Evaluation of a noninvasive continuous cardiac output monitoring system based on thoracic bioreactance

Optimization of preload in severe sepsis and septic shock

In sepsis both under- and overresuscitation are associated with increased morbidity and mortality. Moreover, sepsis can be complicated by myocardial dysfunction, and only half of the critically ill patients exhibit preload responsiveness. It is of paramount importance to accurately, safely, and rapidly determine and optimize preload during resuscitation. Traditional methods of determining preload based on measurement of pressure in a heart chamber or volume of a heart chamber ("static" parameters) are inaccurate and should be abandoned in favor of determining preload responsiveness by using one of the "dynamic parameters" based on respiratory variation in the venous or arterial circulation or based on change in stroke volume in response to an endogenous or exogenous volume challenge. The recent development and validation of a number of noninvasive technologies now allow us to optimize preload in an accurate, safe, rapid and, cost-effective manner.

Utility of impedance cardiography for the detection of hemodynamic changes in stable patients with sickle cell disease

OBJECTIVES

The study sought to assess the potential utility of impedance cardiography (ICG) to detect hemodynamic changes after erythrocytapheresis in stable children with sickle cell disease (SCD).

METHODS

We prospectively monitored cardiac index, systemic vascular resistance index, heart rate, and blood pressure using ICG before and after erythrocytapheresis in 26 stable children with SCD. Echocardiography was carried out in all patients to evaluate left ventricular systolic function. Hemoglobin (Hb), sickle cell hemoglobin (HbS), and ferritin levels were also measured.

RESULTS

Of a total of 78 erythrocytapheresis procedures in 26 children with SCD, 22 (28.2%) had hypotensive episodes defined as a decrease in systolic, diastolic, or mean blood pressure by 10 mmHg. Risk factors for developing hypotension during erythrocytapheresis were identified with logistic regression analysis: lower-body surface area and decrease in cardiac index. In contrast, age, prepheresis Hb and HbS, serum ferritin levels, and left ventricular function at baseline were not associated with hypotension.

CONCLUSIONS

This study demonstrates the feasibility of the ICG technique to detect the hemodynamic changes in children with SCD after an erythrocytapheresis procedure.

Noninvasive continuous hemodynamic monitoring

Monitoring of continuous blood pressure and cardiac output is important to prevent hypoperfusion and to guide fluid administration, but only few patients receive such monitoring due to the invasive nature of most of the methods presently available. Noninvasive blood pressure can be determined continuously using finger cuff technology and cardiac output is easily obtained using a pulse contour method. In this way completely noninvasive continuous blood pressure and cardiac output are available for clinical use in all patients that would otherwise not be monitored. Developments and state of art in hemodynamic monitoring are reviewed here, with a focus on noninvasive continuous hemodynamic monitoring form the finger.

Physiologic and Clinical Principles behind Noninvasive Resuscitation Techniques and Cardiac Output Monitoring

Clinical assessment and vital signs are poor predictors of the overall hemodynamic state. Optimal measurement of the response to fluid resuscitation and hemodynamics has previously required invasive measurement with radial and pulmonary artery catheterization. Newer noninvasive resuscitation technology offers the hope of more accurately and safely monitoring a broader range of critically ill patients while using fewer resources. Fluid responsiveness, the cardiac response to volume loading, represents a dynamic method of improving upon the assessment of preload when compared to static measures like central venous pressure. Multiple new hemodynamic monitors now exist that can noninvasively report cardiac output and oxygen delivery in a continuous manner. Proper assessment of the potential future role of these techniques in resuscitation requires understanding the underlying physiologic and clinical principles, reviewing the most recent literature examining their clinical validity, and evaluating their respective advantages and limitations.

Cardiac output assessed by invasive and minimally invasive techniques

Cardiac output (CO) measurement has long been considered essential to the assessment and guidance of therapeutic decisions in critically ill patients and for patients undergoing certain high-risk surgeries. Despite controversies, complications and inherent errors in measurement, pulmonary artery catheter (PAC) continuous and intermittent bolus techniques of CO measurement continue to be the gold standard. Newer techniques provide less invasive alternatives; however, currently available monitors are unable to provide central circulation pressures or true mixed venous saturations. Esophageal Doppler and pulse contour monitors can predict fluid responsiveness and have been shown to decrease postoperative morbidity. Many minimally invasive techniques continue to suffer from decreased accuracy and reliability under periods of hemodynamic instability, and so few have reached the level of interchangeability with the PAC.

