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PubMed1983

Regional blood flow distribution in dog during induced hypotension and low cardiac output. Spontaneous breathing versus artificial ventilation.

Respiratory muscle blood flow and organ blood flow was studied in two groups of dogs with radioactively labeled microspheres to assess the influence of the working respiratory muscles on the regional distribution of blood flow when arterial pressure and cardiac output were lowered by pericardial tamponade. In one group (n = 6), the dogs were paralyzed and mechanically ventilated (Mv), while in the other (n = 6), they were left to breathe spontaneously (Sb). Cardiac output fell to 30% of control values during tamponade in both groups and was maintained constant. None of the dogs was hypoxic. Ventilation in the Sb group peaked after 50 min of hypotension, but remained unchanged in the Mv group. Duplicate measurements of blood flow were made during a control period and after 50 min of tamponade (corresponding to the peak ventilation in Sb). Blood flow to the respiratory muscles increased significantly (P less than 0.001) during tamponade in Sb (diaphragmatic flow increased to 361% of control values), while it decreased in Mv. Although the arterial blood pressure and cardiac output were comparable in the two groups, blood flow distribution during tamponade was different. In Sb, the respiratory muscles received 21% of the cardiac output, compared with only 3% in the Mv group. Thus, by muscle paralysis and Mv, a large fraction of the cardiac output used by the working respiratory muscles can be made available for perfusion of other organs during low cardiac output state: blood flows to the liver, brain, and quadriceps muscles were significantly higher during tamponade in the Mv group compared with the Sb group. Similarly, blood lactate at all times after the induction of low cardiac output and hypotension was significantly lower in the Mv animals (P less than 0.005).

Animals↗
PubMed1990

Reproducibility of measurements of cardiac output in newborn infants by Doppler ultrasound.

Interobserver reproducibility in deriving cardiac output by measuring aortic blood flow velocity and diameter with imaging and Doppler ultrasound was investigated in 20 healthy infants born at full term. Aortic diameter was measured in three ways. Mean blood flow velocity was measured at three sites with both continuous wave and pulsed Doppler. Two observers carried out each study independently. Intraobserver reproducibility was investigated in 12 infants using the suprasternal site for measuring blood flow velocity. The most reproducible determination of cardiac output was found when the suprasternal site with continuous wave Doppler was used for measurement of blood flow velocity and M mode trailing edge to leading edge echocardiography was used for diameter. Normal mean (2 SD) cardiac output is 231 (77) ml/kg/min. Technical difficulties in measuring aortic diameter accurately limit direct comparison between infants.

Cardiac Output↗
PubMed1997

Beneficial effects of a Ca2+ sensitizer, MCI-154, on the myocardial oxygen consumption-cardiac output relation in patients with left ventricular dysfunction after myocardial infarction: comparison with dobutamine and phosphodiesterase inhibitor.

Although conventional inotropic agents such as catecholamines increase myocardial oxygen consumption, a newly developed inotropic agent, a Ca2+ sensitizer, may be able to increase cardiac output with less myocardial oxygen consumption. By using right-side heart catheterization, we assessed the ratio of the increase in myocardial oxygen consumption per unit increase in cardiac output during beta-adrenergic receptor stimulation (dobutamine, n = 15), phosphodiesterase inhibition (E-1020, n = 10), and Ca2+ sensitization (MCI-154, n = 17) in patients with coronary artery disease. Dobutamine increased cardiac output and myocardial oxygen consumption. E-1020 increased cardiac output but did not change myocardial oxygen consumption. MCI-154 increased cardiac output and decreased myocardial oxygen consumption. The oxygen cost of increasing cardiac output with dobutamine and with E-1020 was different from that with dextran infusion (n = 18); in contrast, the oxygen cost with MCI-154 was significantly smaller. Thus a newly developed Ca2+ sensitizer, MCI-154, may be beneficial for the treatment of heart failure.

Cardiac Output↗
PubMed1994

[Continuous monitoring of cardiac output by analysis of the pulse contour].

