Helium versus oxygen for tracheal gas insufflation during mechanical ventilation

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Objective: To evaluate and compare the effect of tracheal gas insufflation using two gases with different physical properties, helium and oxygen, as an adjunct to conventional mechanical ventilation in patients with respiratory failure. Design: Prospective, intervention study. Setting: General intensive care unit in a tertiary university medical center. Patients: Seven sedated and paralyzed patients with respiratory failure of various etiologies. All patients were ventilated in the volume-control mode (tidal volume 5 to 7 mL/kg). Inclusion criteria were PaCO2 of >or=to50 torr (>or=to6.7 kPa), together with peak inspiratory pressure of >or=to35 cm H2 O and respiratory rate of >or=to14 breaths/min. Interventions: All patients were intubated with an endotracheal tube that had an additional lumen opening at its distal end, through which tracheal gas insufflation was administered. The tracheal gas insufflation was applied continuously throughout the respiratory cycle at three flow rates (2, 4, and 6 L/min) with two gases, oxygen and helium, while the ventilatory settings were maintained constant. Measurements and Main Results: In addition to airway pressures and arterial blood gases, the relative efficacy of tracheal gas insufflation with each gas was estimated using a “coefficient of efficiency” (which we defined as the change in PaCO2/peak inspiratory pressure) compared with baseline measurements. Tracheal gas insufflation with both gases decreased PaCO sub 2 significantly (p < .05) at all flow rates. This effect was accompanied by an increase in airway pressure with both gases (oxygen and helium). However, at flow rates of 6 L/min, tracheal gas insufflation with helium resulted in lower peak inspiratory pressure than with oxygen. Tracheal gas insufflation with helium was more effective (as estimated by the coefficient of efficiency) than with oxygen at all flow rates (p < .05). Conclusion: In volume-controlled, mechanically ventilated patients with respiratory failure, tracheal gas insufflation with helium might be suggested as an alternative to oxygen. (Crit Care Med 1998; 26:290-295) The understanding that conventional mechanical ventilation may induce or aggravate lung injury led to the development of lung-protective modes of ventilation. Permissive hypercapnia, or controlled mechanical hypoventilation, became one of the strategies for the management of mechanically ventilated patients with severe respiratory failure [1,2]. Gradual increases in PaCO2 are well tolerated by many patients. However, there are patients in whom such an increase in PaCO2 is unacceptable. Some critically ill patients are slow in their metabolic response to hypercapnia because of impairment in vital organ function. In addition, pulmonary hypertension resulting from hypercapnia may have a detrimental effect on injured lungs [2]. Tracheal gas insufflation with oxygen had been shown to be effective in reducing deadspace and PaCO2 in such mechanically ventilated patients [3-5]. Helium is a biologically inert gas that has a much lower density than oxygen or air. When given at the same flow rate as a nitrogen-oxygen mixture, a helium-oxygen mixture produces a much lower Reynolds number than a nitrogen-oxygen mixture, and thus will presumably blunt the development of turbulent flow [6,7]. These physical properties of helium have been used to lower resistance to gas flow in patients with obstruction at various airway levels. Previous studies [8-12] have shown the effectiveness of using a helium-oxygen mixture in the treatment of upper airway obstruction, as well as in asthmatic patients. The hypothesis of this study was that tracheal gas insufflation with helium will be more effective than tracheal gas insufflation with oxygen as an adjunct to conventional mechanical ventilation in patients with respiratory failure.

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