Measurement of cardiac output in children by bioreactance

The objective of this study was to evaluate a new bioreactance method for noninvasive cardiac output (CO) measurement (NICOM) in children. Ten patients between 1 and 144 months of age and with no hemodynamic disturbances were studied. Using bioreactance, heart rate (HR), mean arterial pressure (MAP), and cardiac index (CI) measurements were made every 6-8 h. CI was 2.4 ± 1.03 l/min/1.73 m(2) (range 1-4.9 l/min/1.73 m(2)); There were significant correlations between CI and age (r = 0.50, P = 0.003), weight (r = 0.66, P < 0.001), and MAP (r = 0.369, P = 0.037). Significant differences in CI (P < 0.001) were detected between children weighing <10 kg (1.9 ± 0.73 l/min/1.73 m(2); range 1-3.2), 10-20 kg (2.07 ± 0.7 l/min/1.73 m(2); range 1-3.6), and >20 kg (3.7 ± 0.8 l/min/1.73 m(2); range 2.4-4.9). We conclude that the CI measured by bioreactance in children varies with the age and weight of the patients and is lower than the normal range in a large percentage of measurements. These data suggest that this method is not useful for evaluating CI in small children.

Hemodynamic parameters to guide fluid therapy

The clinical determination of the intravascular volume can be extremely difficult in critically ill and injured patients as well as those undergoing major surgery. This is problematic because fluid loading is considered the first step in the resuscitation of hemodynamically unstable patients. Yet, multiple studies have demonstrated that only approximately 50% of hemodynamically unstable patients in the intensive care unit and operating room respond to a fluid challenge. Whereas under-resuscitation results in inadequate organ perfusion, accumulating data suggest that over-resuscitation increases the morbidity and mortality of critically ill patients. Cardiac filling pressures, including the central venous pressure and pulmonary artery occlusion pressure, have been traditionally used to guide fluid management. However, studies performed during the past 30 years have demonstrated that cardiac filling pressures are unable to predict fluid responsiveness. During the past decade, a number of dynamic tests of volume responsiveness have been reported. These tests dynamically monitor the change in stroke volume after a maneuver that increases or decreases venous return (preload) and challenges the patients' Frank-Starling curve. These dynamic tests use the change in stroke volume during mechanical ventilation or after a passive leg raising maneuver to assess fluid responsiveness. The stroke volume is measured continuously and in real-time by minimally invasive or noninvasive technologies, including Doppler methods, pulse contour analysis, and bioreactance.

Impact of VV optimization in relation to left ventricular lead position: an acute haemodynamic study

AIMS

Left ventricular (LV) lead placement to the most delayed segment offers the greatest potential benefit to cardiac resynchronization therapy (CRT). We assessed the impact of interventricular (VV) optimization on acute changes in cardiac output (CO) in patients with and without LV pacing of the most delayed segment.

METHODS AND RESULTS

In 124 patients, the most delayed segment was defined by speckle tracking radial strain and the LV lead position by biplane fluoroscopy. Patients were classified as either a concordant (LV lead at latest site), adjacent (within one segment), or remote (two or more segments away) LV lead. Atrioventricular (AV) and VV delays were optimized by echocardiography. Cardiac output was measured non-invasively and a >20% increase in CO from baseline (intrinsic) defined acute response. Changes in CO in patients with concordant, adjacent, or remote LV leads were recorded following atrioventricular optimization alone (AV OPT) and after combined AV and VV optimization (AV/VV OPT). Compared with AV OPT pacing, AV/VV OPT produced a greater rise in CO (5.45 ± 1.1 vs. 5.76 ± 1.2 L/min, P< 0.001) and higher acute response rates (48.4 vs. 61.3%, P= 0.041). In adjacent patients, compared with AV OPT pacing, AV/VV OPT settings increased the response rate from 36.4 to 63.6% (P= 0.037). VV optimization had no effect on acute response rates in patients with remote (26.7 vs. 33.3%, P = 0.581) or concordant LV leads (65.6 vs. 72.1%, P = 0.438).

CONCLUSION

VV optimization overcomes some but not all of the deleterious effects of a suboptimal LV lead position.