One method of continuous cardiac output monitoring by analysis of the radial pulse contour (Qcp) relates left ventricular stroke volume and systolic blood pressure by calculating the impedence characteristic of the aorta (Zao). It was assessed during haemodynamic monitoring by comparing it with the thermodilation method in the pulmonary artery (Qtd) in 20 patients with cardiac failure due to dilated cardiomyopathy (6 cases) and ischaemic cardiomyopathy (14 cases) treated by inotropic agents or vasodilators. Over an average monitoring period of 35 hours 159 measurements of cardiac output were performed by the two methods. There was an excellent correlation between the two methods (r = 0.90; p < 0.001; Qcp = 0.97 Qtd). The systematic error (bias) between the two methods was about 2.5%. The accuracy of Qcp compared with Qtd was 12.5%. During infusion with a vasoactive agent (Piroximone), the method based on pulse contour analysis did not reflect sudden variations in cardiac output. The systematic error between the two methods rose to 19% of the value measured, reflecting the lack of adaptation of parameters of correction in this situation and which necessitated recalibration of Zao at least once after injection of the drug.

Cardiac Output↗
PubMed1981

Influence of ketamine anesthesia on cardiac output and tissue perfusion in rats subjected to hemorrhage.

Ketamine anesthesia has been considered suitable for use in patients suffering from acute hypovolemia. Using a microsphere technique, fractional distribution of cardiac output and tissue perfusion were determined in rats subjected to moderate (10 ml/kg), or severe (bled to 60 torr systolic arterial pressure) hemorrhage. In the moderate bleeding bleeding experiments, rats under ketamine anesthesia were compared to awake rats as well as to awake normovolemic rats. In the severe bleeding experiments, rats under ketamine anesthesia were compared to rats under barbiturate anesthesia. Following moderate bleeding the ketamine group had a significantly large cardiac output and higher arterial pressure than the unanesthetized group. There were no major differences in the fractional distribution of cardiac output, although tissue perfusion in the ketamine group was significantly larger in heart, kidneys, skin, and small intestine. The shed blood volume necessary to reach 60 torr in systolic arterial pressure was 36 per cent of normal blood volume in the ketamine group, and 23 per cent in the barbiturate group. In spite of the greater blood loss, rats under ketamine anesthesia displayed significantly larger cardiac output and a higher elevation of arterial pressure 20 min after the hemorrhage. In the ketamine group, fractional distribution of cardiac output favored the internal organs as opposed to an increase in the carcass in the barbiturate group. The ketamine anesthetized rats had a significantly larger perfusion to most organs, including heart, kidneys, and brain. It is concluded from this study that in rats experiencing acute hypovolemia blood flow to vital organs and cardiac output are well maintained under ketamine anesthesia.

Anesthesia, General↗
PubMed1981

An evaluation of thermodilution cardiac output measurement using the Swan-Ganz catheter.

Errors in thermodilution cardiac output measurement were quantitated to determine the order of accuracy of routine measurements performed by unskilled personnel. In vitro and in vivo studies were undertaken to examine factors affecting the volume and temperature of the injectate, catheter thermistor and computer performance, effect of respiration, use of cold (0-4 degrees C) versus ambient temperature (20-25 degrees C) injectate, and the interpretation of measurements. Ambient temperature injectate incurred unacceptably large errors; cold injectate (injections were timed with respiration) produced variations in performance by equipment and personnel which accounted for only 2% of the variation between successive measurements. Real changes in cardiac output and inherent variability of the downslope of the thermal curve, necessitating an empirically based calculation, account for up to 10% variation between successive measurements. When cold injectate was used, and the average of three corrected measurements taken, thermodilution cardiac output measurements were within 10% of a simultaneous dye dilution measurement.

Animals↗
PubMed1997

Cardiac output distribution in the chick embryo from stage 36 to 45.

OBJECTIVE: The distribution of cardiac output to different organs is well described in the mammalian fetus. Chick embryos are not often used in perinatal cardiovascular research and therefore it is not known whether they can serve as an animal model for this purpose. In this study we documented cardiac output distribution in chick embryos at increasing incubation time. METHODS: Fertilized eggs from day 10 to 19 with an incubation time of 21 days were studied in 3 increasing incubation time groups (10-13, 14-16 and 17-19 days). For the experiment, the egg was placed in a holder in an incubator. The egg was opened at the air cell and a small vein of the chorioallantoic membrane was catheterized. Twenty thousand fluorescent 15 microns microspheres in 0.2 ml were injected. After 5 min, the embryo was sacrificed and the different organs were dissected and digested for microsphere isolation and subsequent fluorescence analysis. RESULTS: The chorioallantoic membrane, which is the placenta equivalent of the chick embryo, received a relatively large fraction of the combined cardiac output: 52.08% (interquartile range [IQR] 12.67%) on days 10-13 and 40.95% (IQR 27.24%) on days 17-19. Relatively small fractions were distributed: to the heart 2.03% (IQR 1.58) on days 10-13 and 3.18% (IQR 1.95) on days 17-19, and to the brain 3.20% (IQR 1.80) on days 10-13 and 5.02% (IQR 3.39) on days 17-19. As incubation time advanced, the fraction of the combined cardiac output to the chorioallantoic membrane and yolk-sac decreased significantly in favor of the heart and brain. CONCLUSION: This distribution shows great similarity to the one found in the mammalian fetus. The chick embryo is an attractive model for perinatal cardiovascular research.

