THE DETERMINANTS OF RESPIRATORY RATE DURING MECHANICAL VENTILATION

The independent and interactive effect of feedback related to volume, CO2, inspiratory flow, and arousal state on the regulation of respirat ory rate in mechanically ventilated humans is not well characterized. We examined the rate response of eight normal volunteers during both q uiet wakefulness and non-rapid-eye-movement (NREM) sleep, while mechan ically ventilated through a nasal mask in an assist/control mode with a machine back-up rate of 2 breaths/min. Tidal volume (VT) was set sli ghtly above spontaneous VT and then increased by 0.2 L every 3 min up to 1.8 L or 25 ml/kg. Either an inspiratory flow of 40 L/min or an ins piratory time of 2 s (iso-Ti) was set, with CO2 added (FICO2 > 0) or F ICO2 = 0. Measurements were made during both quiet wakefulness and NRE M sleep. We found that as VT increased, the respiratory rate decreased ; the rate decline was observed during wakefulness and sleep, and unde r isocapnic as well as hypocapnic conditions. Increasing inspiratory f low raised the respiratory rate during wakefulness and NREM sleep. Dur ing NREM sleep, hypocapnia resulted in wasted ventilator trigger effor ts. In summary, both VT and inspiratory flow settings affect the respi ratory rate, and depending on state, can affect CO2 homeostasis. Venti lator settings appropriate for wakefulness may cause ventilatory insta bility during sleep.