Low-Flow Apneic Ventilation (Apneic Insufflation)

The need for an absolutely quiet surgical field for short periods often arises during a thoracotomy in which a standard endotracheal tube and two-lung ventilation are used. This can be accomplished relatively safely by using the principle of apneic mass movement oxygenation. Indeed, this method gained considerable popularity around the time that DLTs were being introduced into clinical practice. If ventilation is stopped during the administration of 100% oxygen and the airway is left connected to a fresh gas supply, oxygen will be drawn into the lung by mass movement to replace the oxygen that crossed the alveolocapillary membrane. It is not usually difficulty to maintain an adequate PaO₂ (especially if 5 to 10 cm H₂ O of CPAP is used) during at least 20 minutes of apneic mass movement oxygenation.

If the flow of oxygen into the lungs is relatively low (<0.1 L/kg/min), almost all the carbon dioxide produced is retained, and the arterial carbon dioxide tension (PaCO₂ ) rises approximately 6 mm Hg in the first minute because of the wash-in of venous blood into the arterial compartment (venous blood has a carbon dioxide tension 6 mm Hg higher than that of arterial blood) and then 3 to 4 mm Hg each minute thereafter because of normal carbon dioxide production.^{[407] [408]} On the basis of these considerations, if a patient had normal carbon dioxide production, was hyperventilated to a PaCO₂ of 30 mm Hg, and was then made apneic and had oxygen insufflated into the lungs at low flow, the PaCO₂ after 10 minutes of apnea would be 63 to 72 mm Hg. Indeed, one report describes a series of eight patients in whom apneic mass movement oxygenation was used for 18 to 55 minutes after normal ventilation. ^[407] Although the lowest arterial saturation that resulted was 98%, PaCO₂ in the five patients in whom it was measured ranged from 103 to 250 mm Hg, and pH ranged from 6.72 to 6.97. Despite the fact that severe degrees of hypercapnia and respiratory acidosis may be well tolerated in some healthy patients, it would appear that the safe period of low-flow apneic oxygenation during thoracotomy would be well below 10 minutes. Although it has not been studied, the use of low-flow apneic oxygenation should theoretically be possible when a DLT is used. In all cases in which this technique is used, arterial oxygen saturation monitoring by pulse oximetry is mandatory.