Allantois↗
PubMed1996

Iced temperature injectate for thermodilution cardiac output determination causes minimal effects on cardiodynamics.

OBJECTIVES: Controversy exists regarding the ideal injectate temperature for measuring cardiac output. Iced temperature injectate gives a higher signal/noise ratio and less variability in the measured cardiac output. Thus, less volume and fewer measurements are required. Advocates of room temperature injectate have suggested that iced temperature injectate may perturb cardiodynamics. This concern has remained largely untested. To help resolve this controversy, we examined the effects of 5 mL iced injectate (0 degrees to 4 degrees) infusions on cardiodynamics. DESIGN: Prospective, randomized, controlled study. SETTING: A critical care research laboratory. SUBJECTS: Five domestic pigs, weighing between 20 to 25 kg. INTERVENTIONS: Under barbiturate anesthesia, pigs underwent placement of a) a thermodilution catheter in the right internal jugular vein; b) a right carotid artery catheter for mean arterial pressure; and c) sonomicrometry crystals for dynamic measurements of left ventricular dimensions. Calculations were made of end-systolic and end-diastolic left ventricular volume and ejection fraction. Six cardiac output measurements were performed in each pig. Data were obtained at baseline (just before iced temperature injectate infusion) and every 3 sec for 9 secs. MEASUREMENTS AND MAIN RESULTS: The only significant effect seen with iced temperature injectate infusion was a small, transient decrease in heart rate (-5.9 +/- 1.1 beats/min from a baseline heart rate of 144.8 +/- 20.6 beats/min). Indices of preload, contractile function, and dynamic cardiac geometry were unaffected. CONCLUSIONS: Iced temperature injectate used in clinically relevant volumes causes transient negative chronotropic effects, but reservations regarding other perturbations of cardiodynamics are unfounded. Thus, the use of iced temperature injectate for cardiac output determination is still a viable alternative to room temperature injectate use, especially when a larger signal/noise ratio is required.

Analysis of Variance↗
PubMed1990

Hemodynamics of conscious unrestrained baboons, including cardiac output.

A method is described for comprehensive hemodynamic study of undisturbed baboons (Papio hamadryas) that incorporates cardiac output measurement by thermodilution. Instrumentation includes arterial, aortic, and central venous catheterization by a surgical technique that does not require entry to peritoneal or thoracic cavities. It provides a means for right atrial indicator delivery with aortic temperature recording of thermodilution curves. Accuracy was confirmed by comparison to measurement by Swan-Ganz catheters. Diurnal variations of systemic arterial pressure in long-term study of conscious baboons were shown to result from significant increases in cardiac output by day (P less than 0.001), despite concomitant falls in systemic vascular resistance. The cardiac output values obtained were 0.13 l.min-1.kg-1 at night and 0.16 l.min-1.kg-1 by day. Comparison of these results to previous reports of cardiac output in baboons highlights the inadequacies of methods that require physical restraint or anesthesia. This technique also leaves the baboons intact for subsequent breeding or experimental use after catheter removal without the need for further surgery.

Animals↗
PubMed1989

Changes in cardiac output during epidural anaesthesia for caesarean section.

Serial haemodynamic investigations were performed in 20 women who had epidural anaesthesia before elective Caesarean section. Cardiac output was measured by Doppler and cross-sectional echocardiography at the aortic valve. Measurements were performed after preloading the circulation with Ringer lactate solution (mean volume 805 ml) and then 10, 20, 30 and 40 minutes after administration of 0.5% bupivacaine. Preloading increased mean stroke volume from 84 to 94 ml and mean cardiac output from 7.01 to 7.70 litres/minute. Relative to basal values, mean arterial pressure decreased in all subjects after administration of bupivacaine (mean - 14%, range - 1 to - 33%). Five subjects were given ephedrine because of hypotension. In three of these subjects hypotension was associated with a marked decrease in cardiac output. No significant changes in arterial blood pressure or cardiac output were demonstrated relative to basal values in the remaining 15 subjects. Ephedrine induced consistent increases in blood pressure and cardiac output.

