Comparisons of Continuous-wave Doppler Ultrasound Monitor and Echocardiography in Cardiac Postoperative Pediatric Patients

Introduction

In pediatric patients undergoing cardiac surgery, hemodynamic management is extremely important. Cardiopulmonary bypass induces capillary leakage and inflammatory reaction, thus contributing to postoperative hemodynamic instability. ¹ The ability to measure cardiac output (CO) may allow early detection of circulatory impairment, guide therapeutic decisions, and improve outcome in critically-ill infants. ²

The ultrasonic CO monitor (USCOM, Sydney, Australia) is a noninvasive hemodynamic monitoring system using continuous wave Doppler interrogation of blood flow in the ascending aorta. Benefits include portability, safety, dynamic monitoring, and ease of operation, but accuracy and precision in perioperative and critical care settings vary. Chong et al's meta-analysis calculated pooled mean bias, precision, and percentage error in six adult studies, ³ concluding that USCOM achieved good agreement with bolus thermodilution similar to other minimally invasive methods of perioperative CO monitoring, and may be useful. Literature on role and accuracy of USCOM in children is scarce with few studies assessing its utility in different pediatric populations,^4‐6 mostly with stable hemodynamics; there are fewer evaluations of mechanically ventilated, inotropic supported, cardiac postoperative pediatric patients.

Our aim was to compare and assess USCOM accuracy and precision in young pediatric patients in the immediate post-operative cardiac surgery period.

Materials and Methods

Results

One hundred and thirteen comparison scans were performed on 61 patients. Demographic and clinical characteristics are presented in Table 1. All patients were mechanically ventilated and properly sedated with no additional breaths. Mean tidal volume was 9 cc/kg (±2), mean PEEP was 5 cmH2O (±0), and all patients had sinus rhythm. No mortality occurred during the study period. Summary of the compared parameters is presented in Table 2. Comparing USCOM to Echocardiography yielded a mean CO difference of −0.24 (±0.95) L/min, mean CI difference of −1.01 (±2.6) L/min/m², mean SV difference of −1.53 (±5.86) cc, and mean VTI difference of 0.37 (±4.75) cm. Bland–Altman analyzes demonstrated poor agreement comparing USCOM to echocardiography in CI, SV, and VTI measurements (Figures 1–3). Comparison of the aortic valve diameter measured by echocardiography with aortic valve diameter estimated by the USCOM also yielded poor agreement (Figure 4). Mean AO difference was 0.08 (±0.15) cm. In order to rule-out the confounding effect of AO discrepancy on hemodynamic variable calculations, we performed multiple regression analyzes and calculated a correction formula to the USCOM derived AO. We used USCOM VTI and SV measurements in order to calculate “corrected” USCOM CI. The corrected CI was also found to have poor agreement with the echocardiography derived CI (Figure 5).

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Figure 1.

Bland–Altman analysis comparing USCOM to echocardiography for cardiac index (CI) measurements.

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Figure 2.

Bland–Altman analysis comparing USCOM to echocardiography for stroke volume (SV) measurements.

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Figure 3.

Bland–Altman analysis comparing USCOM to echocardiography for velocity-time integral (VTI) measurements.

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Figure 4.

Bland–Altman analysis comparing USCOM aortic diameter (AO) to echocardiography measurements.

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Figure 5.

Bland–Altman analysis comparing corrected USCOM cardiac index (USCOM corrected CI) to CI by echocardiography. Using multiple regression analyzes we found a correction formula for the USCOM derived aortic diameter. We used USCOM VTI and SV measurements in order to calculate “corrected” USCOM CI.

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Table 1.

Demographic and Clinical Parameters of the Study Cohort.

Parameter mean
Female gender	26 (42.6%)
Age [days], (range, ±SD)	94 (3-430, ±111)
Weight [Kg], (range, ±SD)	4.7 (2-10.5, ±2.1)
STAT 1–2 n (%)	33 (54%)
STAT 3–4	28 (46%)
VIS (range, ±SD)	15.3 (5-68, ±11)
Diagnoses
TOF	20 (32.8%)
TGA	14 (23.0%)
VSD	12 (19.7%)
VSD & Coarctation of the aorta	5 (8.2%)
AVSD	5 (8.2%)
TAPVR	2 (3.3%)
Rastelli operation	2 (3.3%)
Taussig bing	1 (1.6%)

Abbreviations: TOF, Tatrallogy Of Fallot; TGA, transposition of the great arteries; VSD, Ventriculo-Septal Defect; AVSD, Atrio-Ventricular septal defect; TAPVR, Total Anomalous Pulmonary Venous Return.

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Table 2.

Comparison Parameters.

