RESUMEN
BACKGROUND: Carbon dioxide absorbers allow the use of fresh gas flow below minute ventilation (VËE). Models are developed and tested in vitro to quantify their performance with variable carbon dioxide load (VËCO2), fresh gas flow, VËE, end-tidal carbon dioxide (ETco2) fraction, and the type of workstation used. METHODS: First principles are used to derive a linear relationship between fresh gas flow and fractional canister usage or FCU0.5 (the reciprocal of the time for the inspiratory carbon dioxide fraction to reach 0.5%). This forms the basis for two basic models in which VËE was measured by spirometry or calculated. These models were extended by multiplying VËE with an empirical workstation factor. To validate the four models, two hypotheses were tested. To test whether the FCU0.5 intercept varied proportionally with VËCO2 and was independent of VËE, FCU was measured for 10 canisters tested with a fixed 0.3 l/min fresh gas flow and a range of VËCO2 while VËE was either constant or adjusted to maintain ETco2 fraction. A t test was used to compare the two groups. To confirm whether a change in VËCO2 accompanied by a change in VËE to maintain ETco2 fraction would shift the linear fresh gas flow-FCU0.5 relationship in a parallel manner, 19 canisters were tested with different combinations of VËCO2 and fresh gas flow. These measured FCU values were compared to those predicted by the four models using Varvel's performance criteria. RESULTS: With 0.3 l/min fresh gas flow, FCU0.5 was proportional with VËCO2 and independent of whether VËE was adjusted to maintain ETco2 fraction or not (P = 0.962). The hypothesized parallel shift of the fresh gas flow-FCU0.5 relationship was confirmed. Both extended models are good candidate models. CONCLUSIONS: The models predict prepacked canister performance in vitro over the range of VËE, fresh gas flow, and VËCO2 likely to be encountered in routine clinical practice. In vivo validation is still needed.
Asunto(s)
Dióxido de Carbono , Consumo de Oxígeno , EspirometríaRESUMEN
Isocapnic hyperventilation (ICHV) is occasionally used to maintain the end-expired CO2 partial pressure (PETCO2) when the inspired CO2 (PICO2) rises. Whether maintaining PETCO2 with ICHV during an increase of the PICO2 also maintains arterial PCO2 (PaCO2) remains poorly documented. 12 ASA PS I-II subjects undergoing a robot-assisted radical prostatectomy (RARP) (n = 11) or cystectomy (n = 1) under general endotracheal anesthesia with sevoflurane in O2/air (40% inspired O2) were enrolled. PICO2 was sequentially increased from 0 to 0.5, 1.0, 1.5 and 2% by adding CO2 to the inspiratory limb of the circle system, while increasing ventilation to a target PETCO2 of 4.7-4.9% by adjusting respiratory rate during controlled mechanical ventilation. Pa-ETCO2 gradients were determined after a 15 min equilibration period at each PICO2 level and compared using ANOVA. Mean (standard deviation) age, height, and weight were 66 (6) years, 171 (6) cm, and 75 (8) kg, respectively. Capnograms were normal and hemodynamic parameters remained stable. PETCO2 could be maintained within 4.7-4.9% in all subjects at all times except in 1 subject with 1.5% PICO2 and 5 subjects with 2.0% PICO2; data from the one subject in whom both 1.5 and 2.0% PICO2 resulted in PETCO2 > 5.1% were excluded from analysis. Pa-ETCO2 gradients did not change when PICO2 increased. The effect of a modest rise of PICO2 up to 1.5% on PETCO2 during RARP can be readily overcome by increasing ventilation without altering the Pa-ETCO2 gradients. At higher PICO2, airway pressures may become a limiting factor, which requires further study.