DOI: 10.1097/CCM.0b013e3181987db5

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PMID: 19237958

Issn Print: 0090-3493

Publication Date: 2009/03/01


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Functional capillary density measurement: A useful new tool to assess the peripheral circulation in infants?*

Martin Keszler

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In this issue of Critical Care Medicine, Top et al (1) report the findings of an investigation of microvasculature in severely ill infants with respiratory failure before and after treatment with extracorporeal membrane oxygenation (ECMO) using the technique of functional capillary density (FCD) measurement. The article has several strengths. It is clearly written and describes the measurement technique and patient population well. More importantly, it draws attention to an area of pathophysiology that has received limited attention and is not easily monitored in the clinical settings.
When it comes to assessing circulatory status in infants, clinicians have focused for too long on variables that are easy to measure, such as blood pressure and heart rate. Because of small size and the risk of complications, invasive hemodynamic measurements are seldom available in newborn infants, making clinical assessment of circulatory status difficult. It has been shown that systemic blood pressure is a poor indicator of cardiac output and tissue perfusion, especially in infants (2, 3). Thus, a noninvasive means of assessing the status of the peripheral circulation and estimating adequacy of organ perfusion is an important unmet need. FCD measurement may be a useful tool in this regard, although its clinical usefulness at the bedside is yet to be clearly demonstrated.
FCD uses orthogonal polarization spectral imaging to directly visualize the microcirculation. Total length of capillaries with flowing cells (i.e., functionally open capillaries) per field of view is then determined using specialized software and expressed as FCD (4). The clinical utility of this approach may be facilitated by the recent development of automated image analysis techniques (5). Unlike other techniques, this approach provides direct visual evidence of the state of the microcirculation in the area being examined and thus, presumably, the adequacy of peripheral perfusion. Arguably, this should be more informative than blood pressure measurement, heart rate, or assessment of serum lactate.
The authors report that FCD was quite low initially and improved after completion of treatment with ECMO, when the infants were no longer very sick and were actually receiving treatment with vasodilators, in contrast to the vasopressors that they received before ECMO. This finding, of course, is hardly a surprise. The authors go to some length to try to convince us that the changes in FCD are related to ECMO itself. The case is not convincing, because the comparison was between sick infants on high vasopressor support before ECMO and much less ill infants receiving systemic vasodilators, with ECMO just happening to intervene in between. In other words, it is likely that the improved FCD is the result of the general improvement in cardiorespiratory status, whether as a result of ECMO or other therapies. There is nothing unique about ECMO that should have a specific effect on the microcirculation. The second control group that was added to bolster the case is not directly comparable and the patients were not exposed to vasodilators to the same extent.
The specific explanation for the documented improvement in FCD is, in fact, immaterial. The valuable contribution of this report is that it documents, for the first time, the feasibility of FCD measurement in sick preterm infants and its ability to apparently accurately assess the state of the microcirculation. Although it is too early to hail this as a breakthrough technology, the approach seems to offer important insights into functional physiology of the sick preterm infants, which has not been readily available in the past. What role FCD will eventually claim alongside other techniques, such as near-infrared spectroscopy (6, 7), which is also aimed at evaluating adequacy of the peripheral circulation and tissue oxygen delivery remains to be seen.
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