Adult↗
PubMedDate not supplied

Bileaflet mechanical heart valves at low cardiac output.

Several earlier studies have indicated that bileaflet mechanical heart valves behave irregularly at low cardiac output and low pulse rate conditions, and that their hydrodynamic performances are generally inadequate. The authors conducted in vitro experiments in a pulsatile mock circulatory loop to compare the performance of the St. Jude Medical (SJM) valve and a long body bileaflet prosthesis recently introduced by Medical Carbon Research Institute (MCRI) (Austin, TX). The new MCRI mechanical heart valve model was designed with emphasis on improved hydrodynamic efficacy by introducing a long body with parallel leaflets and by leaflets increasing the flow area. Experimental studies were conducted on five test valves (MCRI 19 mm, MCRI 25 mm, SJM 19 mm, SJM 23 mm, and SJM 29 mm) with cardiac outputs of 2.0, 2.5, 3.0, and 3.5 L/min at a pulse rate of 40 beats/min, and 3.5, 4.0, 4.5, and 5.0 L/min at a pulse rate of 70 beats/min. Transvalvular pressure drop and closure volume were assessed by measuring the instantaneous ventricular and aortic pressures and aortic flow. The leaflet motions of the tested valves were observed by direct video recording using a charge coupled device camera while the flow measurements were being conducted. Testing under simulated physiologic ventricular and aortic pressure waveforms, the results of this study show that the MCRI bileaflets remained fully open during the entire ejection phase, even at very low cardiac output conditions (2.0 L/min). The closure volume (defined as percentage of forward flow volume) increased with decreasing cardiac output, as reported earlier by others. Comparative results also indicate that the MCRI design has nearly a two size pressure drop advantage over the SJM, with significantly smaller closure volume.

Blood Flow Velocity↗
PubMed2002

Comparison of continuous thermodilution and bolus cardiac output measurements in septic shock.

OBJECTIVE: To compare continuous (CCO) and bolus (BCO) thermodilution cardiac output measurement techniques over a wide range of cardiac outputs and blood temperatures in a septic sheep model. DESIGN AND SETTING: Prospective experimental study in a university intensive care laboratory. SUBJECTS: Thirty-five anesthetized sheep. INTERVENTIONS: Pulmonary artery catheters allowing measurement of CCO and BCO were placed through the external jugular vein. Cecal ligation and perforation was performed to induce septic shock. In 14 sheep two femoral venous catheters were placed and connected to a hemofiltration system to alter blood temperature. MEASUREMENTS: CCO and BCO were measured every hour during the experiment. Three 10-ml bolus injections of iced normal saline were given through a closed injectate system and then averaged. The CCO readings were collected just before the BCO measurements. The relationship between CCO and BCO was assessed using Bland and Altman's method. RESULTS: In 465 paired data the temperature ranged between 34.0 degrees and 40.9 degrees C, CCO between 1.4 and 17.0 l/min, and BCO between 1.1 and 17.4 l/min. There was a highly significant correlation between CCO and BCO ( r=0.97). The bias (difference between CCO and BCO) was -0.19 l/min, the SD of the difference 0.45 l/min, and the limits of agreement -1.08/0.71 l/min. There were also highly significant correlations between CCO and BCO at the different temperatures (extreme values: 34.0-34.9 degrees C, r=0.90; 40.0-40.9 degrees C, r=0.98). CONCLUSIONS: Thermodilution measurements of CCO are reliable, when compared to BCO measurements, over a large range of cardiac outputs and blood temperatures.

Animals↗
PubMed1996

Relationship between blood pressure and cardiac output in preterm infants requiring mechanical ventilation.

