HFOV).

Data Source:Review of observational studies, controlled trials, case reports, institutional experience, and hospital HFOV guide-lines for adult patients.

Data Summary:Respiratory therapists require unique physical assessment skills and knowledge in managing patients on HFOV. Respiratory therapy procedures relevant to HFOV include setting endotracheal tube cuff leaks, performing lung recruiting

maneu-vers, endotracheal suctioning, and monitoring ventilator param-eters. Respiratory therapists serve as essential team members in the creation and implementation of written HFOV guidelines (e.g., algorithms) to optimize patient care.

Conclusion: Respiratory therapy assessment and procedural skills are essential in providing optimal care to adult patients on HFOV. (Crit Care Med 2005; 33[Suppl.]:S196 –S203)

KEY WORDS: high-frequency oscillatory ventilation; respiratory therapy; acute respiratory distress syndrome; lung recruitment maneuvers; endotracheal cuff leak

formed before initiation?), initial HFOV settings, baseline monitoring measure-ments, and HFOV strategies for a failure-to-respond scenario. An important goal of the HFOV team approach is to facilitate communication between disciplines and to ensure that parameter adjustments are appropriate. The intensive care unit RT is responsible for identification and hands-on care of the patient after initia-tion. He or she is the liaison to the HFOV team and has been trained and demon-strated to be competent in caring for pa-tients on HFOV.

We believe the “team approach” pro-vides our patients with the best care by optimizing HFOV management and pro-viding ongoing education for those indi-viduals not as familiar with the manage-ment of HFOV.

Patient Assessment and Monitoring Techniques

RTs must have a broad understanding of ARDS pathophysiology, mechanical properties of the mode of ventilation be-ing used, and the specific ventilator strat-egy and goals chosen for the patient. A thorough physical assessment is, un-doubtedly, the most important skill needed to provide quality care to patients on HFOV. RTs receive extensive training in auscultation, inspection, palpation, and percussion (6). In addition, they re-ceive thorough training on how to

inter-pret the monitoring indices associated with CV, as well as how to troubleshoot a ventilator (7). At present, literature re-garding physical assessment and specific monitoring parameters for adult patients on HFOV is limited. Patients on HFOV may experience acute changes requiring rapid recognition to provide optimal management.

Identifying Diaphragm Position.Lung inflation during HFOV may be estimated by monitoring diaphragm position on su-pine portable chest radiographs (CXR). Hyperinflation may be suspected on CXR if the apical to diaphragm distance ex-ceeds 24 –25 cm and/or the anterior sixth rib is visible above the diaphragm (8). Because changes in lung inflation may be accompanied by positional changes of the diaphragm, it is beneficial to identify baseline diaphragm position at the initi-ation of HFOV. The traditional technique of percussion to locate diaphragm posi-tion is difficult as a result of the noise generated by the ventilator. To accurately percuss the diaphragm, ventilation may need to be temporarily interrupted by stopping piston oscillation.

At Parkland Hospital, we have used a Doppler technique to quickly identify the position of the diaphragm at the bedside. After initiation of HFOV, and simulta-neously with the initial CXR, a pencil Doppler (9.3 mHz, model 915BL; Parks Medical Electronics, Aloha, OR) is used to locate the diaphragm. Diaphragm

posi-tion is then marked directly on the pa-tient with a surgical marker pen. This mark is used to correlate the position of the diaphragm with its location on the CXR. Once the location of the diaphragm has been confirmed, we then monitor lung displacement as part of routine pa-tient assessment with every ventilator check. If trended consistently, this tech-nique assists with early recognition of changes in lung inflation and may prompt ordering of a CXR if a significant change is suspected. Other techniques that may be used at the bedside to locate the position of the diaphragm are dia-phragmatic auscultation or conventional ultrasound visualization.

