Extracorporeal Arteriovenous Ultrasound Measurement of Cardiac Output in Small Children

Theodor S. Sigurdsson, M.D.; Anders Aronsson, M.D.; Lars Lindberg, M.D., Ph.D.


Anesthesiology. 2019;130(5):712-718. 

In This Article

Materials and Methods

Study Design and Subjects

Forty-seven children undergoing elective cardiac surgery for correction of atrial septal defect and/or ventricular septal defect were enrolled in the study. Inclusion criteria were informed written parental consent, weight of less than 15 kg, and atrial septal and/or ventricular septal defects. Because lack of repeatability of the CO measurement can interfere with the comparison of the two methods, it was important that the CO measurement and the stroke volume were fairly constant during the measurement. Exclusion criteria were, therefore, shunts (undiagnosed extracardiac or significant residual shunt after the surgical correction), perioperative arrythmias (supraventricular, nodal tachycardia, and atrioventricular heart block), and/or significant valvular regurgitations (aortic, mitral, tricuspid, and pulmonary valvular leaks). This study was approved and registered by the Ethics Committee of Lund University, Lund, Sweden (Dnr 2013636).

Cardiac Output Measurements

Calculation of CO by Use of Saline Blood Dilution Detected by Ultrasound Sensors (COUD). The technology is based on the premise that the ultrasound velocity of blood changes linearly with the level of blood dilution caused by injection of a specified bolus volume of body-temperature isotonic saline. The ultrasound velocity in blood is 1,560 to 1,585 m/s and decreases toward the ultrasound velocity of saline (1,530 m/s) after a bolus injection of saline. The device, developed by Transonic Systems Inc. (USA), uses an extracorporeal arteriovenous loop connected between existing arterial and central venous lines. The loop is connected to an external roller pump, which maintains a stable blood flow in the loop of 10 to 12 ml/min. The loop contains specific venous and arterial segments to which external ultrasound sensors fit. The sensors measure the ultrasound velocity and the blood flow at the out-flow and in-flow parts of the loop circuit.

A measurement session begins by entering the patient's weight, length, arterial blood pressure, central venous pressure, and heart rate into the device. The connection stopcock to the out- and in-flow segments of the arteriovenous loop, primed with 5 ml of body-temperature, heparinized isotonic saline, is opened. The roller pump starts, and a small amount of body-temperature physiologic isotonic saline (0.5 to 1.0 ml/kg) is injected at the out-flow segment of the loop on the venous side before the venous ultrasound sensor. The saline is warmed to 37°C body temperature by a bag warmer that is connected to the device. The volume and time of the injected saline is determined by the venous ultrasound sensor. The saline is completely mixed as the blood passes through the cardiopulmonary circulation and gives rise to a homogenous blood dilution on the arterial side. The final blood dilution that occurs in the systemic arterial circulation is detected by the arterial ultrasound sensor at the arterial in-flow segment of the loop, and an ultrasound velocity curve is generated (Figure 1).

Figure 1.

Schematics (A) and monitor display (B) of the tested cardiac output (CO) measurement device. The y axis C (%) represents the percentage concentration of saline in the arterial blood while the x axis is time (seconds). ACVI, active circulation volume index; BSA, body surface area; CBVI, central blood volume index; CI, cardiac index; CVP, central venous pressure; HR, heart rate; SVI, stroke volume index; SVRI, systemic vascular resistance index; TEDVI, total end diastolic volume index; TEF, total ejection fraction.

