June 2002 / Volume 47 / Number 6 / Page 667
Imposed Work of Breathing During Ventilator Failure
INTRODUCTION: Ventilators possess an anti-asphyxia valve that allows spontaneous breathing of ambient air during ventilator failure. This study examined the imposed work of breathing and pressure-time product of 8 critical care and 9 portable ventilators, using a laboratory simulation of spontaneous breathing during ventilator failure. METHODS: A test lung was modified to simulate spontaneous breathing with a tidal volume of 0.5 L and peak inspiratory flow of 60 L/min. A pneumotachograph and pressure tap were placed at the proximal airway between the breathing circuit and endotracheal tube. Flow was derived from the pressure drop across the pneumotachograph. Signals were amplified, integrated, and saved to a spreadsheet program, and imposed work of breathing and pressure-time product were calculated. Also measured were the inspiratory pressure required to open the anti-asphyxia valve (cracking pressure), time to cracking pressure, maximum negative inspiratory pressure, and time to maximum negative inspiratory pressure. RESULTS: For the critical care ventilators the mean ± SD imposed work of breathing ranged from 213.07 ± 3.53 to 890.63 ± 0.88 mJ/L and the pressure-time product ranged from 2.67 ± 0.01 to 13.37 ± 0.01 cm H2O · s/L. For the portable ventilators the mean ± SD imposed work of breathing ranged from 361.37 ± 1.22 to 969.60 ± 22.70 mJ/L and the pressure-time product ranged from 4.52 ± 0.01 to 16.70 ± 0.37 cm H2O · s/L. CONCLUSIONS: Spontaneous breathing during ventilator failure may impose work approximating the physiologic work of breathing. This imposed work may prevent effective breathing through the anti-asphyxia valve during mechanical ventilator failure due to electrical failure. These results reinforce the need to properly monitor mechanically ventilated patients and to have in place sufficient back-up power supplies and a method of manual ventilation.
Key words: imposed work of breathing, ventilator failure, anti-asphyxia valve, pressure-time product, ventilator, test lung, cracking pressure, electrical failure.
[Respir Care 2002;47(6):667–674]
Minimizing the imposed work of breathing (WOBI) is a principal goal of mechanical ventilation as well as of mechanical ventilator design. Numerous investigators have evaluated the WOBI characteristics of ventilators during simulated spontaneous breathing. During our previous evaluation of ventilator operation we noted significant differences in WOBI characteristics of ventilators following power failure. Ventilators designed for transport and home care and, more recently, critical care ventilators, are equipped with battery back-up to continue operation in the event of power failure. Additionally, American Society of Testing and Materials standards (ASTM standards F1100 and F1246) require ventilators to incorporate a mechanical safety valve, often referred to as an anti-asphyxia valve, that allows spontaneous breathing from ambient air in the event of power (electric or compressed gas) failure. Though failure of mechanical ventilators is rare, at least one case of ventilator malfunction during power failure has been described.
Given the variety of environments in which ventilators are used and the possibility of power loss, we conducted a bench study to evaluate WOB imposed by anti-asphyxia valves in both portable and critical care ventilators.
Alarms should indicate ventilator failure, and ventilator failure should be immediately detected in a critical care setting. However, it is not inconceivable that a substantial period of time could elapse between ventilator failure and detection in a subacute or home setting. Thus, patients in those settings may be required to breathe via the anti-asphyxia valve for several minutes, and it is important that the WOBI be easily tolerated by the ventilator-dependent patient.