A comparison of the effects of high frequency--low tidal volume and low frequency--high tidal volume mechanical ventilation.

Ten neonates with severe lung disease were studied while mechanically ventilated with standard volume preset infant ventilators, using two different ventilatory patterns. Slow ventilatory rates and high tidal volumes were alternated with rapid rates and low tidal volumes; minute ventilation, FIo2, PEEP, and I:E ratios were held constant. Peak inspiratory pressure, mean airway pressure, expiratory time, Pao2, Paco2, pH, and arterial blood pressure were measured and compared for each frequency-tidal volume combination. The best arterial oxygenation occurred at the combination of settings that produced the highest mean airway pressure and always during low frequency-high tidal volume ventilation (P less than 0.001). Changes in oxygenation appeared to be directly related to changes in MAP. A second experiment examined two different ventilator systems' responses to changes in ventilatory rate. When the rate of one ventilator (Bourns LS104 volume preset) was increased, MAP increased. When the rate of the other ventilator (Bennett PR2 pressure preset) increased, MAP decreased. These observations suggest that there is a direct relationship between MAP and orterial oxygenation, and that the supposed advantages of one ventilatory pattern over the other may be secondary to inadvertent changes in subtle pressure-time relationships within the respiratory cycle and incidental changes in MAP. These changes may vary from one ventilator to another.