5
thWorld Congress of Echocardiography and Vascular Ultrasound
Feasibility of Transcutaneous Cardiac Output Monitoring Using
Continuous Wave Doppler Ultrasound;
Ultrasonic Cardiac Output Monitor (USCOM
)
Phillips R.A.
Dadd M.J., Gill R.W., West M.J., Burstow D.J.
The University of Queensland and The Prince Charles Hospital, Brisbane, Australia, and CSIRO, Australia
Introduction:
Cardiac output is an important clinical measurement useful in many medical situations. Continuous wave (CW) Doppler interrogation of intracardiac haemodynamics is an accurate, reproducible, noninvasive and inexpensive method of quantitating cardiac output (CO) and diagnosing cardiac disease.
The concept of continuous transcutaneous CO monitoring using CW is an exciting prospective application of quantitative echocardiography (echo).
This presentation describes the science supporting the project, the development of a prototype Ultrasonic Cardiac Output Monitor (USCOM) and the results from a pilot validation study.
Background:
Monitoring CO is important in many medical situations particularly during anaesthesia, and in the acute care and coronary care (1,2,3), for both adults and children (4,5), where early detection of changes in cardiac function allows prompt and accurate therapeutic intervention.
Continuous or serial CO data may also produce prognostic indicators of cardiac events and may be useful to monitor ongoing health.
Other methods:
Currently ECG is the most common method of cardiac monitoring, however this makes no evaluation of CO.
Swan-Ganz catheterisation has been the standard method for continuous measurement of CO in the critically ill, but observational studies have indicated a possible associated increased morbidity and mortality (6,7,8), suggesting that a less invasive method would be useful (9,10).
Ultrasound:
CO is modulated by many variables in the venous circulation, the right heart, the pulmonary vascular system, the left heart, the arterial circulation and the blood itself, however intracardiac flow represents the sum of all these variables.
Doppler ultrasound is used to measure blood flow and interrogation of intracardiac and aortic flow is a non-invasive, accurate and well validated method of measuring CO in humans (11,12,13,14,15,16,17,18,19,20,21) and animals (22,23,24,25,26,27,28).
CW measurement of transpulmonary (PV) and transaortic (AV) derived CO has been in clinical use since the early1980s and remains a routine component of diagnostic echo examinations (29,30).
Sensitivity:
There is substantial evidence to indicate that CW Doppler is sufficiently sensitive to detect changes in CO associated with cardiac pathophysiology.
While there is a physiologic variation of Doppler determined outputs in normal subjects, Doppler echo has been validated to include serial measurements of the small Doppler detected CO changes associated with vasodilation therapy in cardiac failure (31), in patients with coronary artery disease before, during and after dobutamine infusion (32), during incremental pacing and inotropic stimulation and systemic vasodilation (33).
Comparison of Doppler with CO2 rebreathing and Fick methods during a variety of interventions to alter CO has also demonstrated excellent correlations (r=0.99) (34). This validation confirms that serial Doppler echo is sensitive enough to detect the changes associated with cardiac disease (28,35). Averaging of a large number of output samples using automated signal edge detection could increase this sensitivity.
Safety:
The majority of evidence indicates that CW Doppler is biologically safe, based on nearly 30 years of clinical use without detection of adverse bio-effects.
At SPTA levels of less than 94mW/cm2(13), even when used for long periods of time, as would be required in monitoring, exposures fall below safe levels recommended by the AIUM (36).
Systolic function:
PV and AV CW flow profiles demonstrate beat to beat time versus velocity blood flow information which yields reproducible (35) direct measures of cardiac systolic performance including peak velocity (Vpk), mean velocity (Vmn), peak pressure gradient (Ppk), mean pressure gradient (Pmn), time velocity integral (tvi) or stroke distance,cycle duration (Cyc), heart rate (HR), right and left ventricular ejection time (ET), right and left ventricular filling timeand derived functions such as Vpk x HR, Pmn x HR, tvi x HR (minute distance) and ET / Cyc (1,2,4,10,12,14,15,16,17,29,30,37,38).
2D echo can be used to determine the cross sectional area of the valve in advance, allowing determination of stroke volume, CO or cardiac index.
