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The importance of the right ventricle is consistently underestimated. In conventional thinking about the cardiovascular system, the focus is invariably the function of the left ventricle. It is to be applauded that this study of Clark et al recognises the importance of the right ventricle and attempts to quantify measures of function in healthy term and preterm babies.
One reason for the absence of data about the function of the right ventricle is the lack of consistent landmarks that allow standardisation of function measures. Even in the left ventricle, interpretation of myocardial function measures is fraught with problems. In preterm babies, the traditional M mode measures, such as fractional shortening, are of limited use because of the reduced movement of the ventricular septum.1 An often ignored basic truth about the cardiovascular system is that a measurement at any one point in that system can be affected by events not only at the site of measurement but also occurring upstream and downstream of that site. For myocardial function, it is the preload and afterload conditions that most affect the interpretation of the findings. Sometimes myocardial function measures reflect the health of the myocardium, but in the transitional circulation they are more likely to reflect the load conditions.2 For example, a significant ductal shunt will improve measures of left ventricle contractility such as ejection fraction and fractional shortening, because the shunt will both increase the left ventricle preload and reduce the afterload.2 Left ventricular dysfunction has been measured in babies with severe respiratory distress/pulmonary hypertension and interpreted as being a primary problem.3 However, this overlooks the fact that the preload of the left ventricle depends on blood getting through the lungs. If the resistance to pulmonary blood flow is very high (high right ventricular afterload), the right ventricular function will appear poor, and, because it will not be able to pump blood through the lungs, the preload on the left ventricle will be low and left ventricular contractility will also appear poor. In our own (unpublished) studies, nitric oxide can produce dramatic improvements in the function and outputs of both ventricles in some babies.
Clarke et al have shown that right ventricle end systolic and diastolic dimensions fall over the first 24 hours of life to a similar degree in a term and preterm cohort. The preterm babies were healthy and ranged from 31 to 34 weeks gestation; cardiorespiratory adaptation would be expected to occur relatively normally in such babies.4 Therefore these results may not be applicable to very preterm babies (< 30 weeks gestation) in whom most preterm haemodynamic pathology is concentrated.5 The authors speculate that the decrease in dimensions of the right ventricle may reflect increase in size of the left ventricle and also the postnatal fall in pulmonary vascular resistance (afterload). The latter is not measured, but has been previously documented as falling gradually over the first 24 hours. The other factor not considered is the effect of incompetence of the foramen ovale on increasing right ventricular preload. Such incompetence is common in the early postnatal period particularly while there is still a ductal shunt.6
In our studies to date, we have derived little useful understanding from traditional measures of left ventricular contractility in newborns. Our focus has been more on flow measures, which we would argue is the important physiological outcome of myocardial function. Low systemic blood flow is common in the very preterm and is strongly associated with morbidity in these babies.5 Immaturity of myocardial function is likely to be a factor. We are currently analysing data on the relation between this low flow and the load independent measures of velocity of left ventricle circumferential shortening and wall stress.7 However, we do not have data on right ventricular function, which is just as important. The data of Clarke et al may provide an important framework on which to extend understanding of cardiac pathophysiology in the very preterm infant. However, these data also highlight that to interpret any measure in the cardiovascular system, it is important to know the limitations of that measurement and also what is happening upstream and downstream of the site where the measurement is taken.