Myocardial mechanics and energetics during continuous positive airway pressure in sedated pigs.

OBJECTIVE

To test the hypothesis that increased cardiac output with continuous positive airway pressure (CPAP) leads to increased myocardial metabolic cost.

DESIGN

Prospective, repeated-measures, laboratory studies.

SETTING

University-affiliated hospital animal research laboratory.

SUBJECTS

Eight sedated pigs that had been previously instrumented for collection of hemodynamic data.

INTERVENTIONS

Application of CPAP at 0, 5, 10, and 15 cm H2O and recovery under conditions of normal blood volume (normovolemia) and after administration of hetastarch 35 mL/kg (hypervolemia).

MEASUREMENTS AND MAIN RESULTS

We measured mean arterial pressure, cardiac output, systemic vascular resistance index, the first derivative of the left ventricular pressure at a left ventricular 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 normovolemia, 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/cm5.m2, p < .01). With hypervolemia, cardiac output increased at low-level CPAP (5.7 +/- 1.4 L/min at CPAP of 0 cm H2O to 6.4 +/- 1.6 L/min at CPAP of 5 cm H2O, p < .05) and systemic vascular resistance index decreased (2412 +/- 552 to 2033 +/- 436 dyne.sec/cm5.m2, p < .01). There were no associated significant changes in myocardial oxygen consumption, or its major correlates when cardiac output increased with CPAP (hypervolemic conditions).

CONCLUSIONS

In normal pigs, there is no change in myocardial oxygen demand with CPAP, whatever the change in cardiac output. Thus, increased cardiac output with CPAP carries little extra metabolic cost. Increased cardiac output with low-level CPAP in hypervolemia is associated with systemic vasodilation.