Evaluation of pulse contour methods in calculating stroke volume from pulmonary artery pressure curve (comparison with aortic pressure curve)

Right ventricular stroke volume assessed by pulmonary artery pulse contour analysis

Background

Stroke volume measurement should provide estimates of acute treatment responses. The current pulse contour method estimates left ventricle stroke volume. Heart-lung interactions change right ventricular stroke volume acutely. We investigated the accuracy, precision, and trending abilities of four calibrated stroke volume estimates based on pulmonary artery pulse contour analysis.

Results

Stroke volume was measured in 9 pigs with a pulmonary artery ultrasound flow probe at 5 and 10 cmH₂O of PEEP and three volume states (baseline, bleeding, and retransfusion) and compared against stroke volume estimates of four calibrated pulmonary pulse contour algorithms based on pulse pressure or pressure integration. Bland-Altman comparison with correction for multiple measurements and trend analysis were performed. Heart rate and stroke volumes were 104 ± 24 bpm and 30 ± 12 mL, respectively. The stroke volume estimates had a minimal bias: − 0.11 mL (95% CI − 0.55 to 0.33) to 0.32 mL (95% CI − 0.06 to 0.70). The limits of agreement were − 8.0 to 7.8 mL for calibrated pulse pressure to − 10.4 to 11.5 mL for time corrected pressure integration, resulting in a percentage error of 36 to 37%. The calibrated pulse pressure method performed best. Changes in stroke volume were trended very well (concordance rates 73–100%, r² 0.26 to 0.987, for pulse pressure methods and 71–100%, r² 0.236 to 0.977, for integration methods).

Conclusions

Pulmonary artery pulse contour methods reliably detect acute changes in stroke volume with good accuracy and moderate precision and accurately trend short-term changes in cardiac output over time.

Continuous and less invasive central hemodynamic monitoring by blood pressure waveform analysis

Blood pressure waveform analysis may permit continuous (i.e., automated) and less invasive (i.e., safer and simpler) central hemodynamic monitoring in the intensive care unit and other clinical settings without requiring any instrumentation beyond what is already in use or available. This practical approach has been a topic of intense investigation for decades and may garner even more interest henceforth due to the evolving demographics as well as recent trends in clinical hemodynamic monitoring. Here, we review techniques that have appeared in the literature for mathematically estimating clinically significant central hemodynamic variables, such as cardiac output, from different blood pressure waveforms. We begin by providing the rationale for pursuing such techniques. We then summarize earlier techniques and thereafter overview recent techniques by our collaborators and us in greater depth while pinpointing both their strengths and weaknesses. We conclude with suggestions for future research directions in the field and a description of some potential clinical applications of the techniques.

Continuous cardiac output and left atrial pressure monitoring by long time interval analysis of the pulmonary artery pressure waveform: proof of concept in dogs

We developed a technique to continuously (i.e., automatically) monitor cardiac output (CO) and left atrial pressure (LAP) by mathematical analysis of the pulmonary artery pressure (PAP) waveform. The technique is unique to the few previous related techniques in that it jointly estimates the two hemodynamic variables and analyzes the PAP waveform over time scales greater than a cardiac cycle wherein wave reflections and inertial effects cease to be major factors. First, a 6-min PAP waveform segment is analyzed so as to determine the pure exponential decay and equilibrium pressure that would eventually result if cardiac activity suddenly ceased (i.e., after the confounding wave reflections and inertial effects vanish). Then, the time constant of this exponential decay is computed and assumed to be proportional to the average pulmonary arterial resistance according to a Windkessel model, while the equilibrium pressure is regarded as average LAP. Finally, average proportional CO is determined similar to invoking Ohm's law and readily calibrated with one thermodilution measurement. To evaluate the technique, we performed experiments in five dogs in which the PAP waveform and accurate, but highly invasive, aortic flow probe CO and LAP catheter measurements were simultaneously recorded during common hemodynamic interventions. Our results showed overall calibrated CO and absolute LAP root-mean-squared errors of 15.2% and 1.7 mmHg, respectively. For comparison, the root-mean-squared error of classic end-diastolic PAP estimates of LAP was 4.7 mmHg. On future successful human testing, the technique may potentially be employed for continuous hemodynamic monitoring in critically ill patients with pulmonary artery catheters.