Because the technology records the ultrasound velocity simultaneously at both the out-flow and in-flow segments of the loop at a constant blood flow rate, not only can the area under the curve be analyzed, but the time of occurrence and form of the dilution curve after it passes through the lungs and heart can be used to calculate total end-diastolic cardiac volume, central blood volume, and active blood volume and to determine and detect cardiac shunts.[12–17] CO is calculated by analyzing the area under curve based on the Stewart–Hamilton indicator dilution principle.[18–20]

CO Measurement with Perivascular Flow Probes (COPVFP). AU-series confidence perivascular flow probes (Transonic Systems Inc.) were used in this study. The flow probe is custom-designed to fit around vessels to measure blood flow in real time by ultrasound transit-time technology. Transit-time technology uses four crystals and wide-beam illumination to send ultrasonic signals back and forth across the vessel, alternately intersecting the blood in upstream and downstream directions. The flowmeter derives an accurate measure of the "transit time" it takes for the wave of ultrasound to travel from one transducer to the other. The difference between the upstream and downstream integrated transit times is a measure of true volume flow, not velocity.

The flow probes are available in different diameter sizes (8 to 24 mm) and can be used multiple times because they can undergo standard sterilization. The probes come ready to use and calibrated from the manufacturer with a certified length of use for more than 1 yr.

In our study, the flow probe was applied to the aorta approximately 1 cm distal to the origin of the coronary arteries (Figure 2). The flow probe was then connected to an Optima dual-channel HT363 Flow-QC meter (Transonic Systems Inc.). AureFlo diagnostic software (Transonic Systems Inc.) was used to visualize a good signal of pulsating aortic blood flow waveform and record CO. Transit-time ultrasound perivascular flow probes are considered the standard reference method for cardiac output estimation and have been verified in number of studies.[21,22]

Figure 2.

Placement of the perivascular flow probe around aorta.

Experimental Protocol

Anesthesia was induced using fentanyl (5 μg/kg) and penthothal (5 mg/kg) and maintained with isoflurane (0.5 to 1.0%). Pancuronium (0.2 mg/kg) was given to facilitate intubation with a cuffed endotracheal tube. As is routine in children undergoing cardiac surgery, all subjects had a peripheral arterial catheter placed in the radial artery and a central venous catheter placed in the right internal jugular vein. The catheters were connected to the arteriovenous loop of the CO device and were primed with heparinized (2 units/ml) 37°C physiologic saline. Ultrasound sensors were placed on the venous and arterial segments of the arteriovenous loop before surgery. After surgical correction and weaning from cardiopulmonary bypass, transesophageal echocardiography was performed to exclude residual intracardiac shunts or valve regurgitations. When a stable sinus rhythm and normal body temperature were achieved, the surgeons applied the perivascular flow probe around the aorta, and measurements were initiated. Each measurement session consisted of five consecutively repeated CO measurements with injections of body temperature physiologic saline boluses and, simultaneously, five readings of aortic blood flow measured with the transit-time ultrasound periaortic flow probe.

Statistical Analysis

Statistical analysis was performed using Statistica version 12 (Statsoft, USA). No statistical power calculation was conducted before the study, because the bias and SD of the bias between the two methods were unknown, and the CI for the 95% limits of agreement was not possible to estimate. The sample size was based on previous experience with this design. All data are expressed as mean ± SD unless indicated otherwise.

The degree of variation of each technique was presented as the coefficient of error (CE) of average repeated measurements, calculated as ratio of the coefficient of variation (CV) divided with the square root of the number (n) of repeated measurements (CE = CV/√n). Precision of a technique is considered to be two times the coefficient of error as suggested by Cecconi et al.[23]

Bland–Altman analysis was used to estimate bias between the different techniques while accounting for the repeated measurements within each individual.[24] The mean difference (bias) between cardiac output with saline dilution and ultrasound detection (COUD) minus cardiac output with perivascular flow probe around aorta (COPVFP) was calculated and plotted against the average of the comparison (COUD + COPVFP)/2. The 95% limits of agreement were calculated as mean bias ± 1.96 × SD (SD of the bias). Limit of agreement analysis was performed to determine whether the two methods agreed sufficiently with each other so that one could replace the other. The 95% CI of the bias and the limits of agreement were determined after testing for normal distribution using Levene's test.[25]

According to Critchley and Critchley,[26] the percentage error (PE) was calculated as 1.96 × SD of the bias/mean cardiac output of the reference method × 100%.