(8) Detection of pulmonary air embolism  

A rapid decrease of PETCO₂ in the absence of changes in  blood pressure, central venous pressure and heart rate indicates an air embolism without systemic hemodynamic consequences.^1,2 However, as the size of air embolism increases, a reduction in cardiac output occurs which further decreases PETCO₂ measurement. A reduced cardiac output by itself can decrease PETCO₂. Therefore, in the event of a rapid decrease in PETCO₂ associated with a reduction in cardiac output, a rise in the pulmonary arterial pressure confirms the occurrence of pulmonary embolism.^1,2  

Drummond et al³ defined the relative sensitivities of end tidal carbon dioxide analysis, end-tidal nitrogen analysis, and pulmonary artery pressure (PAP) monitoring in detection of venous air embolism (VAE) in a study. Serial injections of air  (0.25, 0.5, 0.75,  1.0, and 1.5 ml/kg) were performed in six mongrel dogs. The frequency with which positive responses (PAP increase >2 mm Hg; ETCO₂ decrease >0.2%; ETN₂ increase 0.04%) were observed following VAE) was not different for three methods.  However, the response time (time to maximum following VAE) was significantly more rapid for PAP and ETN₂ than for ETCO₂; although the range for the three methods was narrow. The time to return to baseline levels was significantly more rapid for ETN₂ and ETCO₂ which was in turn significantly faster than PAP.   

 References

1. Smelt WLH. Capnography and air embolism. Can Anaesth Soc J 1986;33:113-5.

2. Symons NLP, Leaver HK. Air embolism during craniotomy in the seated position: a comparison of methods of detection. Can Anaesth Soc J 1985;32:17407.

3. Drummond JC, Prutow RJ, Scheller MS.  A comparison of the sensitivity of pulmonary artery pressure, end-tidal carbon dioxide, and end-tidal nitrogen in the detection of venous air embolism in the dog. Anesth Analg 1985;64:688-92.