Measuring superior vena cava flow as part of echocardiography examinations performed by neonatologists

Field: Medicine (Health Sciences)

Reliable methods for haemodynamic monitoring play an important role in identifying underlying pathophysiological processes in critically ill term and preterm infants and are prerequisites for selecting adequate therapy. The ideal haemodynamic monitoring system should offer accurate and reproducible measurement of the relevant variables and provide data that can be interpreted. It should be easy to use, readily available and work independent of the operator. It must have a rapid response time, must cause no harm and must be cost-effective. Finally, yet importantly, the monitoring system should provide useful information to guide optimal therapy 1. Because such an ideal system does not exist, we have to choose the most appropriate system according to the target population and the clinical problem that has to be solved. Neonatologists can use bedside echocardiography for focused assessment of cardiovascular function, to obtain real-time physiological information for decision-making and intervention. This method has spread from centres in Australia and New Zealand to centres in North America and Europe. Clinicians and researchers have debated different terminologies, such as functional echoes, point-of-care ultrasound, targeted neonatal echocardiography, clinician-performed ultrasound and neonatologist-performed echocardiography 2-6. Several guidelines for structured training programmes with accreditation in neonatal echocardiography have been published 3-5. For example, in 2016, a working group within the European Society for Paediatric Research and European Society for Neonatology published recommendations for a training programme for neonatologist-performed echocardiography in Europe 6. The differences between the programmes are primarily related to the extent and amount of training required. The main concern that has been debated is the risk that neonatologists may overlook congenital heart defects. As a result, all the guidelines recommend close links and good cooperation between neonatologists and paediatric cardiologists, so that their skills complement each other rather than replacing each other. Most of the guidelines recommend the superior vena cava (SVC) flow measurement as a substitute for, or an additional measure for, systemic blood flow, because ductal and atrial shunting can affect the assessment of cardiac output from the left or right ventricle 7. The techniques for measuring SVC flow using echocardiography have not yet been directly validated against invasive measures. While validation against left ventricular output measurements in babies with no shunting was found to be good in one study, a magnetic resonance imaging study found that the method was inaccurate and casted doubt on the use of this measurement for diagnostic decision-making 8. That study has, however, been criticised for not fulfilling standard criteria for repeated measurement validations 9. Obviously, the technique for measuring SVC flow by ultrasound has several limitations. Echocardiography does not offer continuous measurements over time. Intraobserver and interobserver variability may be of significant importance. Although Doppler measures of arterial blood flow in the central circulation have good reproducibility, and correlate well with invasive measures of blood flow, it is debated whether this is also the case for venous flow. An inappropriate angle of insonation will underestimate the blood flow velocity. The calculation uses the assumption of laminar flow, with a parabolic flow profile that is representative of long, straight blood vessels under steady flow conditions. Current methods assessing SVC flow hardly meet these assumptions, because they recommend assessing the SVC velocity at the entrance of the vessel into the right atrium. The most important methodological problems with the assessment of SVC flow relate to the uncertainty associated with calculating the cross-sectional area of the vein. The formula π·d2/4 presupposes a perfect round vessel and uses the square of the vessel diameter to calculate a static cross-sectional area. These geometrical presuppositions, and the squaring of linear data, will amplify measurement errors. Despite these limitations, a review by McGovern and Miletin in this issue of Acta Paediatrica concludes that, although the SVC flow method is not perfect, it is currently the best available method for monitoring central perfusion in the neonatal population 10. SVC flow measured by Doppler ultrasound is an operator-dependent snapshot estimation of flow. Optimally obtained SVC flow measurement is just one piece of the puzzle that may provide valuable information to the neonatologist about the central circulation. Kluckow and Evans recommend measuring both SVC flow and right ventricular output, as a cross-check against each other, to minimise risks of measurement error 7, 9. As echocardiography is also particularly useful for assessing cardiac function, newer modalities for the myocardial function assessments should probably be included in the overall evaluation of the central haemodynamic monitoring system. An ideal monitoring system should give information about the microcirculation as well as the macrocirculation and assess whether the cells receive enough oxygen to sustain cellular mitochondrial respiration. And last, but not least, we must not forget that monitoring itself cannot improve outcomes, but it can help us to develop a better understanding of the haemodynamic situation and guide us to the most appropriate therapy.