Determination of physiological dead space in anaesthetized horses: a method-comparison study

To compare two methods of Bohr–Enghoff physiological dead space to tidal volume ratio (Vd/VtBohr–Enghoff) determination using a mixing chamber and an E-CAiOVX metabolic monitor. Prospective, clinical, method-comparison study. Twenty horses anaesthetized for elective orthopaedic procedures. Horses were anaesthetized with isoflurane in oxygen and the lungs were mechanically ventilated (Vt 15±2 mL kg−1). Arterial blood was sampled to provide arterial partial pressure of carbon dioxide (PaCO2) for dead space calculation using a metabolic monitor. Mixed expired partial pressure of carbon dioxide (PēCO2) obtained from the custom-made mixing chamber was recorded at the time of arterial blood sampling. Dead space fraction was calculated using the Enghoff modification of the Bohr equation. Agreement between the methods was assessed by Bland–Altman test. A clinically acceptable error was defined to be ≤ 10%. Forty-nine simultaneous Vd/VtBohr–Enghoff results were obtained. There was no clinically significant bias between the mixing chamber and E-CAiOVX. The limits of agreement were within a priori defined error (bias±95% limits of agreement: −0.022±0.078). Acceptable agreement was found between the two methods. The E-CAiOVX metabolic monitor might be a suitable device for measuring Vd/VtBohr–Enghoff in anaesthetized horses.