Validation of the Radford Nomogram to Estimate the Minute Volume Required to Attain Normocapnia in Patients Undergoing General Anesthesia: A Single-Center Retrospective Study

Abstract

Objective: The Radford nomogram, an old mathematical chart device to estimate the required ventilation for maintaining normocapnia, remains unvalidated in patients undergoing modern, balanced anesthesia. This study aims to investigate the performance of the Radford nomogram in patients undergoing general anesthesia and derive a simple equation to estimate the minute volume required to attain normocapnia (MVnorm). Methods: This single-center retrospective study enrolled 78 patients (age ≥ 18 years) undergoing cerebral revascularization for Moyamoya disease. We defi ned MVnorm as the median of all values of the minute volume during normocapnia (estimated PaCO2: 38–42 mmHg). We examined the agreement level between the estimated minute volume using the Radford nomogram and MVnorm using the Bland–Altman analysis. Furthermore, we developed and validated a simple equation predicting MVnorm based on gender and a multiple of body weight, using a split-sample validation technique. Result: The Radford nomogram tended to overestimate MVnorm with a mean bias of 560 mL/min (95% limits of agreement, -848–1, 968 mL/min). The equation developed using data from the development group (n = 52): required minute volume (mL/min) = 85 × body weight (kg) in male patients and 70 × body weight (kg) in female patients. In the validation group (n = 26), the mean bias of this simple equation was 224 mL/min (95% limits of agreement, -1, 264–1, 712 mL/min). Conclusion: The Radford nomogram overestimates MVnorm in modern, balanced anesthesia. The simple equation using gender and a multiple of body weight yields similar predictive performance to the Radford nomogram.