Continuous cardiac output and left atrial pressure monitoring by pulmonary artery pressure waveform analysis

We introduce a novel technique for continuous (i.e., automatic) monitoring of cardiac output (CO) and left atrial pressure (LAP) by mathematical analysis of a pulmonary artery pressure (PAP) waveform. To obtain an initial evaluation of the technique, we applied it to PAP waveforms obtained from nine critically ill patients and compared the resulting CO and LAP estimates with standard operator-dependent thermodilution and pulmonary capillary wedge pressure measurements, respectively. We report that the technique achieved an overall CO error of 17.2% and an overall LAP error of 15.8%. With further testing, the technique may ultimately be employed so as to permit, for the first time, continuous CO and LAP monitoring in critically ill patients.

A right ventricular pressure waveform based pulse contour cardiac output algorithm in canines

Tracking changes in stroke volume or cardiac output (CO) can be useful in the diagnosis and treatment of various cardiac illnesses. Existing arterial pressure waveform based pulse contour CO algorithms perform poorly during altered systemic hemodynamics. In this study, a right ventricular pressure waveform based pulse contour CO algorithm was developed to estimate the amplitude and duration of a hypothetical triangular flow waveform in the pulmonary artery. This algorithm was tested against gold standard blood flow measurements in ten canines during acute perturbations to preload (inferior vena caval occlusion (IVCO), rapid saline infusion), afterload (descending aortic occlusion (DAO), serotonin, angiotensin II, sodium nitroprusside infusion), and cardiac contractility (dobutamine and propranolol infusion). The algorithm correctly predicted the changes in CO (r2 = 0.82) that varied from - 45 to 31% of the baseline levels. To explain this finding both the pulmonary arterial (PA) and the ascending aortic (AA) input impedances were modeled as three element windkessels. In the AA the peripheral resistance (from - 61 to 191%), characteristic impedance (from - 59 to 20%) and total arterial compliance (from - 49 to 34%) varied significantly with these perturbations. In contrast, these parameters in the PA changed little. In particular, except serotonin infusion, the characteristic impedance of the PA deviated only 6% (SD/mean) from baseline values. This suggests right ventricular pressure waveform based estimate of CO is possible during acute changes in left ventricular hemodynamics.

Towards automating the pulmonary artery catheter: a canine validation study

We have recently introduced a unique technique to automatically estimate both cardiac output (CO) and left atrial pressure (LAP) by pulmonary artery pressure (PAP) waveform analysis. In this contribution, we review the technique and present its evaluation with respect to gold standard, but highly invasive, reference aortic flow probe CO and direct LAP catheter measurements from two dogs during various pharmacological interventions. We report that the technique achieved an overall CO error of 15.3% and an overall LAP error of 17.1% over a wide hemodynamic range. For comparison, the overall LAP error of classic end-diastolic PAP estimates was about three times as large. With further successful testing, the technique may ultimately be employed so as to effectively automate the pulmonary artery catheter.

Assessment of cardiac output from systemic arterial pressure in humans

OBJECTIVE

To evaluate the reliability, by comparison with established techniques, of a new method to assess cardiac output, called pressure recording analytical method (PRAM), deriving from the analysis of the arterial pressure profile in the time domain the arterial-pressure-blood flow relationship.

DESIGN

Criterion standard.

SETTING

Hemodynamics laboratory at an university medical center.

PATIENTS

Twenty-two hemodynamically stable cardiac patients scheduled for diagnostic right and left heart catheterization.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Cardiac index was simultaneously estimated by direct-oxygen Fick method, thermodilution, and PRAM applied to pressure signals recorded either invasively from an aortic catheter (PRAMa) or noninvasively at the finger (PRAMf) by photoplethysmography. Cardiac index values obtained by established techniques were significantly correlated with those estimated by PRAM: Fick method vs. PRAMa, r(2) =.88, vs. PRAMf, r(2) =.94; thermodilution vs. PRAMa, r(2) =.77, vs. PRAMf, r(2) =.77. The Bland-Altman analysis showed agreement between the Fick method and PRAM, with all data points comprised within the limits of agreement (+/-2SD) (mean difference +/- SD: -0.012 +/- 0.187 L x min(-1) x m(-2) for PRAMa; 0.024 +/- 0.167 L x min(-1) x m(-2) for PRAMf). Agreement was also found between thermodilution and PRAM, with all but one data point lying within the limits of agreement (mean difference +/- SD: -0.154 +/- 0.348 L x min(-1) x m(-2) for PRAMa; -0.108 +/- 0.348 L x min(-1) x m(-2) for PRAMf).

CONCLUSIONS

In the range evaluated (cardiac index from 1.65 to 3.91 L x min(-1) x m(-2) by the Fick method), PRAM provides reliable invasive and noninvasive estimates of cardiac output in hemodynamically stable cardiac patients. PRAM may prove clinically useful for the beat-to-beat monitoring of cardiac output.

