Application of CO₂ clearance to estimate cardiac output in patients with venoarterial extracorporeal membrane oxygenation

Dear Editor,

Patients supported by VA-ECMO are characterized by end-organ hypoperfusion due to low CO (cardiac output) and hypotension. The measurement of CO in these patients plays an important role in the evaluation and treatment of the disease. However, several hemodynamic monitoring methods used to assess the patient's own CO have not been validated, and their effectiveness is still controversial. In addition to using ultrasound to estimate CO, other methods, such as Pulmonary artery catheter (PAC), Transpulmonary thermodilution (TPTD), and Arterial pressure waveform analysis (APWA) have some defects in VA-ECMO patients, which makes it impossible to estimate CO normally [1].

The Fick equation applied to CO₂ indicates that the CO₂ clearance (equivalent to CO₂production in a steady state) equals the product of cardiac output by the difference between CCO₂ in mixed venous blood (CvCO₂) and arterial blood (CaCO₂): VCO₂ = CO × (CvCO₂−CaCO₂); CO = VCO₂/(CvCO₂−CaCO₂) [2]. Our group has observed that this applies in patients with VA-ECMO who have partial cardiac output and no intracardial shunts or valvular regurgitation (with blood supplied from the right radial artery). A total of 24 eligible patients with VA ECMO were observed by our team. VCO₂ was measured with the use of a Mindray SV850 ventilator. The ventilator has a built-in CO₂ monitor, which uses the mainstream absorptiometry method to accurately monitor the concentration of CO₂ in the exhaled gas, and the VCO₂ is measured according to the minute ventilation. Regarding mixed venous blood, we obtained it through a pulmonary artery catheter. All blood gas samples were analyzed by an ABL90 FLEX blood gas analyzer. The Mchardy–Visser formula was used to calculate the CCO₂ results. Because the formula was corrected for HB, PH, and SO₂, the results were highly reliable [3]. CO was estimated by having a professional sonographer use ultrasound equipment to measure CO, and the patient was also subjected to arterial-venous blood gas analysis and VCO₂ recording. We compared the CO measured by the two methods and found that the error between them was less than 10% (Fig. 1).

Fig. 1

Comparison of cardiac output measured by VCO2 and Ultrasound

The method of measuring CO by VCO₂ has its unique advantages over the method of measuring CO by ultrasound. Its data are collected by precision instruments, which eliminate the interference of human factors.CO can be assessed relatively accurately by a simple ventilator setting and arterial and venous blood gas analysis. It makes it easier and faster for ICU doctors to evaluate the cardiac function of patients. However, we must admit that there are some limitations in estimating CO with VCO₂. The human heart is subject to neural, and humoral regulation. Cardiac output is adjusted to meet systemic metabolic demands in response to changes in basal metabolic rate. As the metabolic rate changes, the CO₂ produced by the body will also change, thus changing the VCO₂ and CCO₂. Using VCO₂ to assess CO requires that the body's metabolic rate is stable to improve the reliability of the results. In the included observation cases, due to the operation of VA ECMO, we did adequate sedation and analgesia during the measurement of CO to avoid the huge fluctuations in the body's metabolic rate in a short period and reduce errors.Also we need to acknowledge that the sample size of the present study is relatively small, and the data presented in this paper are primary observations. Larger and deeper clinical trials are already underway. Our team believes that routine VCO₂ measurement in patients with VA-ECMO is beneficial to disease monitoring and accurate treatment.