Räsänen J
Department of Anesthesiology, University of South Florida, College of Medicine, Tampa 33612.
Crit Care Med. 1992 Sep;20(9):1335-40. doi: 10.1097/00003246-199209000-00023.
To determine whether continuous gas flow in the breathing circuit or an airleak around the tracheal tube cuff will introduce errors into the measurement of oxygen consumption (VO2) with indirect calorimetry.
Nonrandomized, controlled trial.
Experimental laboratory.
Ten healthy, anesthetized mongrel dogs, weighing 8 to 12 kg.
Data were recorded at seven levels of flow, from 0 to 12 L/min in excess of minute ventilation, through a continuous breathing circuit. Data were recorded at five levels of tracheal tube cuff leak from 0% to 40% of inspiratory minute volume.
VO2 was measured using an indirect calorimeter with constant internal gas flow and calculated from results of blood gas analysis, cooximetry, and thermodilution cardiac output determinations at all levels of continuous breathing circuit flow and cuff leak. BP, heart rate, respiratory rate, arterial and mixed venous blood gases, and body temperature were measured to assess stability of cardiopulmonary function. Continuous breathing circuit flow did not affect the accuracy of indirect calorimetry until the total flow reached a critical value (11.5 L/min) that was slightly below the internal flow constant of the metabolic monitor (12.4 L/min). At higher circuit flows, measured VO2 decreased in a linear fashion, while calculated VO2 remained unchanged. Above the critical flow, the error of indirect calorimetry correlated significantly only with the total circuit flow (r2 = .64), not with the exhaled concentration of CO2 (r2 = .005) or the inspiratory-expiratory oxygen difference (r2 = .004). The continuous flow rate at the critical circuit flow was 66 +/- 15% of the subjects' peak inspiratory flow. Increasing tracheal tube cuff leak produced a progressive decrease in measured VO2 but not in calculated VO2. The difference between measured and calculated VO2 was linearly related to the magnitude of the leak (r2 = .56), and was statistically significantly larger at all levels of cuff leak, when compared with measurements during complete cuff seal.
An indirect calorimeter in which measurement of VO2 is based on internal constant flow rather than spirometry can be used to accurately measure VO2 from a continuous-flow breathing circuit, if the total circuit flow is less than the internal flow. This limitation may restrict the use of continuous flow to a level below the subject's peak inspiratory flow. The accuracy of indirect calorimetry cannot be guaranteed for any amount of tracheal tube cuff leak.
确定呼吸回路中的持续气流或气管导管套囊周围的漏气是否会在间接测热法测量氧耗量（VO₂）时引入误差。
非随机对照试验。
实验实验室。
10只体重8至12千克的健康麻醉杂种犬。
通过持续呼吸回路，在超过分钟通气量0至12升/分钟的七个气流水平下记录数据。在气管导管套囊漏气占吸气分钟量的0%至40%的五个水平下记录数据。
使用内部气流恒定的间接测热仪测量VO₂，并根据所有持续呼吸回路气流和套囊漏气水平下的血气分析、血液氧合血红蛋白测定及热稀释法心输出量测定结果进行计算。测量血压、心率、呼吸频率、动脉血和混合静脉血气以及体温，以评估心肺功能的稳定性。在总气流达到临界值（11.5升/分钟）之前，持续呼吸回路气流不影响间接测热法的准确性，该临界值略低于代谢监测仪的内部气流常数（12.4升/分钟）。在更高的回路气流下，测量的VO₂呈线性下降，而计算的VO₂保持不变。高于临界气流时，间接测热法的误差仅与总回路气流显著相关（r² = 0.64），与呼出二氧化碳浓度（r² = 0.005）或吸呼氧差值（r² = 0.004）无关。临界回路气流时的持续气流速率为受试者峰值吸气气流的66±15%。气管导管套囊漏气增加会导致测量的VO₂逐渐下降，但计算的VO₂不受影响。测量的VO₂与计算的VO₂之间的差异与漏气程度呈线性相关（r² = 0.56）, 并且与套囊完全密封时的测量值相比，在所有套囊漏气水平下该差异均具有统计学意义。
如果总回路气流小于内部气流，基于内部恒定气流而非肺量计测量VO₂的间接测热仪可用于准确测量持续气流呼吸回路中的VO₂。这一限制可能会将持续气流的使用限制在受试者峰值吸气气流以下的水平。对于任何程度的气管导管套囊漏气，间接测热法的准确性都无法得到保证。