Suppression of cerebral hemodynamics is associated with reduced functional capacity in patients with heart failure

This investigation elucidated the underlying mechanisms of functional impairments in patients with heart failure (HF) by simultaneously comparing cardiac-cerebral-muscle hemodynamic and ventilatory responses to exercise among HF patients with various functional capacities. One hundred one patients with HF [New York Heart Association HF functional class II (HF-II, n = 53) and functional class III (HF-III, n = 48) patients] and 71 normal subjects [older control (O-C, n = 39) and younger control (Y-C, n = 32) adults] performed an incremental exercise test using a bicycle ergometer. A recently developed noninvasive bioreactance device was adopted to measure cardiac hemodynamics, and near-infrared spectroscopy was employed to assess perfusions in the frontal cerebral lobe (Δ[THb](FC)) and vastus lateralis muscle (Δ[THb](VL)). The results demonstrated that the Y-C group had higher levels of cardiac output, Δ[THb](FC), and Δ[THb](VL) during exercise than the O-C group. Moreover, these cardiac/peripheral hemodynamic responses to exercise in HF-III group were smaller than those in both HF-II and O-C groups. Although the change of cardiac output caused by exercise was normalized, the amounts of blood distributed to frontal cerebral lobe and vastus lateralis muscle in the HF-III group significantly declined during exercise. The HF-III patients had lower oxygen-uptake efficiency slopes (OUES) and greater Ve-Vo(2) slopes than the HF-II patients and age-matched controls. However, neither hemodynamic nor ventilatory response to exercise differed significantly between the HF-II and O-C groups. Cardiac output, Δ[THb](FC), and Δ[THb](VL) during exercise were directly related to the OUES and Vo(2peak) and inversely related to the Ve-Vco(2) slope. Moreover, cardiac output or Δ[THb](FC) was an effect modifier, which modulated the correlation status between Δ[THb](VL) and Ve-Vco(2) slope. We concluded that the suppression of cerebral/muscle hemodynamics during exercise is associated with ventilatory abnormality, which reduces functional capacity in patients with HF.

The effect of exercise on large artery haemodynamics in cystic fibrosis

BACKGROUND

Adult patients with cystic fibrosis (CF) have resting abnormal large artery haemodynamics. Here, we obtain further insight in patients with CF by evaluating haemodynamic response to physiological stress.

METHODS

Thirty-six stable CF patients mean (SD) age 28.9 (9.0)years and 25 controls matched for age, gender and body mass index were studied. Central haemodynamic parameters; including augmentation index (AIx) and wasted left ventricular pressure energy (∆E(W)) were determined pre, during and post light intensity cycle ergometry.

RESULTS

During exercise, despite a similar heart rate and blood pressure, patients had comparatively greater ∆E(W) (P=0.03) and trend towards greater AIx (P=0.07) than controls. Exercise ∆E(W) was greatest in patients with CF related diabetes (n=11). In all subjects, exercise ∆E(W) was related to age (r=0.54, P<0.001) and FEV(1)% predicted (r=-0.32, P=0.01).

CONCLUSIONS

Adults with CF have an abnormal haemodynamic response to exercise. This finding has deleterious implications for myocardial performance.

Noninvasive measurement of cardiac performance in recovery from exercise in heart failure patients

OBJECTIVE

To examine the association between cardiac performance during recovery and the severity of heart failure, as determined by clinical and cardiopulmonary exercise test responses.

METHODS

As part of a retrospective cohort study, 46 heart failure patients and 13 normal subjects underwent cardiopulmonary exercise testing while cardiac output was measured using a noninvasive device. Cardiac output in recovery was expressed as the slope of a single exponential relationship between cardiac output and time; the recovery-time constant was assessed in relation to indices of cardiac function, along with clinical, functional, and cardiopulmonary exercise responses.

RESULTS

The recovery time constant was delayed in patients with heart failure compared with normal subjects (296.7 ± 238 vs. 110.1 ±27 seconds, p <0.01), and the slope of the decline of cardiac output in recovery was steeper in normal subjects compared with heart failure patients (p<0.001). The slope of the decline in cardiac output recovery was inversely related to peak VO(2) (r = -0.72, p<0.001) and directly related to the VE/VCO(2) slope (r = 0.57, p,0.001). Heart failure patients with abnormal recovery time constants had lower peak VO(2), lower VO(2) at the ventilatory threshold, lower peak cardiac output, and a heightened VE/VCO(2) slope during exercise.

