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.