Fig. 3From: The use of head helmets to deliver noninvasive ventilatory support: a comprehensive review of technical aspects and clinical findingsThe amount of "fresh" gas flowing through the helmet (MVtotal) determines the average CO2 concentration within the helmet (hCO2). Circles represent measured data, while lines are the theoretical curves obtained by the equation reported in the graph at different levels of CO2 production. Reproduced from [59]Back to article page