Objective: Previous studies have shown a significant difference between ventilator-measured tidal volume and actual-delivered tidal volume. However, these studies used external methods for measurement of compression volume. Our objective was to determine whether tidal volume could be accurately measured at the expiratory valve of a conventional ventilator using internal computer software to compensate for circuit compliance with a dual control mode of ventilation.
Design: Clinical study during an 8-month period.
Setting: Pediatric intensive care unit.
Patients: All patients admitted to the pediatric intensive care unit during the enrollment period who were mechanically ventilated using the Servo I (Maquet, Bridgewater, NJ) were eligible for this study.
Interventions: Patients were ventilated using a dual-control mode of ventilatory support and either an infant or adult circuit (with and without circuit compensation).
Measurements and main results: Tidal volume measured at the endotracheal tube using a pneumotachometer was compared with ventilator-displayed tidal volume. Sixty-eight patients were studied between September 2004 and April 2005. Age range was 2 days to 18 yrs (median, 23 mos) and weight range was 2.3 kg to 103 kg (median, 14.5 kg) with 41 male patients (60%). We found ventilator-displayed tidal volume, without circuit compensation, generally overestimates true-delivered tidal volume and, with circuit compensation, generally underestimates true-delivered tidal volume. However, agreement between tidal volume measured at the patient's airway and that measured with and without compensation for circuit compliance was good. The error in both cases, without and with circuit compensation, is relatively greater in infants and small children.
Conclusions: There is an underestimation of delivered tidal volume when compensating for circuit volume loss measured at the ventilator. There is no improvement in measured tidal volume using circuit compensation in small infants and children.