OBJECTIVE: To assess the contribution of cardiac output in determining the blood pressure of preterm infants and to identify other factors that may be important. METHODS: Sixty-seven preterm infants requiring mechanical ventilation (median birth weight, 1015 gm: median gestational age, 28 weeks) underwent on echocardiographic study at on average age of 19 hours (range, 7 to 31 hours). Measurements taken included left ventricular ejection fraction, left and right ventricular outputs by means of pulsed Doppler and the diameter of both the ductal and atrial shunt jets with the use of color Doppler as a measure of the size of shunt. Simultaneous measurements of intraarterial blood pressures, mean airway pressure, and inspired fraction of oxygen were recorded. RESULTS: After we allowed for the influence of ductal shunting, the correlation between the left ventricular output and mean arterial blood pressure was significant but weak (r = 0.38). There were infants with low blood pressures and normal cardiac outputs, and conversely there were infants with low cardiac outputs and normal blood pressure. The infants with a mean arterial blood pressure of less than 30 mm Hg had a significantly lower gestational age (27 vs 28 weeks), higher mean airway pressure (9.0 vs 7.0 cm H2O), larger ductal diameter (1.6 mm vs 0.7 mm) and a lower systemic vascular resistance (163 vs 184 mm Hg/L per minute per kilogram of body weight). Multilinear regression identified higher mean airway pressure and larger ductal diameter as significant negative influences on mean arterial blood pressure, with higher gestational age and higher left ventricular output as significant positive influences. CONCLUSIONS: Normal blood pressure cannot necessarily be equated with normal systemic now. These data emphasize the importance of other influences, and in particular that of varying systemic vascular resistance, in the determination of blood pressure in preterm infants.

Blood Pressure↗
PubMed1994

Multicenter trial of a new thoracic electrical bioimpedance device for cardiac output estimation.

OBJECTIVE: To evaluate the capacity of a new thoracic electric bioimpedance system to estimate cardiac output compared with the conventional thermodilution method. DESIGN: Prospective, multicenter study. SETTING: A university-run county hospital, a university-run U.S. Veterans Affairs hospital, and a university-affiliated U.S. military hospital. PATIENTS: A series of 68 critically ill patients whose conditions required pulmonary artery catheter insertion. MEASUREMENTS AND MAIN RESULTS: A total of 842 simultaneous pairs of cardiac output estimations by conventional thermodilution and a new thoracic electric bioimpedance system that uses an improved signal processing technique based on an all-integer-coefficient filtering technology, using a time-frequency distribution that provides a high signal/noise ratio were evaluated. The r value was .86, r2 = .74, and p < .001 by regression analysis; the mean difference between the two methods relative to their average value was 16.6 +/- 12.9 (SD) %; the precision was 1.4 L/min or 0.8 L/min/m2; the bias was -0.013 L/min. The mean difference between successive pairs of thermodilution measurements was 8.6 +/- 0.6 (SD) %, which was about half the difference between simultaneous pairs of measurement by the two methods. The changes in impedance estimations were close to simultaneously measured changes in thermodilution estimates of cardiac output. CONCLUSIONS: The new bioimpedance system satisfactorily estimated cardiac output as measured by the thermodilution technique. The difference between the two estimations is more than made up for by the continuous noninvasive capability of the impedance system.

Adult↗
PubMed1987

Continuous Fick cardiac output compared to continuous pulmonary artery electromagnetic flow measurement in pigs.

A system for continuous Fick cardiac output measurement (CFCO) is described and compared to continuous electromagnetic pulmonary artery flow (EMCO) and intermittent thermodilution (TDCO) measurements. Oxygen consumption was determined from continuous respiratory gas exchange analysis and arterio-venous oxygen difference from fiberoptic oximetry. A computer calculated cardiac output and other variables every 20 s. Seven pigs were monitored for a total of 10 h, during which cardiac output was manipulated by obstructing venous return or infusing epinephrine. 1748 pairs of EMCO and CFCO values were compared. The best correlation was obtained when CFCO was advanced 20 s with respect to EMCO (R = .89, CFCO = .99 EMCO + .16). TDCO was compared to EMCO during periods of steady state (R = .85, TDCO = .89 EMCO + 1.25, N = 139). TDCO was also compared with simultaneous CFCO (R = .87, TDCO = .89 CFCO + 1.01, N = 251). CFCO is recommended as a reliable standard of continuous cardiac output measurement. It is not a real time measurement; the technique has a time lag of approximately 20 s which is the result of the time constant of the VO2 measurement. Sources for error are discussed with suggestions for improving quality control.

Animals↗
PubMed1999

Comparison of pulmonary artery and arterial thermodilution cardiac output in critically ill patients.