Noninvasive cardiac output measurement in orthostasis: pulse contour analysis compared with acetylene rebreathing

We tested the reliability of noninvasive cardiac output (CO) measurement in different body positions by pulse contour analysis (CO(pc)) by using a transmission line model (K. H. Wesseling, B. De Wit, J. A. P. Weber, and N. T. Smith. Adv. Cardiol. Phys. 5, Suppl. II: 16-52, 1983). Acetylene rebreathing (CO(rebr)) was used as a reference method. Twelve subjects (age 21-34 yr) were studied: 1) six in whom CO(rebr) and CO(pc) were measured in the standing and 6 degrees head-down tilt (HDT) postures and 2) six in whom CO was measured in the 30 degrees HDT, supine, 30 degrees head up-tilt (HUT), and 70 degrees HUT postures on a tilt table. The CO(rebr)-to-CO(pc) ratio in (near) the supine position during rebreathing was used as the calibration factor for CO(pc) measurements. Calibrated CO(pc) (CO(cal sup)) consistently overestimated CO in the upright posture. The drop in CO with upright posture was underestimated by approximately 50%. CO(cal sup) and CO(rebr) values did not differ in the 30 degrees HDT position. Changes in the CO(rebr)-to-CO(pc) ratio are highly variable among subjects in response to a change in posture. Therefore, CO(pc) must be recalibrated for each subject in each posture.

Positive pressure inspiration differentially affects right and left ventricular outputs in postoperative cardiac surgery patients

PURPOSE

The purpose of this study was to determine the dynamic changes in right ventricular (RV) and left ventricular (LV) output during positive airway pressure inspiratory hold maneuvers so as to characterize the interaction of processes in creating steady-state cardiac output during positive pressure ventilation.

MATERIALS AND METHODS

We examined the disparity of RV and LV outputs at 5 seconds (early) and 20 seconds (late) into a 24-second inspiratory hold maneuver in 14 subjects in the intensive care unit immediately following coronary artery bypass surgery. RV output was measured by the thermodilution technique, whereas LV output was measured by the arterial pulse contour method. RV and LV volumes were also measured by thermal and radionuclide ejection fraction techniques, respectively.

RESULTS

As P(aw) was progressively increased from 0 to 20 cm H2O in sequential inspiratory hold maneuvers, both RV and LV outputs changed differently both at 5 seconds and 20 seconds into the inspiratory hold maneuvers. When expressed as change in cardiac output (L/min) for every cm H2O P(aw) increase relative to end-expiratory values, RV output increased at 5 seconds (0.05 +/- 0.15 L/min) then decreased at 20 seconds (-0.08 +/- 0.21, P < .05). LV output decreased slightly at 5 seconds (-0.14 +/- 0.22) and did not change from this minimal depressed level at 20 seconds (P < .05). Changes in RV and LV output were paralleled by changes in RV and LV end-diastolic volumes, respectively.

CONCLUSION

Positive pressure inspiration induces time-dependent changes in central hemodynamics, which are dissimilar between RV and LV function. Initially, inspiration increases RV output but decreases LV output, such that intrathoracic blood volume increases. However, sustained inspiratory pressures induce proportionally similar decreases in both RV and LV outputs. Thus, the hemodynamic effects of positive pressure ventilation will depend on the degree of lung inflation, the inspiratory time, and when measurements are made within the ventilatory cycle. These data also suggest that positive pressure ventilation with up to 20 cm H2) P(aw) does not significantly impair ventricular performance in humans.

Hemodynamic monitoring in childhood

Hemodynamic monitoring is indicated in children with impending or manifest cardiocirculatory failure. Since cardiocirculatory failure is characterized by an imbalance between oxygen delivery and oxygen demand due to perfusion failure, the parameters monitored should aid in the assessment of these oxygen variables. Oxygen delivery depends on oxygen content and cardiac output. Cardiac output is determined by heart rate and stroke volume; stroke volume by preload, afterload and contractility. Since the direct measurement of oxygen consumption routinely is almost impossible, global oxygen utilization represented by mixed venous oxygen saturation may be used to quantify the relationship between oxygen delivery and oxygen consumption. Justification of invasive hemodynamic monitoring depends among other things on an optimal balance between usefulness of information and complications associated with the techniques used. In future, the development of further noninvasive techniques and the scientific evaluation of recommended monitoring techniques are prospects in cardiovascular monitoring in childhood.