CONCLUSIONS

Impaired cardiac output recovery kinetics can identify heart failure patients with more severe disease, lower exercise capacity, and inefficient ventilation. Estimating cardiac output in recovery from exercise may provide added insight into the cardiovascular status of patients with heart failure.

Minimally invasive monitoring of cardiac output in the cardiac surgery intensive care unit

Cardiac output monitoring in the cardiac surgery patient is standard practice that is traditionally performed using the pulmonary artery catheter. However, over the past 20 years, the value of pulmonary artery catheters has been challenged, with some authors suggesting that its use might be not only unnecessary but also harmful. New minimally invasive devices that measure cardiac output have become available. In this paper, we review their operative principles, limitations, and utility in an integrated approach that could potentially change patients' outcome. However, it is now clear that it is how the monitor is used (ie, the protocol or therapy associated with its use, or its lack thereof), and not the monitor per se, that should be questioned when a patient's outcome is being evaluated.

Hemodynamic changes as a diagnostic tool in acute heart failure--a pilot study

OBJECTIVES

To examine whether posturally induced changes in cardiac output differentiate patients presenting with dyspnea to the emergency department (ED) with acute heart failure (AHF) from other causes.

METHODS

This was an observational study of patients presenting to the ED with dyspnea. Exclusion criteria included ischemic chest pain, electrocardiographic changes diagnostic of acute myocardial infarction, pericardial effusion or chest wall deformities causing dyspnea, or heart transplant. Hemodynamic variables of cardiac index (CI), total peripheral resistance index, and thoracic fluid content (TFC) were determined in upright seated and supine positions 3 minutes apart using bioreactance technology (Cheetah Medical Inc, Portland, Ore). Acute heart failure was defined as either B-type natriuretic peptide 100 to 500 pg/mL and discharge diagnosis of AHF or a B-type natriuretic peptide greater than 500 pg/mL.

RESULTS

Of 92 patients, 25 had AHF, 23 had asthma/chronic obstructive pulmonary disease (COPD), and 44 had dyspnea related to other conditions; 41 (44.1%) were male, 56 (60.2%) were African American, and the mean age was 58 ± 15.0 years. Mean baseline TFC was higher in AHF vs asthma/COPD (59.3 ± 26.0 vs 39.7 ± 14.8 1/kW, P = .003) and trended higher compared to other patients with dyspnea (49.2 ± 22.0, P = .10). Postural changes in mean CI were lower in AHF (-0.20 ± 0.84 L min(-1) m(-2)) vs asthma/COPD (1.20 ± 1.23 L min(-1) m(-2); P = .002) and other dyspnea patients (0.82 ± 0.91 L min(-1) m(-2); P = .007).

CONCLUSION

Patients with AHF have greater TFC but lower CI responses to postural changes compared to patients with asthma and COPD. Knowledge of these changes may help rapidly differentiate AHF from asthma and COPD in the ED.

Peak cardiac power measured noninvasively with a bioreactance technique is a predictor of adverse outcomes in patients with advanced heart failure

Peak oxygen consumption (VO(2) ) during cardiopulmonary exercise testing (CPET) is a powerful predictor of survival, providing an indirect assessment of cardiac output (CO). Noninvasive indices of CO derived from bioreactance methodology would add significantly to peak VO(2) as a means of risk-stratifying patients with heart failure. In this study, 127 patients (53 ± 14 years of age, 66% male) with heart failure and an average ejection fraction of 31% ± 15% underwent symptom-limited CPET using a bicycle ergometer while measuring CO noninvasively by a bioreactance technique. Peak cardiac power was derived from the product of the peak mean arterial blood pressure and CO divided by 451. Follow-up averaged 404 ± 179 days (median, 366 days) to assess endpoints including death (n=3), heart transplant (n=10), or left ventricular assisted device implantation (n=2). Peak VO(2) and peak power had similar areas under the curve (0.77 and 0.76), which increased to 0.83 when combined. Kaplan-Meier cumulative survival curves demonstrated different outcomes in the subgroup with a VO(2) <14 mL/kg/min when stratified by a cardiac power above or below 1.5 W (92.2% vs 82.1% at 1 year and 81.6% vs 58.3% at last follow-up, P=.02 by log-rank test). Among patients with heart failure, peak cardiac power measured with bioreactance methodology and peak VO(2) had similar associations with adverse outcomes and peak power added independent prognostic information to peak VO(2) in those with advanced disease (eg, VO(2) <14 mL/kg/min).