Auscultation. Breath sounds should be auscultated with every ventilator check and during patient compromise. Recommendations for frequency of HFOV ventilator checks by the RT are every 30 mins during the first hour after initia-tion, then every hour for 2 hrs, and then every 2 hrs (Table 2). Although bilateral auscultation of the chest may not reveal adventitious breath sounds, it can assist in the identification of lung inflation, di-aphragm position, and may be helpful with detection of other complications such as pneumothorax, atelectasis, endo-tracheal tube (ETT) obstruction, or mu-cus plugging. Unlike the assessment of breath sounds during CV, the clinician may not be able to identify wheezes, rhonchi, or crackles during HFOV be-cause of the small tidal volumes delivered and the noise of the oscillator piston. For this reason, breath sounds should be aus-cultated whenever HFOV is interrupted for manual ventilation. During HFOV, the clinician should listen closely to the quality of the percussions delivered by the ventilator. Breath sounds should be auscultated over all accessible regions of the chest and compared with the opposite side for symmetry. Unilateral decreased breath sounds may be detected with pneumothorax, mucous plugging, atelec-tasis, mainstem intubation, and pleural effusions. Bilaterally decreased breath sounds may be observed with accidental extubation, ETT occlusion (partial or complete), alveolar collapse, alveolar overdistension, fluid overload/pulmonary edema, and obesity.

Visual and Tactile Inspection. It is very important to visualize and palpate the chest with each HFOV assessment. Visual inspection of the chest for “chest wiggle” and movement of the abdomen may assist with identifying changes in

Table 1.High-frequency oscillatory ventilation (HFOV) consultation team

Responsibility Action/circumstance

Intensive care unit respiratory therapist

Identify patients that meet the indications for implementation of HFOV

Contact RC team leader when patient is identified Record current settings on conventional ventilation before

implementation of HFOV

Contact RC team leader when ventilator changes are indicated

Consult with TL, consultation team members, and physician staff regarding all setting changes

Respiratory care team leader Assure that HFOV is available and properly calibrated Play an active role in the management of the HFOV Contact identified consultation team member when

ventilator changes are indicated

Consult with intensive care unit therapists, consultation team members, and physician staff regarding all setting changes

Consultation team members Play an active role in the management of the HFOV Consult with team leaders, therapists, other team members,

and physician staff regarding all setting changes Respond promptly when on-call for consultation

Provide pertinent articles, troubleshooting guides, and other material to interested therapists and physicians

lung compliance, lung resistance, and/or airway resistance. If the chest is not bouncing as much as it was 2 hrs previ-ously and there appears to be slight changes in ventilator parameters, there may be significant changes occurring in lung mechanics. Serial evaluations of rib spaces may also assist in determining lung inflation. Rib spaces that have in-creased over time may indicate the lung has been recruited to the point of overd-istension. Obesity or chest wall edema may make these monitoring techniques difficult.

ENDOTRACHEAL TUBES

Proper ETT management is of extreme importance in all critically ill patients, especially those receiving HFOV. ETT size, position, and patency have direct effects on gas exchange independent of alterations in the patients’ underlying lung pathology. Smaller-diameter ETTs (e.g.,ⱕ7 mm internal diameter) attenu-ate delivered tidal volume and make ef-fective ventilation of large adults more difficult.

Endotracheal Tube Position

ETT position should be checked regu-larly and maintained. As a result of high levels of mean airway pressure (mPaw) during HFOV, migration of the ETT prox-imally in the trachea may occur. The po-sition of the ETT relative to a fixed ana-tomic site (e.g., upper front teeth or gum) should be recorded and monitored frequently. Migration of the ETT as little as 2 to 3 cm can adversely affect the ability to ventilate the patient.

Tracheal Suctioning

Gross pulmonary edema, hemorrhage, or foaming into the airway, ETT, and/or oscillator circuit will impede the ability to oxygenate and ventilate during HFOV. Obvious filling of the ETT tube with edema, blood, or foam must be cleared by tracheal suction (usually combined with vigorous manual ventilation with an at-tached positive end-expiratory pressure valve) before initiation of HFOV. Simi-larly, excessive secretions or mucus plug-ging in the distal airways or ETT can

