Contribution of lung stretch depressor reflex to nonlinear fall in cardiac output during PEEP

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Abstract
The hypothesis that lung stretch reflexes elicit negative cardiovascular effects during positive end-expiratory pressure (PEEP) application, in a ramp procedure up to 15 cmH2O, was tested in piglets under steady-state anesthesia and muscle relaxation. The effects of lung stretch on hemodynamics were studied by comparing the differences in responses during PEEP application with 2 different tidal volumes. In both ventilatory conditions, cardiac output and aortic pressure decreased nonlinearly in 3 phases with the rise of PEEP: a gradual decrease in phase I, a sharp decrease in phase II and again a more gradual decrease in phase III. Heart rate decreased significantly in phase II. In the series with the larger tidal volume, implying more lung stretch during insufflation, phase II was between a PEEP of 2.6 and 9 cmH2O. In the series with the smaller tidal volume, phase II occurred betwen 5.7 and 10.5 cmH2O. To assess the contribution of lung stretch reflexes to the decrease in cardiac output, changes in central venous pressure were studied. Again, a nonlinear response was observed, indicating that an additional effect besides the rise in mean central venous pressure was involved in the decrease in cardiac output. During ventilation with the smaller tidal volume, phase II of the decrease in cardiac output was also shifted to higher values of mean central venous pressure, which could only be ascribed to the differences in lung stretch at insufflation. It appeared that under circumstances of artificial ventilation the onset of the reflex is determined by a characteristic threshold of lung stretch. In addition to the mechanical effects of intrathoracic pressure on venous return and secondary elicited cardiovascular compensatory mechanisms, the hemodyanamic responses to PEEP are also determined by a negative lung stretch vasopressor reflex. This results in an extra cardiac output decrease, which explained the nonlinear response.