Fluctuations of inspired concentrations of nitric oxide and nitrogen dioxide during mechanical ventilation

A sharp decline in the NO concentration was found between the respirator's inspiratory outlet and more distal points along the inspiratory limb of the circuit. This finding was most evident when an active heating system was mounted between those points.The concentrations of NO and NO2 should be measured as near to the patient as possible, as significant fluctuations of these concentrations might be found along the inspiratory limb of the respiratory circuit especially when an active heating system is used.Inhalation of nitric oxide (NO) has been shown to selectively dilate the pulmonary vascular bed in animals as well as in humans [1,2,3,4]. Therefore, it has been used to reduce pulmonary hypertension in neonates [5,6] or after cardiac surgery [7,8]. In contrast to intravenously administered vasodilators, inhaled NO does not exert any vasodilating effect on the systemic circulation due to its rapid inactivation by haemoglobin when it enters the bloodstream [9,10]. When given via inhalation in severe acute respiratory distress syndrome (ARDS), NO predominantly produces vasodilation in the ventilated areas of the lung. Therefore, it does not only reduce pulmonary hypertension but it also redistributes blood flow towards the ventilated areas, thereby reducing intrapulmonary shunt and improving arterial oxygenation [11].For these reasons NO inhalation may become a widespread adjunctive treatment for severe hypoxaemia or pulmonary hypertension. However, the administration of gaseous NO is complicated by the fact that NO reacts with oxygen (O2) to form nitrogen dioxide (NO2) [9,10], which is known to be a toxic agent causing pulmonary epithelial damage [12,13]. Since the conversion of NO to NO2 is dependent on the concentration of NO and O2 as well as on their contact time, the concentrations of both gases and their contact time should be generally minimized to avoid potential toxic NO2 concentrations [14]. For the clinical use of inhaled NO it is therefore necessary to m