Effects of normoxic and hypoxic exercise regimens on cardiac, muscular, and cerebral hemodynamics suppressed by severe hypoxia in humans

Hypoxic preconditioning prevents cerebrovascular/cardiovascular disorders by increasing resistance to acute ischemic stress, but severe hypoxic exposure disturbs vascular hemodynamics. This study compared how various exercise regimens with/without hypoxia affect hemodynamics and oxygenation in cardiac, muscle, and cerebral tissues during severe hypoxic exposure. Sixty sedentary males were randomly divided into five groups. Each group (n = 12) received one of five interventions: 1) normoxic (21% O(2)) resting control, 2) hypoxic (15% O(2)) resting control, 3) normoxic exercise (50% maximum work rate under 21% O(2); N-E group), 4) hypoxic-relative exercise (50% maximal heart rate reserve under 15% O(2); H-RE group), or 5) hypoxic-absolute exercise (50% maximum work rate under 15% O(2); H-AE group) for 30 min/day, 5 days/wk, for 4 wk. A recently developed noninvasive bioreactance device was used to measure cardiac hemodynamics, and near-infrared spectroscopy was used to assess perfusion and oxygenation in the vastus lateralis (VL)/gastrocnemius (GN) muscles and frontal cerebral lobe (FC). Our results demonstrated that the H-AE group had a larger improvement in aerobic capacity compared with the N-E group. Both H-RE and H-AE ameliorated the suppression of cardiac stroke volume and the GN hyperemic response (Delta total Hb/min) and reoxygenation rate by acute 12% O(2) exposure. Simultaneously, the two hypoxic interventions enhanced perfusion (Delta total Hb) and O(2) extraction [Delta deoxyHb] of the VL muscle during the 12% O(2) exercise. Although acute 12% O(2) exercise decreased oxygenation (Delta O(2)Hb) of the FC, none of the 4-wk interventions influenced the cerebral perfusion and oxygenation during normoxic/hypoxic exercise tests. Therefore, we conclude that moderate hypoxic exercise training improves cardiopulmonary fitness and increases resistance to disturbance of cardiac hemodynamics by severe hypoxia, concurrence with enhancing O(2) delivery/utilization in skeletal muscles but not cerebral tissues.

Comparison of monitoring performance of Bioreactance vs. pulse contour during lung recruitment maneuvers

INTRODUCTION

This study was designed to test the hypothesis of equivalence in cardiac output (CO) and stroke volume (SV) monitoring capabilities of two devices: non invasive transthoracic bioreactance (NICOM), and a pulse contour analysis (PICCO PC) coupled to transpulmonary thermodilution (PICCO TD).

METHODS

We included consecutive patients of a single ICU following cardiac surgery. Continuous minute-by-minute hemodynamic variables obtained from NICOM and PICCO PC were recorded and compared in 20 patients at baseline, during a lung recruitment maneuver (20 cmH2O of PEEP) and following withdrawal of PEEP. PICCO TD measurements were also determined. We evaluated the accuracy of these two technologies at baseline using PICCO TD as reference and we estimated the precision by the fluctuation around the mean value (2SD/mean). Then, we assessed time response, amplitude response and reliability for detecting expected decreases when PEEP was applied. Type I and type II errors were analyzed.

RESULTS

CO values (PICCO TD) ranged from 1.6 to 8.0 L.min-1. At baseline, CO values were comparable for NICOM, PICCO PC and PICCO TD: 5.0 +/- 1.2, 4.7 +/- 1.4 and 4.6 +/- 1.3 L.min.-1, respectively (NS). Limits of agreements with PICCO TD were 1.52 L.min.-1 for NICOM and 1.77 L.min.-1 for PICCO PC, NS. The 95% statistical power gives an equivalence with a threshold of 0.52 L.min.-1 for NICOM vs. PICCO PC. The CO precision was 6 +/- 3% and 6 +/- 5% for NICOM and PICCO PC, respectively, NS. When PEEP was applied, CO was reduced by 33 +/- 12%, 31 +/- 14% and 32 +/- 13%, for NICOM, PICCO PC and PICCO TD, respectively (NS). Time response was 3.2 +/- 0.7 minute for NICOM vs. 2 +/- 0.5 minute for PICCO PC (NS). SV results were comparable to those for CO.

