Arterial to end-tidal CO2 tension and alveolar dead space in halothane- or isoflurane-anesthetized ponies.

The research article studies the correlation between end-tidal and arterial CO2 tension in ponies under halothane or isoflurane anesthesia. It suggests that isoflurane may be superior to halothane for prolonged anesthesia due to less alveolar dead space increase and CO2 tension.

Understanding the Parameters

• The study considers end-tidal partial pressure of CO2 (PETCO2) and arterial (PaCO2) CO2 tension in anesthetized ponies. PETCO2 refers to the maximum amount of CO2 present at the end of exhalation, while PaCO2 is the amount of CO2 in the arterial blood.
• The ponies were spontaneously breathing under two types of anesthesia – halothane and isoflurane.
• Alveolar dead space is a part of the lung where the gas exchange does not occur, although it is ventilated.

Observations Made

• PETCO2 could be used as an indicator of PaCO2, providing trend information during the first 60 minutes of either halothane or isoflurane anesthesia when the PaCO2 values were less than 60 to 70 mm of Hg.
• However, halothane anesthesia lasting over 90 minutes led to exceeding PaCO2 values, large differences in PaCO2-PETCO2, and significant increase in alveolar dead space.
• Such effects were not observed during similar durations of isoflurane anesthesia, which points towards isoflurane being potentially safer for longer anesthesia.

Suggestions and Conclusions

• The authors recommend arterial blood gas analysis during halothane anesthesia when the PETCO2 is greater than 60 to 70 mm of Hg. This is due to the observed correlation between the elevated PETCO2 and larger alveolar dead space and PaCO2-PETCO2 differences.
• The likely reason for the increase in alveolar dead space during halothane anesthesia, as per the study, is a decrease in alveolar capillary perfusion relative to alveolar ventilation. This means that the blood flow to the capillaries surrounding the alveoli is reduced, compared to how much air is reaching the alveoli.
• Given these findings, the authors conclude that isoflurane may be a superior choice to halothane for prolonged anesthesia in spontaneously breathing horses.