Determination of ventilatory minute volumes for normocapnic ventilation under anaesthesia in healthy adults.

BACKGROUND

Mechanical ventilation under anaesthesia needs to be controlled to maintain normal oxygen and carbon dioxide tensions in the blood and to economize on fresh gas flows. Various ventilation nomograms such as Radford's nomogram and Nunn's CO2 predictor are based on data from studies, some of which do not mimic the conditions which prevail under anaesthesia. We, therefore, planned a study to formulate nomograms for normocapnic ventilation for anaesthetized adult subjects.

METHODS

Two hundred and fifty-three patients with normal pulmonary function tests, scheduled for elective non-thoracic surgery were studied. Subjects were ventilated with a Siemens Servo 900 B ventilator using CO2 analyser 930 (Siemens-Elema Sweden) to adjust the minute volume sufficient to maintain end-tidal carbon dioxide fraction (FE,CO2) at around 5.5 normocapnia (PaCO2 5.06-5.6 kPa). This was confirmed with arterial blood gas analysis.

RESULTS

The mean (SD) ventilation required for male patients was 6.123 (0.91) L [105 (13.1) ml/kg]. This was significantly higher than the requirement for female patients [5.262 (0.82) L; 98.7 (13.3) ml/kg]. The minute volume requirements showed a significant correlation with weight (W), height (H), body surface area, body mass index and other combinations of weight and height such as W x H, W/H, W2, W3 and H/W1/3. Nomograms were constructed for different weights and heights of males and females using multiple regression analysis. These minute volumes were found to be significantly higher than those calculated according to Radford's nomogram as well as Nunn's CO2 predictor and significantly lower than the Adelaide ventilation guide.

CONCLUSION

The differences obtained in our nomograms are probably because the earlier ones were based on minute carbon dioxide production and physiological dead space data obtained from widely differing studies, some of which did not resemble conditions prevailing under anaesthesia. None of these used strict inclusion criteria such as pulmonary function tests. These may also be due to a difference in body structure between the subjects studied. Therefore, minute volume requirements calculated based on available western nomograms should not be applied to Indian subjects for normocapnic ventilation under anaesthesia.