adversely affect adequate gas exchange during HFOV. Because the mechanism of injury associated with ARDS predisposes to alveolar collapse, the RT must be aware that tracheal suctioning (TS) can also be detrimental by creating negative carinal pressure, which promotes addi-tional alveolar derecruitment. For this reason, TS should be performed only when clinically indicated (e.g., visible se-cretions), especially in patients with mar-ginal oxygenation requiring high mean airway pressures (mPaw). To ensure ETT patency, the inline TS catheter can be passed (without turning on suction) ev-ery 2 to 4 hrs along with instillation of a small volume of sterile saline (2–3 mL). We perform tracheal suction on CV just before initiation of HFOV and then briefly clamp the ETT during transition to min-imize alveolar derecruitment. It should be noted that this clamping technique is used in any instances the patient requires a ventilator disconnect (e.g., bronchos-copy, TS, transporting). If possible, TS is avoided during the first 12 hrs on HFOV to allow alveolar recruitment. After this

Table 2.High-frequency oscillatory ventilator (HFOV) check sheet HFOV checks to be done every 30 mins⫻2, every 1 hr⫻2, then every 2 hrs or as ordered

Date and time3 (30 Mins) (60 Mins) (2 Hrs) (3 Hrs) (5 Hrs) (7 Hrs) (9 Hrs)

FIO2 mPaw Power ⌬P Hz % I-time System temperature Bias flow Cuff leak SpO2 Heart rate Blood pressure OI Diaphragm monitored Bilateral BS

Spon resp rate

⌬s in mPaw

⌬s in⌬P 1mPaw alarm 2mPaw alarm Pressure limit

Consult Team Notification Parameters ●SpO2drop of⬎3–5% without recovery

●mPaw drift⬎ ⫾5 cm H2O (with spontaneous breathing) 䡩May need increase in paralytic or sedation

●⌬P driftⱖ5 cm H2O with significant diaphragm change or⌬P driftⱖ10 cm H2O without significant diaphragm change ●ABG parameters

䡩pH⬎7.45 or⬍7.20 or a⌬ofⱖ10

䡩PaCO2⬎60 mm Hg or⬍30 mm Hg or a⌬ofⱖ10 mm Hg 䡩PaO2⬎90 mm Hg or⬍55 mm Hg or a⌬ofⱖ10 mm Hg Changes made3

initial timeframe, in-line, closed TS is performed only when necessary and not on a “routine” scheduled interval. Addi-tional indications for TS include an abrupt increase in proximal oscillatory amplitude (⌬P) coupled with decreased “chest wig-gle,” unexplained hypercapnia or increas-ing oxygen requirements, and followincreas-ing prone positioning. If the patient’s respira-tory status continues to deteriorate in the presence of excessive pulmonary secretions after TS, bronchoscopy may be considered. Patients who desaturate (SpO₂dropsⱖ5%) after TS should be considered for perfor-mance of a lung recruiting maneuver (LRM; see subsequently).

Endotracheal Tube Obstruction

ETT narrowing caused by inspissated mucus or blood clot accumulation will result in increased airway resistance sec-ondary to a decreased lumen size. This increase in airway resistance can result in increased proximal oscillatory pressure amplitude (⌬Pproximal) displayed on the

ventilator and decreased carinal oscilla-tory pressure amplitude (⌬Pcarinal).

Par-tial occlusion of the ETT should be sus-pected if a previously stable PaCO₂is now

increasing. Identification may also be re-flected by an acute or gradual increase in ⌬Pproximal (⬎5 cm H2O) coupled with a

decrease in chest wiggle, a decrease in breath sounds bilaterally, and an increas-ing oxygen requirement. Acute occlusion of the ETT from mucous plugging or a kinked tube presents with a sudden in-crease in ⌬Pproximal (⬎10 cm H2O),

de-creased chest wiggle, dede-creased breath sounds bilaterally, and rapid oxygen de-saturation with hypercapnia. It is imper-ative the RT understands that decreasing the power setting in an attempt to main-tain an ordered⌬Pproximalwill only mask

the underlying problem. Moreover, de-creasing the power may result in further reduction of ⌬Pcarinal, which may

com-promise the patient more, even though the proximal amplitude appears to be un-changed.