OBJECTIVE: We studied the agreement between cardiac output measurements via pulmonary artery thermodilution [CO(PA)], regarded as the current clinical gold standard, and aortic transpulmonary thermodilution [CO(AORTA)]. DESIGN: Prospective clinical study. SETTING: Surgical intensive care unit of a university hospital. PATIENTS: 37 patients with sepsis or septic shock (n = 34) and subarachnoid haemorrhage (n = 3). MEASUREMENTS AND RESULTS: We analysed 449 simultaneous cardiac output measurements. All patients were deeply sedated and mechanically ventilated in a pressure controlled mode. Each patient received a 7.5-F five-lumen pulmonary artery catheter and a 4-F aortic catheter with an integrated thermistor. The thermistors of the two different catheters were connected to one computer system (COLD-Z021, Pulsion Medical Systems, Munich, Germany). Linear regression analysis revealed: CO(AORTA) = 0.96. CO(PA) + 1.02 (l/min) (r = 0.97, p < 0.0001). CO(AORTA) was consistently higher than CO(PA) with a bias of 0.68 (l/min) and a standard deviation of 0.62 (l/min). CONCLUSION: Cardiac output derived from aortic transpulmonary thermodilution is suitable for measurement in the intensive care unit. Measurements of CO(AORTA) are consistent with, but slightly higher than, those obtained from pulmonary artery thermodilution.

Adult↗
PubMed1995

Comparison of transtracheal and extravascular Doppler determinations of stroke volume and cardiac output at various states of volume loading in piglets.

OBJECTIVE: To assess the applicability of a new technology in neonates. Transtracheal Doppler and extravascular Doppler determinations of stroke volume and cardiac output were compared with thermodilution measurements at various states of volume loading in an animal model. DESIGN: Prospective, descriptive study. SETTING: Animal research laboratory at a university medical center. SUBJECTS: Fourteen newly weaned piglets, weighing 2.8 to 6.5 kg. INTERVENTIONS: Doppler probes were placed on the endotracheal tube tip (transtracheal Doppler) and directly on the aortic adventitia (extravascular Doppler). A 4-Fr thermodilution catheter was inserted in the pulmonary artery. Stroke volume and cardiac output determinations were recorded at baseline, after a 15-mL/kg volume load and after successive 15-mL/kg blood withdrawals to exsanguination or a systolic blood pressure of < 20 mm Hg. MEASUREMENTS AND MAIN RESULTS: Transtracheal and extravascular Doppler measurements of cardiac output were not significantly different from thermodilution at any physiologic state. These techniques were able to measure stroke volumes and cardiac outputs at the low levels seen in severe hemorrhagic shock. CONCLUSIONS: Transtracheal Doppler and extravascular Doppler measurements of cardiac output compare favorably with thermodilution. These methods effectively followed trends from alterations in intravascular volume, even at very high heart rates and small stroke volumes. Transtracheal Doppler and extravascular Doppler should yield useful information in critically ill neonatal patients, where data regarding stroke volume and cardiac output may be useful in clinical management.

Animals↗
PubMed2004

Load dependence of cardiac output in biventricular pacing: right ventricular pressure overload in pigs.

BACKGROUND: The effect of biventricular pacing on stroke volume is believed to be dependent on right ventricular/left ventricular delay, but effects in individual patients are unpredictable. This variability may reflect relative right and left ventricular volume and/or pressure overloads. Accordingly, we tested the hypothesis that the relation of cardiac output to right ventricular/left ventricular delay is load dependent in a pig model of pulmonary stenosis. METHODS: After median sternotomy in 6 anesthetized, domestic pigs, complete heart block was induced by ethanol ablation. During epicardial, atrial tracking DDD biventricular pacing, atrioventricular delay was varied between 60 and 180 ms in 30-ms increments. Right ventricular/left ventricular delay was varied at each atrioventricular delay from +80 ms (right ventricle first) to -80 ms (left ventricle first) in 20-ms increments. Aortic flow, right ventricular pressure, peripheral arterial pressure, and electrocardiogram were measured in the control state and during pulmonary stenosis, created by tightening a snare around the pulmonary artery until cardiac output decreased by 50%. RESULTS: Atrioventricular and right ventricular/left ventricular delay had no effect on cardiac output during the control state, but during pulmonary stenosis there was a statistically significant (P =.0001, repeated-measures analysis of variance) right ventricular/left ventricular delay-related trend toward higher cardiac output with right ventricular pacing first. This effect was more pronounced when the optimal atrioventricular delay was determined first, resulting in a 20% increase in cardiac output when the optimal right ventricular/left ventricular delay was compared with simultaneous biventricular pacing. CONCLUSIONS: Optimized biventricular pacing in swine is associated with increased cardiac output during acute pulmonary stenosis, but not during the control state. Further studies are needed to determine whether specific types of right ventricular and left ventricular overload predictably affect the relation between right ventricular/left ventricular delay and cardiac output.

Animals↗