Noninvasive Assessment of Cardiac Output: Accuracy and Precision of the Closed‐Circuit Acetylene Rebreathing Technique for Cardiac Output Measurement

Abstract

Background Accurate assessment of cardiac output is critical to the diagnosis and management of various cardiac disease states; however, clinical standards of direct Fick and thermodilution are invasive. Noninvasive alternatives, such as closed‐circuit acetylene (C 2 H 2 ) rebreathing, warrant validation. Methods and Results We analyzed 10 clinical studies and all available cardiopulmonary stress tests performed in our laboratory that included a rebreathing method and direct Fick or thermodilution. Studies included healthy individuals and patients with clinical disease. Simultaneous cardiac output measurements were obtained under normovolemic, hypovolemic, and hypervolemic conditions, along with submaximal and maximal exercise. A total of 3198 measurements in 519 patients were analyzed (mean age, 59 years; 48% women). The C 2 H 2 method was more precise than thermodilution in healthy individuals with half the typical error (TE; 0.34 L/min [ r =0.92] and coefficient of variation, 7.2%) versus thermodilution (TE=0.67 [ r =0.70] and coefficient of variation, 13.2%). In healthy individuals during supine rest and upright exercise, C 2 H 2 correlated well with thermodilution (supine: r =0.84, TE=1.02; exercise: r =0.82, TE=2.36). In patients with clinical disease during supine rest, C 2 H 2 correlated with thermodilution ( r =0.85, TE=1.43). C 2 H 2 was similar to thermodilution and nitrous oxide (N 2 O) rebreathing technique compared with Fick in healthy adults (C 2 H 2 rest: r =0.85, TE=0.84; C 2 H 2 exercise: r =0.87, TE=2.39; thermodilution rest: r =0.72, TE=1.11; thermodilution exercise: r =0.73, TE=2.87; N 2 O rest: r =0.82, TE=0.94; N 2 O exercise: r =0.84, TE=2.18). The accuracy of the C 2 H 2 and N 2 O methods was excellent ( r =0.99, TE=0.58). Conclusions The C 2 H 2 rebreathing method is more precise than, and as accurate as, the thermodilution method in a variety of patients, with accuracy similar to an N 2 O rebreathing method approved by the US Food and Drug Administration.

Peak Cardiac Output Determined Using Inert Gas Rebreathing: A Comparison of Two Exercise Protocols

“…The "gold standard" methods for determining cardiac output (Q) are the direct Fick and thermodilution techniques (1). Both are highly technical and very invasive, requiring arterial catheterization.…”

“…Photoacoustic gas analyzers monitor the expired air and measure the disappearance rate of the blood soluble gas relative to the blood insoluble gas over the course of the rebreathing period to estimate Q. IGR-derived Q using the Innocor correlates well with the direct Fick (r=0.95) and thermodilution (r=0.94) methods during maximal exercise (3,4). A recent review comprising 10 clinical studies found that the correlation between IGR and the Fick method was higher than the correlation between the thermodilution and Fick method during submaximal steady state exercise (IGR vs Fick: r=0.84 [95% CI 0.74-0.90]; thermodilution vs Fick: r=0.73 [95% CI 0.61-0.82]), and that IGR demonstrated good Bland-Altman Limits of Agreement with the Fick method (1). Moreover, IGR had a higher reliability than the thermodilution method (IGR typical error…”

Isolated knee extensor exercise training improves skeletal muscle vasodilation, blood flow, and functional capacity in patients with HFpEF

“…Peak V̇O 2 was defined as the highest oxygen uptake measured from at least a 30 s Douglas bag collection. Q̇c was measured using a modified acetylene rebreathing technique that is validated, reproducible, and equivalent to invasive measures of Q̇c during maximal exercise (Hardin et al, 2020 ). Arterial–venous oxygen difference (a‐vO 2 difference) was calculated from the ratio between Q̇c and oxygen uptake using the Fick Equation (Hearon Jr. et al, 2019 ).…”

Influence of rapidly oscillating inspired O2 and N2 concentrations on pulmonary vascular function and lung fluid balance in healthy adults

“…C 2 H 2 is an inert, soluble gas that enters the blood stream via pulmonary diffusion but does not bind to hemoglobin. Therefore, the disappearance rate of C 2 H 2 is proportional to pulmonary blood flow and in participants without lung disease, the pulmonary blood flow is equal to systemic blood flow ( Hardin et al, 2020 ). Stroke volume (SV) was then determined via continuously monitored heart rate (HR).…”