The possibility of complete ETT ob-struction should initially be assessed by passage of a suction catheter and can be definitively diagnosed with emergent fi-beroptic bronchoscopy (FOB). If a suc-tion catheter cannot be passed and man-ual ventilation produces no air movement, emergent reintubation is re-quired. An attempt may be made to pass an ETT exchange catheter or stylet in an effort to open the occlusion while

equip-ment is readied to reintubate. “Quick-look” bronchoscopy should be considered before initiation of HFOV to ensure ETT and airway, especially if on CV_⬎3 days. Patients who desaturate significantly af-ter bronchoscopy may benefit from LRM (see subsequently) before resumption of the desired mPaw setting.

Cuff Leaks

Cuff leaks during HFOV may promote PaCO₂ clearance by several mechanisms

and may allow for the use of lower⌬P and higher Hz (which are conceptually more lung protective) (9 –11). A small cuff leak, approximately 5–7 cm H2O, may be tried

when refractory hypercapnia (pH⬍7.20) occurs despite maximal _⌬P and lowest Hz. Failure of a cuff leak to lower PaCO₂

may indicate upper airway edema around the ETT and may respond to placement of an additional oropharyngeal airway to al-low gas egress (12). Some centers use ETT cuff leaks at the initiation of HFOV in all patients. Whether to reserve use of a deliberate cuff leak for refractory hyper-capnia or to use a leak in all patients to facilitate use of lower⌬P and higher Hz strategies should be investigated. Before creating a cuff leak, the mouth and pos-terior pharynx should be suctioned. The low mPaw and high mPaw alarm should be reset to avoid triggering by a drop or rise in mPaw with initial setting of the cuff leak. At Parkland Hospital, our ap-proach is to initiate a cuff leak by increase bias flow by 5 L/min, then slowly remove air from the ETT cuff pilot balloon while monitoring for a 5- to 7-cm H2O drop in

mPaw on the ventilator. Once the appro-priate leak has been applied, the mPaw control is readjusted to return the mPaw to the original setting. It should be noted that increasing bias flow after institution of a cuff leak to achieve a set mPaw may result in an elevated mPaw as a result of a decreased cuff leak. This is of clinical importance because the magnitude of the cuff leak may change as a result of tra-cheal edema, secretion accumulation, and body positioning. The mPaw alarms and mPaw pressure limit should be set appropriately (e.g., bracket desired mPaw by 5–7 cm H2O) to protect the patient in

the event of a decreasing cuff leak. The technique used to create a cuff leak at Wilford Hall Medical Center is detailed in Appendix 1.

HUMDIFICATION

Humidification is often overlooked as an important aspect during any form of mechanical ventilation. Because endotra-cheal intubation bypasses the upper air-way, it becomes necessary for inspired gases to be heated and humidified artifi-cially to mimic normal respiratory phys-iology (13). Complications that may oc-cur as a result of ineffective heat and humidification are, but not limited to, hypothermia, inspissation of airway se-cretions, destruction of airway epithe-lium, and atelectasis (14). There are cur-rently two forms of delivering heat and humidification to patients requiring me-chanical ventilation: external active hu-midifiers and passive heat and moisture exchangers (HME). HMEs have not been adequately studied with HFOV and should not be used.

During HFOV, the bias flow circuit is connected directly to an external active humidifier to provide humidified gas en-tering the inspiratory limb of the circuit. Temperature settings should resemble those normally used during conventional mechanical ventilation and should be set to establish the desired gas temperature at the patient airway temperature port (15). We suggest maintaining tempera-ture settings at 37°C to 39°C. Water lev-els in the chamber should always be maintained at the appropriate levels to prevent the chamber from becoming dry. This will result in the patient receiving only heated gas without proper humidi-fication and may result in complications. The HFOV circuit has two temperature ports on the inspiratory limb, one near the patient’s airway and one near the pressure limit valve. Temperature should always be monitored as close to the pa-tient’s airway opening as possible. Be-cause ambient air temperatures can affect the temperature and relative humidity in the circuit, caution should be exercised if ambient temperatures exceed 84°F (e.g., burn intensive care units). Also, to pre-vent excessive rainout in the circuit, a heated wire circuit should be used. At our institution, active external humidifiers are checked with each ventilator check and appropriate documentation is per-formed.

