Sensitivity of end-tidal nitrogen in venous air embolism detection in dogs

Abstract

Embolized nitrogen appears in alveolar gas during clinical and experimental venous air embolism (VAE). Since early detection of VAE is believed to reduce morbidity and mortality, this study was done to compare the sensitivity of end-tidal nitrogen (ET(N2)) monitoring with other detection methods in current clinical use - precordial Doppler (PD), end-tidal CO2 (ET(CO2)), and pulmonary artery pressure (PAP). Ten mongrel dogs (10-17 kg) were anesthetized, placed in the supine position, immobilized, and ventilated (FI(O2) 1.0; Pa(CO2) 35-40 mmHg). Anesthesia and muscle relaxation were maintained with constant infusions of thiamylal and pancuronium. Maintenance fluids were administered at 5 ml/kg-1.h-1. Mean arterial pressure (MAP), PAP, and ET(N2) and ET(CO2) (Medspec II® mass spectrometer) were displayed on a strip chart recorder. The dogs were divided into two equal groups and given either a step-wise sequence of 1-min air infusion (0.1-1.5 ml.kg-1.min-1) or 5-s bolus air injections (0.25-1.0 ml.kg-1). Changes in PD sounds occurred in all animals at all air doses. Changes in cardiovascular variables and Pa(O2) were minimal. The threshold dose for ET(CO2) and ET(N2) to reach significance was 0.1 and 0.25 ml.kg-1, respectively, while PAP increases were significant at >0.5 ml.kg-1 air doses. The time to maximum change (Δmax) ET(N2) was 30-90 s earlier than Δmax ET(CO2) (P<0.05) and 6-105 s earlier than Δmax PAP. The Δmax for all variables was dose related and statistically significant except for the smallest infusion VAE, where only ET(CO2) was significantly changed. At 0.5 ml.kg-1 and greater air doses, increase in ET(N2) after bolus VAE were double those following infusion of the same amount of air. The magnitude of the changes in ET(CO2) and PAP were comparable following bolus and infusion VAE. It is concluded that changes in ET(N2) following low-dose infusion VAE are less sensitive than changes in ET(CO2), while during the bolus doses studied, they are equally sensitive. Changes in ET(N2) and ET(CO2)) are more sensitive than changes in PAP during bolus and infusion VAE. Continuous mass spectrometry monitoring of ET(N2) may not provide early warning of air entry after small emboli, however, when a significant increase in ET(N2) occurs following VAE, it precedes changes in ET(CO2) and PAP.