Continuous oxygen insufflation in addition to IPPV causes air trapping in a mechanical lung model

A new technique for avoiding barotrauma-induced complications in apnea testing for brain death

Prompted by our experience with complications occurring with apnea testing (AT), we discuss complications reported in the literature. AT is an integral part of brain death assessment. Many complications of AT have been described, including hypoxemia, arterial hypotension, tension pneumothorax and cardiac arrest. We conclude that a commonly used technique in conducting AT can create auto-positive end expiratory pressure (PEEP) and contributes to many complications. The mechanism of occult auto-PEEP in AT is discussed. Intensive care unit patients may have a compensated and asymptomatic relative hypovolemia that can be decompensated by a small amount of auto-PEEP produced by air trapping during insufflating oxygen (O2) through a 7.0 endotracheal tube (ETT). It could then lead to decreased preload, decreased stroke volume, decreased cardiac output and thus, to hypotension and a compensatory tachycardia. The placement of the standard O2 tubing (6mm outside diameter [OD]) inside the 7.0 ETT (7mm inside diameter [ID]) greatly decreased the ETT lumen (73%). We changed our practice to instead use readily available small pressure tubing to insufflate O2 for AT to avoid excessive reduction in the ETT lumen. The change from standard O2 tubing (6mm OD) to pressure tubing (3mm OD) will greatly decrease the reduction in cross-sectional area of 7.0 ETT lumen from 73 to 18% and avoid potential complications of air trapping, auto-PEEP and barotrauma. We have successfully used this new simple technique with readily available equipment to eliminate auto-PEEP in AT while preserving oxygenation.

IPPV plus low-flow intermittent oxygen insufflation (end-exhalation to beginning inhalation) does not improve CO2 elimination

It has been previously reported that continuous insufflation of low-flow O2 (0.05 to 0.20 L/kg/min), both supracarinally and subcarinally, in addition to intermittent positive-pressure ventilation (IPPV) (IPPV + O2 at a specific flow rate) caused progressive hemodynamic deterioration in patients. As demonstrated in a subsequent mechanical lung model, the hemodynamic deterioration was most probably due to lung hyperexpansion. The purpose of this study was to test the hypothesis that the O2 retarded the outflow of gas from the lung during exhalation and that if the insufflation were limited to the period of time from the end of tidal exhalation (EE) to the beginning of the next IPPV tidal inspiration (BI), lung hyperexpansion would not occur. The use of intermittent O2 in addition to IPPV was studied in both a mechanical lung model and in patients under general anesthesia; the mechanical lung model permitted direct examination of lung volume, and the patient study allowed determination of gas exchange effects. In the mechanical lung model and in the patients, a wide range of EE-BI O2 flow rates were used; respectively, 1 to 40 L/min and 0.05 to 0.20 L/kg/min. In the mechanical lung model, lung pressure and volume at EE and end-inspiration did not increase as long as the O2 flow was kept at or below 10 L/min. In the patients, airway pressure and hemodynamics did not change appreciably, but there was also no increase in CO2 elimination.(ABSTRACT TRUNCATED AT 250 WORDS)