LUNG RECRUITMENT MANEUVERS

Lung recruitment maneuvers are used to improve oxygenation after

derecruit-ing events (e.g., suction, bronchoscopy, circuit disconnects) or for patients who continue to have marginal oxygenation during HFOV. LRMs should be consid-ered for an acute drop in SpO₂ⱖ5% after

initial HFOV transition, TS, bronchos-copy, or circuit disconnect. When using an in-line closed suction catheter, perfor-mance of a recruitment maneuver during TS has been shown to prevent alveolar derecruitment during CV (16). Lung re-cruiting maneuvers are typically per-formed by briefly (40 – 60 secs) raising mPaw approximately 10 cm H2O above

the original set mPaw. Before performing the maneuver, the mPaw high alarm must be reset (e.g., to 50 cm H2O) and

any ETT– cuff leak removed. The oscilla-tor piston is turned off during the ma-neuver to minimize additional distal transmission of the_⌬P while the mPaw is elevated. Suggested LRM guidelines for HFOV are detailed in Appendix 1.

PRONE VENTILATION

Prone positioning is becoming a more widely used therapeutic modality in pa-tients with refractory hypoxemia. Studies show an improvement in oxygenation in approximately 60% of patients with ARDS (17, 18). Case reports have observed im-provements in oxygenation and ventila-tion with the combinaventila-tion of HFOV and prone ventilation (19). When patients re-ceiving HFOV are placed in the prone position, patient and ventilator assess-ment become extremely important. Care-ful adherence to a detailed proning algo-rithm is essential (20). Like with all patients receiving mechanical ventila-tion, maintaining a patent airway is of extreme importance. The actual turning of the patient usually requires brief dis-connect from the ventilator circuit and manual ventilation with a positive end-expiratory pressure valve. ETT placement must be confirmed and the circuit should be resecured after prone positioning. HFOV parameters need to be verified and documented immediately before and im-mediately after prone positioning. Lung mechanics and ETT leaks can rapidly change with patient turning, and RTs must be prepared to recognize these changes and address them rapidly. LRMs may be performed in the prone position, particularly if a circuit disconnect was required and desaturation persists.

TENSION PNEUMOTHORAX

Like with all forms of positive pressure ventilation, tension pneumothorax may develop as a manifestation of “vo-lutrauma” or secondary to the cystic na-ture of the underlying lung disease. Ad-ditionally, vigilance must be maintained for pneumothorax in patients undergoing central line placement (e.g., subclavian or internal jugular) and thoracentesis. Studies have demonstrated pneumotho-rax occurrence rates during HFOV simi-lar to those observed with conventional ventilation. However, in contrast to CV, the presence of a pneumothorax can be particularly difficult to detect in patients on HFOV because no alarms on the ven-tilator will reliably signal that tension is developing (21). Quick assessment of ETT placement, hemodynamic parameters, tracheal position, visualization of the chest for unilateral hyperinflation, de-creased chest movement, auscultation for breath sounds, diaphragmatic position, and palpation to identify the presence of subcutaneous emphysema assist the RT in detection of pneumothorax. If time permits and the patient is relatively sta-ble, the diagnosis can be confirmed by a “stat” portable CXR. After placement of a chest tube, the RT should anticipate that adjustments in mPaw and_⌬P:Hz will be required. The degree of leak (e.g., from a bronchopleural fistula) should be quanti-tated on the chest tube suction chamber device, and changes in the leak should be recorded in response to changing HFOV settings and as part of the routine sched-uled ventilator checks. Air leak through a bronchopleural fistula can be minimized by using the highest Hz, the lowest⌬P, the lowest mPaw, and the shortest in-spiratory time (IT%) allowable to achieve acceptable oxygenation and ventilation (22).

