Effects of high-frequency jet ventilation on arterial baroreflex regulation of heart rate

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Abstract
Fifteen anesthetized mechanically ventilated patients recovering from multiple trauma were studied to compare the effects of high-frequency jet ventilation (HFJV) and continuous positive-pressure ventilation (CPPV) on arterial baroreflex regulation of heart rate. Systolic arterial pressure and right atrial pressure were measured using indwelling catheters. Electrocardiogram (ECG) and mean airway pressure were continuously monitored. Lung volumes were measured using two linear differential transformers mounted on thoracic and abdominal belts. Baroreflex testing was performed by sequential intravenous bolus injections of phenylephrine (200 .mu.g) and nitroglycerin (200 .mu.g) to raise or lower systolic arterial pressure by 20-30 Torr. Baroreflex regulation of heart rate was expressed as the slope of the regression line between R-R interval of the ECG and systolic arterial pressure. In each mode of ventilation the ventilatory settings were chosen to control mean airway pressure and arterial PCO2 (PaCO2). In HFJV a tidal volume of 159 .+-. 61 ml was administered at a frequency of 320 .+-. 104 breaths/min, whereas in CPPV a tidal volume of 702 .+-. 201 ml was administered at a frequency of 13 .+-. 2 breaths/min. Control values of systolic arterial pressure, R-R interval, mean pulmonary volume above apneic functional residual capacity, end-expiratory pulmonary volume, right atrial pressure, mean airway pressure, PaCO2, pH, PaO2, and temperature before injection of phenylephrine or nitroglycerin were comparable in HFJV and CPPV. Baroreflex regulation of heart rate after nitroglycerin injection was significantly higher in HFJV (4.1 .+-. 2.8 ms/Torr) than in CPPV (1.96 .+-. 1.23 ms/Torr). Baroreflex regulation of heart rate after phenylephrine injection was not statistically different between HFJV (9.03 .+-. 4.8 ms/Torr) and CPPV (7.75 .+-. 5.1 ms/Torr). These results demonstrate that the administration of small tidal volumes at high frequencies enables a more efficient arterial baroreflex regulation of heart rate than does the administration of large tidal volumes at physiological frequencies.

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