Diastolic function:
Transmitral flow (MV) can be used to measure CO (39) and is routinely used to evaluate left ventricular (LV) diastolic function, an early manifestation of LV dysfunction, occurring before abnormalities of systolic dysfunction (40) and often in the absence of systolic dysfunction (41). Continuous transmitral monitoring may detect early cardiac dysfunction not detected by conventional systolic indices.
Derived indices:
This streaming CO information could be combined with other physiologic measures such as oximetry to create new indices more accurately reflecting tissue perfusion.
Technical:
Improvements in transducer design, signal analysis, computer capabilities and telecommunications potentially allows for a small CW Doppler device to be attached to the chest producing a hands free beat to beat CO signal.
This low bandwidth signal could be transmitted wirelessly to a computer programmed to dial an internet accessed remote signal analysis centre.
This would be particularly useful for monitoring the elderly, the at risk and recovering patient in a cost-effective home environment, where deterioration of CO indices could instigate a planned intervention.
Reliability and reproducibility study:
The feasibility of the USCOM concept was tested using a commercial high quality non-image guided CW Doppler device to acquire PV and AV flow data (uscom) from 40 supine free breathing subjects (normals and patients with cardiac and pulmonary disease) and transfer it to an offline computer for storage and analysis.
PV and AV digital strips were acquired from the same subjects in the conventional (con) left lateral decubitus position and stored for comparison.
688 stroke cycles were analysed and standard haemodynamic values determined for each subject, each method and each valve.
Reliability was tested by comparison of methods and reproducibility tested by comparison of repeated observation of unselected subjects made by observer one and by a second observer, unfamiliar with echocardiography but after 5 minutes instruction.
Results - Reliability:
There was excellent correlation between uscom and con measurements of both PV and AV flow in 40 subjects and 688 cycles with p<0.001 and excellent regression statistics showing no systematic deviation of values.
Results - Reproducibility:
There was excellent correlation of uscom measurements from repeated observations and independent observers with p<0.001 and excellent regression statistics showing no systematic deviation of values.
Normal USCOM values:
10 adults; age =20.3±2.5yrs, Height= 172±8.5cm, Weight=69.7±10.5kgs, BSA=1.81±0.17m2, AVcycles=47, PVcycles=4810 children; age =9.9±1.8yrs, Height= 144±7.7cm, Weight=36.1±9.1kgs, BSA=1.21±0.19 m2, AVcycles=49, PVcycles=47
Vpk (m/s) Pmn (mmHg) tvi (cm) HR (bpm) ET (s)
AVadults 1.27±0.14 3.51±0.79 25.1±3.1 72.2±9.9 0.29±0.01
AVchild 1.13±0.10 2.83±0.55 21.6±2.7 86.9±12.6 0.27±0.02
PVadults 1.07±0.16 2.61±0.64 22.9±3.1 73.8±9.6 0.30±0.02
Vpk x HR Pmn x HR tvi x HR(m/min) ET/Cyc AVadults 85.3±11.1 254±69 18.12±2.89 0.34±0.04 AVchild 81.6±9.9 245±53 18.65±2.10 0.39±0.03 PVadults 74.1±13.3 197±64 17.07±3.31 0.36±0.04 PVchild 63.9±8.9 157±41 15.33±1.89 0.39±0.05 Summary:
1. USCOM signal acquisition from supine free breathing subjects was feasible in all cases in this study, including the elderly and those with cardiac and pulmonary disease.
2. USCOM is reliable and reproducible when compared to the conventional CW method to determine CO measurements, suggesting that USCOM will detect physiologic and pathophysiologic changes already validated for diagnostic CW.
3. Analysis of output signals can be taught in a relatively short time with excellent results, suggesting the possibility of pre-analysis by inexpert observers.
Conclusion:
There is good scientific evidence to support the desirability and feasibility of developing a simple transcutaneous CW ultrasound device that can deliver an inexpensive and accurate signal reflecting beat to beat CO.
The spectral output signal from such a device is ideally suited to cardiac monitoring with the potential for remote signal telemetry.
If proven this device may have multiple applications in the delivery of safe, improved and cost effective medical care.
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