HEMODYNAMICS, ARTERIAL PRESSURE TRACINGS, AND PULSE OXIMETRY

Central vein and pulmonary artery pressure monitoring, peripheral arterial catheters, and pulse oximetry are forms of real-time monitoring that assess pa-tient hemodynamic stability during HFOV. Hemodynamic status is extremely important before and after initiation of HFOV. HFOV maintains alveolar recruit-ment by sustaining an essentially con-stant mPaw (in contrast to the cyclic pressure excursions of conventional

vol-ume-cycled ventilation). This increases mean intrathoracic pressure, reduces central venous return (e.g., right heart preload), and may cause potential adverse effects on the cardiovascular system. Therefore, patients requiring HFOV should be hemodynamically stabilized be-fore initiation and a fluid bolus and/or vasopressors should be readily available if hypotension occurs. We commonly use central venous pressure monitoring (e.g., internal jugular or subclavian central vein) or pulmonary artery flotation cath-eter monitoring to ensure optimal hemo-dynamics in unstable patients. In the ab-sence of a pulmonary artery flotation catheter, heart rate and blood pressure can be useful. Dampened arterial and pulse oximetry waveforms may indicate compromised cardiac output and require special attention by the RT to trend (7). In hypovolemic patients, heart rate read-ings on pulse oximetry may be observed that reflect the oscillatory frequency be-ing delivered. This phenomenon may suggest that cardiac output is being com-promised as a result of high intrathoracic pressures. In addition, waveform tracings of central venous pressure or pulmonary capillary wedge pressure (PCWP) may sometimes show low-amplitude superim-posed waves that correspond to the HFOV frequency (Hz). Brief interruption of the oscillator piston (while maintaining mPaw) may be considered when trying to obtain an accurate assessment of central venous pressure or PCWP waveforms. Changes in mPaw may produce similar changes in central venous pressure or PCWP, suggesting that at least some of the mPaw is being transmitted to the intravascular pressures. The response of blood pressure, central venous pressure, or PCWP trends in response to fluid chal-lenges may be a better indicator of the patient’s intravascular volume status than any absolute value, particularly in hypotensive patients requiring high mPaw.

OPTIMIZING HIGH-FREQUENCY OSCILLATORY VENTILATION SETTINGS

To optimally manage critically ill pa-tients on HFOV, it is important to under-stand the machine’s capabilities and lim-itations. HFOV has been considered a “decoupling device.” By definition, a de-coupling device uses individual controls that affect only certain parameters and nothing else. For example, mPaw, FIO₂,

and inspiratory time % (IT%) primarily affect oxygenation, whereas increasing oscillatory pressure amplitude (⌬P), de-creasing frequency (Hz), and creating ETT cuff leaks primarily increase ventila-tion. Clinical observations, however, sug-gest that HFOV is not an absolute decou-pling device. To maximize gas exchange while minimizing ventilator-associated lung injury, it is imperative to maintain an optimal mPaw while avoiding excess tidal volume delivery. Alveolar overdis-tension or underdisoverdis-tension from an im-properly set mPaw or⌬P:Hz combination can adversely affect both oxygenation and ventilation respectively and potentiate further lung injury.

Optimizing Mean Airway Pressure

Determining an appropriate mPaw setting can prove to be particularly chal-lenging, especially during recruitment phases. Optimal mPaw can be described as that mPaw which causes sufficient lung inflation to maximize gas exchange while protecting the lung from alveolar overdistension, underdistension, or im-peding hemodynamics. In hemodynami-cally stable adults, mPaw is typihemodynami-cally started at 2–5 cm H2O above the mPaw

observed during CV. Subsequent in-creases in mPaw by 1–2 cm H2O every

30 – 60 mins are used (up to a maximum of 40 – 45 cm H2O) to achieve a target

SpO₂ⱖ88% with an FIO₂ⱕ60%. Whether

to wean mPaw before reducing FIO₂ in patients who require high mPaw (e.g.,

ⱖ35 cm H2O) for optimal lung protection

remains unclear. One approach to using combinations of mPaw, FIO₂, and lung

recruiting maneuvers for oxygenation and weaning is outlined in Appendix 1.

Application of technologies such as re-spiratory inductance plethysmography and electrical impedance tomography during HFOV may offer more precise bed-side tools for assessing lung inflation and are reviewed elsewhere in this supple-ment.

OSCILLATORY PRESSURE

AMPLITUDE (⌬P)—RANDOM

DRIFT OR PATIENT FEEDBACK?

During HFOV, “real-time” feedback and trending of airway and lung mechan-ics may be available through monitoring changes in the displayed ⌬P (21, 23). Understanding that a number of clinical variables may cause changes in ⌬P can provide the bedside clinician with a use-ful monitoring tool.

