Reliability of the Acetylene Single-Breath Method For Measuring Cardiac Output
Holm, Christopher David
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Advances in technology have now made it possible to analyze cardiac output (Q) with only a single-breath, making measurements during exercise quicker and less invasive for the subject. Certain non-invasive techniques allow for measurement of the disappearance of a soluble inert gas as it diffuses across the blood-gas barrier in the lungs. The rate of disappearance of the gas is directly proportional to the flow of blood past the lungs and subsequently provides an estimate of pulmonary capillary blood flow (Qc), or Q. The SensorMedics® Corporation (Yorba Linda, CA) has developed a single-breath acetylene (C₂H₂) technique (SensorMedics Vmax 229TM), which includes a simple device to linearize expiratory flow rate by increasing the time by which the sensors can measure the disappearance of the marker gas and improve quantification. The purpose of this investigation was to determine the reproducibility of the C₂H₂ single-breath technique during ramping exercise testing with the addition of a starling resistor in 11 apparently healthy, sedentary volunteers (7 male and 4 female). Subjects performed three maximal ramping exercise test sessions over a 6-week period and Qc was measured at rest and at three time points during the exercise test. The C₂H₂ single-breath Qc measurement technique was shown to be repeatable when systematically related to VO₂ (Qc/VO₂ relation highly correlated r² = 0.72-.74), but slightly lower than previously reported. Means and 95% confidence intervals revealed the precision of the technique over repeated testing days. This method was able to capture Qc measurements at intensities greater than 75% VO₂pk in all subjects with the use of the Starling resistor. Bland-Altman plots reveal Qc measures to be about 50% more variable than highly reproducible measures such as VO₂ and HR. Intraclass reliability coefficients (r) found through repeated measures ANOVA were found to perform low (rx,x= -0.11-0.31) from rest throughout all intensities of exercise. This device is limited in the ability of the sensors to accurately analyze Qc with subjects who are unfamiliar and have difficulty with the single-breath maneuver. Such instances make it difficult for objective, accurate determinations to be made by the clinician. The C₂H₂ single-breath method was found to capture Qc at higher intensities and a high level of precision with the addition of the starling resistor. However, more evidence needs to be analyzed before use of this device can be put into clinical practice.
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