DETECTION AND HEMODYNAMIC CONSEQUENCES OF VENOUS AIR-EMBOLISM - DOES NITROUS-OXIDE MAKE A DIFFERENCE

Volume expansion of intravascular air by nitrous oxide (N2O) may improve the sensitivity of monitors used to detect venous air embolism (VAE) and/or exacerbate hemodynamic changes following VAE. The purpose of this study was to determine if the administration of N2O alters the sensitivity (i.e., threshold of detection) of monitors used to detect VAE or the hemodynamic consequences of VAE. Twenty-one dogs were monitored for VAE with precordial Doppler ultrasound, transesophageal echocardiography (TEE), changes in end-tidal carbon dioxide tension (ET(CO2)), and changes in pulmonary artery pressure (PAP). Venous air was infused at rates between 0.005 and 0.4 ml.kg -1. min-1 during 1 MAC (total anesthetic level) of isoflurane with and without 50% N2O (group 1, n = 7) or isoflurane with and without 75% N2O (group 2, n = 7). The mean quantity of infused air necessary to elicit a positive response in both the presence and absence of N2O was calculated for each monitor. Positive responses were defined as follows: unmistakable audible change in frequency on Doppler ultrasound, visualization of densities consistent with air bubbles in the right cardiac chambers or outflow tract on TEE, a decrease in ET(CO2) greater than or equal to 2 mmHg, and an increase in mean PAP greater than or equal to 3 mmHg. In group 3 (n = 7), venous air was infused at rates between 0.1 and 0.8 ml.kg -1.min-1 during 1 MAC (total anesthetic level) of isoflurane with and without 50% N2O. In group 3, N2O administration was discontinued immediately upon Doppler detection of VAE and air infusion continued until mean arterial pressure (MAP) decreased by 10 mmHg. The volume of VAE necessary to elicit a positive response with the TEE and Doppler was not significantly different in the presence and absence of N2O. However, VAE was detected by changes in ET(CO2) and PAP with smaller volumes of infused air in the presence of both 50% and 75% N2O in oxygen (O2) compared to 100% O2. In group 3, the volume of VAE necessary to decrease the MAP by 10 mmHg was not influenced by N2O. The authors conclude that administration of N2O does not improve the sensitivity (i.e., reduce the threshold of detection of air emboli) of the TEE or precordial Doppler but does improve the sensitivity of ET(CO2) and PAP monitoring in the detection of VAE. In isoflurane-anesthetized dogs, during continuous intravenous infusion of air at rates up to 0.8 ml.kg-1.min-1, hemodynamic compromise is not worsened in animals receiving 50% N2O if N2O administration is discontinued immediately upon Doppler detection of VAE.