MYOCARDIAL MECHANICS AND ENERGETICS DURING CONTINUOUS POSITIVE AIRWAYPRESSURE IN SEDATED PIGS

Objective: To test the hypothesis that increased cardiac output with c ontinuous positive airway pressure (CPAP) reads to in creased myocardi al metabolic cost. Design: Prospective, repeated measures, laboratory studies. Setting: University-affiliated hospital animal research labor atory. Subjects: Eight sedated pigs that had been previously instrumen ted for collection of hemodynamic data. Interventions: Application of CPAP at 0, 5, 10, and 15 cm H2O and recovery under conditions of norma l blood volume (normovolemia) and after administration of hetastarch 3 5 mL/kg (hypervolemia). Measurements and Main Results: We measured mea n arterial pressure, cardiac output, systemic vascular resistance inde x, the first derivative of the left ventricular pressure at a left ven tricular pressure of 50 mm Hg, rate-pressure product, left Ventricular tension-time index, stroke work index, myocardial pressure-myocardial segment length area, coronary artery blood flow and coronary vascular resistance, and myocardial oxygen consumption (four pigs). With normo volemia, cardiac output decreased with CPAP (4.9 +/- 1.2 L/min at CPAP of 0 cm H2O to 4.5 +/- 1.3 L/min at CPAP of 15 cm H2O, p < .005) and systemic Vascular resistance index increased (2509 +/- 702 to 3095 +/- 1080 dyne . sec/cm(5) . m(2), p < .01). With hypervolemia, cardiac ou tput increased at low-revel CPAP (5.7 +/- 1.4 L/min at CPAP of 0 cm H2 O to 6.4 +/- 1.6 L/min at CPAP of 5 cm H2O, p < .05) and systemic Vasc ular resistance index decreased (2412 +/- 552 to 2033 +/- 436 dyne . s ec/cm(5) . m(2), p < .01). There were no associated significant change s in myocardial oxygen consumption, or its major correlates when cardi ac output increased with CPAP (hypervolemic conditions). Conclusions: In normal pigs, there is no change in myocardial oxygen demand with CP AP, whatever the change in cardiac output. Thus, increased cardiac out put with CPAP carries little extra metabolic cost. Increased cardiac o utput with low-level CPAP in hypervolemia is associated with systemic vasodilation.