Measurement of pulmonary CO2 elimination must exclude inspired CO2 measured at the capnometer sampling site

Objective. The pulmonary elimination of the volume of CO₂ per breath (VCO₂/br, integration of product of airway flow (\(\dot V\)) and PCO₂ over a single breath) is a sensitive monitor of cardio-pulmonary function and tissue metabolism. Negligible inspired PCO₂ results when the capnometry sampling site (SS) is positioned at the entry of the inspiratory limb to the airway circuit. In this study, we test the hypothesis that moving SS lungward will result in significant inspired CO₂ (VCO₂[I]), that needs to be excluded from VCO₂/br.Methods. We ventilated a mechanical lung simulator with tidal volume (V_T) of 800 mL at 10 breaths/min. CO₂ production, generated by burning butane in a separate chamber, was delivered to the lung. Airway\(\dot V\) and PCO₂ were measured (Capnomac Ultima, Datex), digitized (100 Hz for 60 s), and stored by microcomputer. Then, computer algorithms corrected for phase diferences between\(\dot V\) and PCO₂ and calculated expired and inspired VCO₂ (VCO₂[E] and VCO₂[I]) for each breath, whose difference equalled overall VCO₂/br. The lung and Y-adapter (where the inspiratory and expiratory limbs of the circuit joined) were connected by the SS and a connecting tube in varying order.Results. During ventilation of the lung model (V_T = 800 ml) with SS adjacent to the inspiratory limb, VCO₂[E] was 16.8± 0.4 ml and VCO₂[I] was 1.1 ±0.1 ml, resulting in overall VCO₂/br (VCO₂[E] —VCO₂[I]) of 15.7 ± 0.4 ml. If VCO₂[I] was ignored in the determination of VCO₂/br, then the %error that VCO₂[E] overestimated VCO₂/br was 7.2± 0.3%. This %error significantly increased (p < 0.05, Student's t-test) when V_T was decreased to 500 mL (%error = 12.4 ± 0.8%) or when SS was moved to the lungward side of a 60 mL connecting tube (VCO₂[I] = 2.8 ± 0.2, %error = 18.2 ± 1.6) or a 140 mL tube (VCO₂[I] = 5.9±03 mL, %error = 37.5±3.3).Conclusions. When the SS was moved lungward from the inspiratory limb, instrumental dead space (VD_INSTR) increased and, at end-expiration, contained exhaled CO₂ from the previous breath. During the next inspiration, this CO₂ was rebreathed relative to SS (i.e. VCO₂[I]), and contributed to VCO₂[E]. Thus, VCO₂[E] overestimated VCO₂/br (%error) by the amount of rebreathing, which was exacerbated by largerVD_INSTR (increased VCO₂[I]) or smaller V_T (increased VCO₂[I]-to-VCO₂/br ratio).