Infrared end‐tidal CO₂ measurement does not accurately predict arterial CO₂ values or end‐tidal to arterial P CO 2, gradients in rabbits with lung injury

End‐tidal P_co2 (P ET co 2) measurements from two commercially available neonatal infrared capnometers with different sampling systems and a mass spectrometer were compared with arterial P co 2 (P a co 2) to determine whether the former could predict the latter in mechanically ventilated rabbits with b d without lung injury. The effects of tidal volume, ventilator frequency and type of lung injury on the gradient between P ET CO 2 and P a co 2 (ΔP (a‐ET) CO 2) were evaluated. Twenty rabbits were studied: 10 without lung injury, 5 with saline lavage and 5 with lung injury by meconium instillation. Paired measurements of P ET CO 2 by two infrared capnometers and a mass spectrometer were compared to P a co 2. In the rabbits without lung injury, the values from the infrared capnometers and mass spectrometer correlated strongly with P a co 2 (r ± 0.91) despite differences in the slopes of the linear regression between P ET CO 2 and ±P a co 2 and in ΔP (a‐ET) CO 2. (P < 0.05). Values from the mainstream IR‐capnometer more closely approdmated the line of identity than the regression between the sidestream IR‐capnometer values or the mass spectrometer and P a co 2 but tended to overestimate P a co 2. The ΔP (a‐ET)  CO 2 was similar at all tidal volumes and ventilator frequencies, regardless of capnometer type. In the rabbits with induced lung injury, while there was a positive correlation between the slopes of the regression between P ET CO 2 and P a co 2 for both capnometers (r ≥ 0.70), none of the regression slopes approximated the line of identify. The ΔP (a‐ET) CO 2 was greater in rabbits with injured than noninjured lungs (P < 0.05). The ΔP (a‐ET) co 2 was similar among capnometers regardless of tidal volume, ventilator frequency, or type of lung injury. The 95% confidence interval of plots P a co 2 against P ET co 2 was large for rabbits with injured and noninjured lungs. The ΔP (a‐ET) CO 2 changed in an erratic manner with changing P a co 2. We conclude that P ET co 2measurements by infrared capnometer or mass spectrometer does not accurately predict P a co 2 or ΔP (a‐ET) CO 2 in the presence of lung injury, and should be used with caution in the management of critically ill infants. Pediatr Pulmonol. 1994; 17:189–196. © 1994 Wiley‐Liss, Inc.