Article Review

Measurements of cardiac output (CO) are frequently done in surgical and critically ill patients as part of optimisation strategies. The clinical standard is considered to be intermittent thermodilution technique based on the Stewart–Hamilton equation. This technique has its limitations, including variability in serial measurements of CO and rare, but potentially serious, complications.

Aim of the study
This systematic review [1] aimed to examine the evidence of echocardiography for measurements of CO using:
–    Difference of end-systolic and end-diastolic volume to measure stroke volume (SV) 
–    Velocity of time integral (VTi) of the blood flow by the cross-sectional area of the left ventricular outflow tract (LVOT) to measure CO

Study selection
1.    Hospitalised patients 
2.    Prospective clinical studies, systematic reviews, or meta-analysis 
3.    Adults 
4.    Use of echocardiography-derived data to assess SV or CO
5.    Use of thermodilution technique as the reference technique
6.    Use of the Bland–Altman method to compare CO monitoring techniques

Results
–    24 studies included (19 TEE, 5 TTE).
–    Heterogeneous population (15 of the TEE studies were performed during elective cardiac surgery under haemodynamically stable conditions, 8 studies were performed in ICU populations including septic shock, acute myocardial infarction, pulmonary hypertension, subarachnoid haemorrhage, and after liver transplantation).
–    Homogeneous CO measurements with thermodilution with some limitations: 5 studies it was not described if patients with arrhythmia were included; mitral regurgitation and tricuspid regurgitation were observed in some studies. 
–    The investigations were blinded and not randomised in the majority of studies, in 3 studies a random allocation process was used.  

Discussion
–    Only 2 studies had a design that allowed a fully unbiased comparison of the two techniques [2,3].
–    Only one study had a design that allowed a fully unbiased comparison of trending ability between echocardiography and thermodilution, and this showed agreement between the two techniques in the directional changes in CO [2].

Conclusion 
The small sample size, heterogeneity of the studies, and inadequate statistical assessment did not allow to make any definitive statement. Current evidence does not support interchangeability between these techniques in measuring cardiac output.

Article review was submitted by Stephane Ledot and Antonella Tosi on behalf of the ESICM Journal Review Club.

REFERENCES

1.    Wetterslev M, Møller-Sørensen H et al (2016). Systematic review of cardiac output measurements by echocardiography vs. thermodilution: the techniques are not interchangeable. Intensive Care Medicine. Systematic Review; Volume 42, Issue 8 / August, 2016; Pages 1223 – 1233

2.    Moller-Sorensen H, Graeser K, Hansen KL et al (2014). Measurements of cardiac output obtained with transesophageal echocardiography and pulmonary artery thermodilution are not interchangeable. Acta Anaesthesiol Scand 58:80–88

3.    Axler O, Tousignant C, Thompson CR et al (1996). Comparison of transesophageal echocardiographic, fick, and thermodilution cardiac output in critically ill patients. J Crit Care 11:109–116