Parameter a Mean (SD) n = 113	USCOM	Echocardiography	Percent difference
CO [L/min]	1.09 (0.67)	1.33 (1.21)	−1.6% (49.3)
CI [L/minute/m2]	3.66 (1.56)	4.7 (2.6)	−9.6% (45.6)
SV [ml]	6.8 (4.04)	8.36 (7.49)	−2.0% (48.4)
VTI [cm]	13.46 (4.49)	13.2 (5.1)	8.9% (34.7)
AO diameter [cm]	0.76 (0.14)	0.85 (0.22)	6.51% (17.4)

Abbreviations: CO, cardiac output; CI, cardiac index; SV, stroke volume; VTI, velocity time integral.

^a

Left ventricular outflow measurements.

Discussion

This study demonstrated that USCOM underestimated CI in comparison with echocardiography, and had poor agreement of VTI, SV, and AO in the setting of mechanically ventilated, immediate postoperative pediatric cardiac intensive care population. These finding cannot be explained solely by diameter discrepancy, as poor agreement was repeated after AO correction by multiple regression analysis.

Literature on the role and accuracy of USCOM in children is scarce with few studies assessing its utility in different pediatric populations. Studies in the neonatal population found that agreement between the USCOM device and conventional echocardiography is poor. Patel et al ⁹ found a mean percentage error of 43% for left ventricular output, higher by USCOM. Fraga et al ⁴ demonstrated overestimation in CO by USCOM, as compared with echocardiography. Both studies concluded that USCOM is not adequate for use as an absolute estimate of CO in healthy neonates. Both studies demonstrated higher VTI as assessed by USCOM, a similar finding to our study. Both studies enrolled healthy neonates aged ∼2 weeks old with a mean weight of 3–3.5 kg. One study had 46% mechanical ventilated patients, and the other no data regarding ventilation status. Chaiyakulsil et al ¹⁰ studied 121 children in a critical care setting, mean age of 4.9 years and weight 19.8 kg, stable hemodynamics, and 31.4% of them mechanically ventilated. They found that USCOM underestimated CI compared with echocardiography and electrical velocimetry, with 21% error. SVV had 257.1% error, VTI 12.7% error, and aortic valve diameter was smaller with 9.2% error. They concluded that the three noninvasive methods might be used interchangeably in pediatric critical care settings with stable hemodynamics. Wongsirimetheekul et al ⁶ compared CO estimation between USCOM and echocardiography in 34 pediatric intensive care unit (PICU) patients with mean age 7.8 years and weight of 27.1 kg. Their study confirmed that CO from the USCOM tended to be overestimated in comparison with surface echocardiography, especially in pediatric septic shock patients. Percentage error of CI and VTI were 42.3% and 32%, respectively. They concluded that the USCOM could not be used as a reliable tool for the absolute value measurement of CO and cardiac index in a pediatric population, especially in patients with septic shock. Beltramo et al ¹¹ studied 31 patients, with mean age of 12 years and weight of 46 kg. Only two patients were mechanically ventilated. They assessed the accuracy of CO measurements obtained by USCOM as compared to pulmonary artery thermodilution in hemodynamically stable children with normal cardiac anatomy. The mean percentage error of CO between techniques was 11%. The authors concluded that USCOM is a reliable tool to assess hemodynamic status in pediatric patients with no intracardiac shunts, and that further studies are needed to validate its usefulness in neonates, patients with congenital heart defects, and pediatric patients with both high and low COs as in shock states. Our study demonstrated similar results with studies conducted on young, hemodynamically unstable, mechanically ventilated pediatric patients. This literature review raises a concern that younger patients, mechanically ventilated and possibly hemodynamically unstable, may cause inaccuracy in USCOM CO measurements.

When inspecting the Bland-Altman plots of VTI, CI and AO diameter, one can interpret that USCOM underestimated AO-diameter in comparison with echocardiography in patients with AO-diameter >1.0 cm and overestimated AO-diameter in patients with AO-diameter less than 0.8 cm and the same pattern with VTI interpretation. In order to rule-out the confounding effect of extreme variables on hemodynamic calculations, and put the analyzes within the daily practical range, we performed the above analyzes on patients with “average physiological” range of CO – 1.5–4.5 L/min, or CI 2–7 L/min/m². ¹² The BA plots of CI were found to have poor agreement with the echocardiography derived CI.

In order to overcome a systematic mistake generated from a predefined table-derived USCOM AO diameter, we used Echo-derived AO-diameter and performed multiple regression analyzes and calculated a correction formula to the USCOM derived AO. We used USCOM VTI and SV measurements in order to calculate “corrected” USCOM CI. The corrected CI was also found to have poor agreement with the echocardiography derived CI (Figure 5).

As the clinical purpose of the USCOM monitor is to analyze the current CI of the critically ill child at the bedside, and the AO diameter, SV and VTI are only the measures, in the practical clinical setting, USCOM had poor agreement with echocardiography.

Conclusion

Our study shows that USCOM underestimates CI in comparison with echocardiography; therefore USCOM should be used with great caution as an absolute estimate or surrogate of CI in neonates and infants in the immediate post-operative, congenital heart surgery period.