Introduction: The helmet may be an effective interface for the delivery of noninvasive positive pressure ventilation. The high internal gas volume of the helmet can act as a 'mixing chamber', in which the humidity of the patient's expired alveolar gases increases the humidity of the dry medical gases, thus avoiding the need for active humidification. We evaluated the temperature and humidity of respiratory gases inside the helmet, with and without a heated humidifier, during continuous positive airway pressure (CPAP) delivered with a helmet.
Methods: Nine patients with acute respiratory failure (arterial oxygen tension/fractional inspired oxygen ratio 209 +/- 52 mmHg) and 10 healthy individuals were subjected to CPAP. The CPAP was delivered either through a mechanical ventilator or by continuous low (40 l/min) or high flow (80 l/min). Humidity was measured inside the helmet using a capacitive hygrometer. The level of patient comfort was evaluated using a continuous scale.
Results: In patients with acute respiratory failure, the heated humidifier significantly increased the absolute humidity from 18.4 +/- 5.5 mgH2O/l to 34.1 +/- 2.8 mgH2O/l during ventilator CPAP, from 11.4 +/- 4.8 mgH2O/l to 33.9 +/- 1.9 mgH2O/l during continuous low-flow CPAP, and from 6.4 +/- 1.8 mgH2O/l to 24.2 +/- 5.4 mgH2O/l during continuous high-flow CPAP. Without the heated humidifier, the absolute humidity was significantly higher with ventilator CPAP than with continuous low-flow and high-flow CPAP. The level of comfort was similar for all the three modes of ventilation and with or without the heated humidifier. The findings in healthy individuals were similar to those in the patients with acute respiratory failure.
Conclusion: The fresh gas flowing through the helmet with continuous flow CPAP systems limited the possibility to increase the humidity. We suggest that a heated humidifier should be employed with continuous flow CPAP systems.