Background:Neonatal end tidal CO2 adaptors for side stream air sampling have been introduced into practice with the assumption that the very small instrumental dead space of the adaptor does not influence ventilation of infants. We wished to test this assumption in an animal model. Methods:7 infant olive baboons (Papio anubis) were sedated with a ketamine/acepromazine mixture, intubated with a cuffed endotracheal tube and allowed to breathe spontaneously while been attached to AVEA ventilator. We used the FilterLine adaptor (A) for the experiment. The ventilation parameters were recorded as follows: spontaneous breathing with no A, with A placed between ETT and ventilator's hot wire flow sensor (but with no air sampling), and with A sampling air at 50 mL/min. The statistical analysis was performed using ANOVA. Results:Average weight of baboons was 1.9 kg. Placement of A without sampling the air decreased frequency from 45 at baseline to 42.9 (by 4.7%) breaths/min (P<0.01), increased tidal volume (VT) from 7.6 at baseline to 8.2 mL/kg (by 7.9%, P<0.01) and increased minute ventilation (VE) from 466 at baseline to 502 mL/kg/min (by 7.7%, P<0.01). Running the CO2 sampling device increased air leak from the circuit from 1.4 to 7.2%. Active sampling of the air from A mitigated the effect of A placement: frequency was minimally increased, VT decreased to 8.0 mL/kg (P=0.1) but significantly reduced VE to 497 mL/kg/min (by 1%, P<0.01). The difference was non-significant for all parameters when baseline was compared with A in actively sampling mode. Conclusions:Contrary to initial assumption, the adaptor showed small but statistically significant influence on the ventilation parameters. The changes are physiologically expected in this kind of intervention. Increasing VE in response to the increase in instrumental dead space and in response to the increase in resistance, the frequency decreased while the VT increased. Sampling the air from the circuit mitigated the influence of A mainly on MV, but not frequency. This is not surprising, taking in consideration that the sampling did not change resistance of the system, but reduced effective dead space by removing void air from the instrumental dead space. The results of this study could be confidently extrapolated to premature infants. The clinical significance of the changes in ventilation, imposed by the end tidal CO2 adaptors, needs to be evaluated.
Ivanov et al. (Mon,) studied this question.