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.
Ultrasonic flow probes have been used to optimize biventricular pacing immediately after cardiopulmonary bypass, improving cardiac output (CO) by 10%; however, flow probes must be removed with chest closure. The PulseCO system (LiDCO Limited, Cambridge, UK) may extend optimization into the postoperative period, but controlled validations have not been reported. Six anesthetized pigs were instrumented for right heart bypass. Flow was varied from 3 to 1 L/min and then back to 3 in 0.5 L/min increments for 60 second intervals. CO was measured by ultrasonic flow probe on the aorta and by PulseCO using a femoral arterial line. PulseCO and flow probe accurately measured CO (PulseCO R2: 0.79-0.95; flow probe R2: 0.96-0.99). At flow of 2 L/min, when the heart was paced 30 bpm over the sinus rate, PulseCO falsely indicated an increase in CO (2.13 vs. 2.30 L/min, p = 0.014). When mean arterial pressure was increased by 20% using a phenylephrine infusion, PulseCO falsely indicated an increase in CO (2.13 vs. 2.47 L/min, p = 0.014). When mean arterial pressure was decreased by 20% using a nitroprusside infusion, PulseCO falsely indicated a decrease in CO (2.13 vs. 1.79 L/min, p = 0.003). PulseCO appears to be useful for assessing acute changes in CO if its limitations are recognized.
We tested the hypothesis that optimized biventricular pacing (BiVP) enhances cardiac output (CO) during critical pulmonary stenosis (PS) by attenuating distortions in left ventricular (LV) geometry. Following median sternotomy in six anesthetized pigs, heart block was induced by ethanol ablation. During epicardial, DDD BiVP, atrioventricular delay (AVD) was varied from 60 ms to 180 ms in 30 ms increments. At the AVD with the highest CO right-left delay (RLD) was varied from (+) 80 ms (RV first) to (-) 80 ms (LV first) in 20 ms increments. At each pacing setting, aortic flow, ECG, and LV diameter were measured in the control state (CON) and during PS, created by snaring the pulmonary artery until CO decreased 50%. Short axis LV echocardiograms were obtained at (+) and (-) 80 ms. In CON, RLD had no effect on function or geometry. During PS optimum BiVP resulted in significant increases in CO (1.12 L/min +/- 0.13 SEM at RLD =+ 40 ms versus 0.92 +/- 0.12 at RLD = 0 and 0.73 +/- 0.08 at RLD =-80), and LV fractional shortening (8.97%+/- 0.51% at RLD =+ 40 ms versus 7.34%+/- 0.58% at RLD = 0 and 6.21%+/- 0.66% at RLD =-80). In addition, LV eccentricity with (-) RLD was significantly different versus CON at both end-diastole (0.79 +/- 0.07 vs 1.02 +/- 0.03, P = 0.011 Student's t-test) and end-systole (0.83 +/- 0.05 vs 1.00 +/- 0.02, P = 0.017). However, with (+) RLD differences versus CON were not significant at either end-diastole (0.88 +/- 0.06 vs 0.99 +/- 0.03) or end-systole (0.92 +/- 0.03 vs 1.01 +/- 0.03). In swine hearts with PS, optimized BiVP increases CO, fractional shortening, and LV symmetry. BiVP warrants further study as treatment for acute postoperative heart failure.
Left ventricle-first biventricular pacing in swine significantly increased cardiac output during acute tricuspid insufficiency but not during the control state. Trials are warranted to develop clinical biventricular pacing for treatment of perioperative right ventricular dysfunction.
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