Increases in ⌬P (assuming constant power setting) may occur with increases in ETT resistance, bronchospasm, or mainstem intubation (21). Thoracic com-pliance changes may have variable effects on ⌬P and may not be distinguishable from airway resistance effects. Similarly, variable ETT cuff leaks or spontaneous breathing may cause fluctuations in⌬P. As a result of the many contributing fac-tors associated with changes in ⌬P, pa-tient care decisions should not be made solely on this measurement. Rather, ⌬P should be included as an additional mon-itoring parameter in conjunction with other patient assessment techniques, he-modynamic parameters, and arterial blood gas analysis. We routinely record the observed⌬P during ventilator checks

and monitor trends as an early indicator of possible changes in pulmonary me-chanics. In situations of acute decompen-sation, an understanding of the variables affecting ⌬P may give the clinician an additional tool to determine whether the cause of decompensation is pulmonary or nonpulmonary. Table 3 depicts various clinical situations and their possible ef-fects on⌬P.

HIGH-FREQUENCY

OSCILLATORY VENTILATION AND AEROSOL MEDICATION DELIVERY

Very few studies have examined meth-ods to optimize aerosolized medication delivery during HFOV. Metered dose in-halers (MDI) appear relatively ineffective in delivering optimal drug amounts. MDI delivery during HFOV has been shown to deliver approximately 1% to 2% of the aerosolized drug in a neonatal lung model and 2.5% to 6.3% in a pediatric lung model (24). In this study, the low deposition was attributed to turbulent flow in conjunction with the high bias flow in the system and the small diameter of the ETT. No difference was noted be-tween differing settings on the HFOV.

Use of flow-driven nebulizers in-line during HFOV has not been thoroughly studied. This form of aerosol generator provides an additional amount of flow to the circuit that will result in alterations of the mPaw and the⌬P. During delivery of aerosolized medication, the RT must reduce oscillator bias flow to maintain constant mPaw (25).

New aerosol generators that use a vi-brational element to generate a

low-Table 3.Affects of clinical situations on proximal amplitude (⌬Pproximal) and possible treatments

Condition Compliance Vt ⌬Pcarinal ⌬Pproximal Treatment

Alveolar overdistension Decreased Decreased Increased Decreased Decrease mPaw incrementally Tension pneumothorax Decreased Decreased Increased Decreased Decrease mPaw for adverse reactions

Chest tube placement Mucous plugging Decreased or same Decreased Increased Decreased Bag, saline lavage and suction

Bronchoscopy Bronchoconstriction Decreased Decreased Increased Decreased Bronchodilator

Steroids Fulminating pulmonary edema

(endotracheal tube frothing)

Decreased Decreased Decreased Increaseda

Increase mPaw Increasing endotracheal tube

resistance (partial occlusion)

No Change Decreased Decreased Increased Bag, saline lavage, and suction Bronchoscopy

Endotracheal tube occlusion No Change Decreased Decreased Increaseda _{Bag, saline lavage, and suction}

Reintubation

Alveolar recruitment Increased Increased Decreased Increased Monitor for over-distension

a

velocity, small droplet medication have showed promising results in optimizing drug delivery. This form of aerosol gen-erator delivers up to the three times the amount of drug without effecting venti-lator parameters when compared with traditional flow-driven aerosol generators (26). Aerosol drug delivery during HFOV should be investigated further to deter-mine optimal techniques. The use of aerosolized selective pulmonary vasodila-tor medications is reviewed elsewhere in this supplement.

CONCLUSION

Integration of respiratory therapy ex-pertise into the management of adult pa-tients on HFOV is an essential compo-nent to successful outcomes with this novel mode of ventilation. Respiratory therapists serve as team leaders in the development and implementation of HFOV treatment algorithms. In addition, the RT plays a vital role in monitoring patients on HFOV, in early recognition of changing clinical conditions (e.g., ob-structing ETTs, pneumothorax, hyperin-flation), and in HFOV-related procedures (e.g., creating ETT cuff leaks, lung re-cruitment maneuvers). Optimal tech-niques to deliver aerosol medications during HFOV remain unclear and require further study.

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