CONCLUSIONS

Although limited to 20 patients, this study has enough power to show comparable CO and SV monitoring capabilities of Bioreactance and pulse contour analysis calibrated by transpulmonary thermodilution.

Measuring impedance in congestive heart failure: current options and clinical applications

Measurement of impedance is becoming increasingly available in the clinical setting as a tool for assessing hemodynamics and volume status in patients with heart failure. The 2 major categories of impedance assessment are the band electrode method and the implanted device lead method. The exact sources of the impedance signal are complex and can be influenced by physiologic effects such as blood volume, fluid, and positioning. This article provides a critical review of our current understanding and promises of impedance measurements, the techniques that have evolved, as well as the evidence and limitations regarding their clinical applications in the setting of heart failure management.

Comparison between Flotrac-Vigileo and Bioreactance, a totally noninvasive method for cardiac output monitoring

INTRODUCTION

This study was designed to compare the clinical acceptability of two cardiac output (CO) monitoring systems: a pulse wave contour-based system (FloTrac-Vigileo) and a bioreactance-based system (NICOM), using continuous thermodilution (PAC-CCO) as a reference method.

METHODS

Consecutive patients, requiring PAC-CCO monitoring following cardiac surgery, were also monitored by the two other devices. CO values obtained simultaneously by the three systems were recorded continuously on a minute-by-minute basis.

RESULTS

Continuous recording was performed on 29 patients, providing 12,099 simultaneous measurements for each device (417 +/- 107 per patient). In stable conditions, correlations of NICOM and Vigileo with PAC-CCO were 0.77 and 0.69, respectively. The bias was -0.01 +/- 0.84 for NICOM and -0.01 +/- 0.81 for Vigileo (NS). NICOM relative error was less than 30% in 94% of the patients and less than 20% in 79% vs. 91% and 79% for the Vigileo, respectively (NS). The variability of measurements around the trend line (precision) was not different between the three methods: 8 +/- 3%, 8 +/- 4% and 8 +/- 3% for PAC-CCO, NICOM and Vigileo, respectively. CO changes were 7.2 minutes faster with Vigileo and 6.9 minutes faster with NICOM (P < 0.05 both systems vs. PAC-CCO, NS). Amplitude of changes was not significantly different than thermodilution. Finally, the sensitivity and specificity for predicting significant CO changes were 0.91 and 0.95 respectively for the NICOM and 0.86 and 0.92 respectively for the Vigileo.

CONCLUSION

This study showed that the NICOM and Vigileo devices have similar monitoring capabilities in post-operative cardiac surgery patients.

Advances in the cardiovascular assessment of patients with chronic kidney disease

Cardiovascular mortality is grossly elevated in patients with chronic kidney disease (CKD), and is associated with a wide variety of structural and functional abnormalities. These issues have driven additional attempts to further characterise these abnormalities to elucidate the pathophysiology involved, assess individual risk and/or target and monitor therapies specifically directed at the cardiovascular (CV) system. This review aims to assess the techniques that are currently available for the study of the CV system. This includes conventional assessments of the whole CV system from heart to peripheral microcirculation (although not deal with VC assessment), as well as the key functional consequences relating to stress induced cardiovascular reserve, perfusion and vasoregulation. In addition this review will introduce a variety of techniques aiming to expand the envelope of conventional measurements.

The arterial Windkessel

Frank's Windkessel model described the hemodynamics of the arterial system in terms of resistance and compliance. It explained aortic pressure decay in diastole, but fell short in systole. Therefore characteristic impedance was introduced as a third element of the Windkessel model. Characteristic impedance links the lumped Windkessel to transmission phenomena (e.g., wave travel). Windkessels are used as hydraulic load for isolated hearts and in studies of the entire circulation. Furthermore, they are used to estimate total arterial compliance from pressure and flow; several of these methods are reviewed. Windkessels describe the general features of the input impedance, with physiologically interpretable parameters. Since it is a lumped model it is not suitable for the assessment of spatially distributed phenomena and aspects of wave travel, but it is a simple and fairly accurate approximation of ventricular afterload.