One large multicenter study found no evidence that there was any difference in UAC placement, high versus low, in relation to IVH or death.10
High placement refers to the area in the aorta corresponding to thoracic vertebrae 6 through 9; low placement is usually referred to as lumbar 2 through 5. A Cochrane review of 5 randomized controlled trials and 1 alternate assignment study found that there is a lower incidence of vascular complications with high catheters, without any difference in other adverse effects.11
The specific study that we examined was chosen because of the dependent variable of IVH. No protocol was set that established a consistent rate of draw and flush, or that addressed the amount of blood drawn. It should be noted that there has been an increase in survival of smaller infants since this study was published.
Three other studies were designed to evaluate CBF, although they did not measure the incidence of IVH. They all used experimental designs. These studies specifically examined CBFV, changes in cerebral blood volume (CBV) and oxygenation, and tissue oxygenation. Definitions of these terms and others used in the medical research studies to follow are listed in .
Changes in the velocity of blood flow in the anterior cerebral artery during the sampling cycle in both high and low UACs was first described in 1996.12
This study used Doppler ultrasonography while flushing the UAC and demonstrated increased systolic and diastolic blood pressure, turbulence and microbubbles in the aorta. Turbulence was decreased with a slower rate of flush infusion. This study demonstrates a direct, positive relationship between the rate of flush and the change in blood flow velocity that occurs. The same volume was used in this study; however, the effect of volume in relation to infant weight and its effect on CBF were not investigated.
Although Doppler ultrasonography measures CBF velocities in the larger cerebral arteries, it cannot assess oxygenation to the brain. The next step that advanced knowledge about the effects of UAC sampling on VLBW infants examined changes in CBV and oxygenation.13
This study demonstrated a significant decrease in oxygenated hemoglobin (O2
Hb) that persisted beyond the time of the blood draw. Decreased levels of O2
Hb indicate that there is less oxygen available for tissue delivery. Cerebral blood flow was decreased in addition to a persistent decrease in cerebral oxygenation. This combination of physiologic events may indicate the potential for ischemia to vulnerable areas of the premature infant brain. Ischemia in watershed areas of the white matter can cause PVL,14
whereas ischemia to the germinal matrix could lead to ischemia/reperfusion injury, resulting in hemorrhage and damage to oligodendroglial precursors. lists the unique characteristics of premature infant's brains that make them especially susceptible to conditions and events that are better tolerated in mature neonates. This study did not control for the rate of aspiration, flushing, or the volume withdrawn and flushed. The description of this sample included VLBW infants. It would be useful to consider the differences in results when UAC sampling is performed on babies of different weights, controlling for rate and volume. This would provide information on the effects the same sampling procedure has on infants with different circulating blood volumes and different vessel size. Considering the percentage of cardiac output to a specific volume and rate of aspiration or flushing and the effect on CBF measures could be one approach to obtain better information about infant weights and safe methods of sampling. Careful consideration would have to be given to the ethical aspects of such a study because of the potential sequelae that could be associated with decreased CBV and cerebral oxygenation. Follow-up of the sample might be useful in linking these results to later IVH or PVL, although many other events might affect those outcomes.
Comparative Characteristics Predisposing the Preterm Infant Brain to IVH and PVL2
A later study did control for the variables of volume and rate, although not for infant weight.15
In addition to measuring O2
Hb, and deoxygenated hemoglobin, a tissue oxygenation index (TOI) was calculated. These investigators chose 20 seconds and 40 seconds as comparative speeds of withdrawing fluid and flushing the catheter to evaluate any differences these rates might impact on cerebral oxygenation. The authors found that drawing 2.3 mL of blood in 20 seconds from a UAC significantly decreased the O2
Hb and TOI while a draw time of 40 seconds had no effect. They recognized that the volume represents a different percentage of total circulating volume in infants of different sizes and that gestational age may have an effect.
Although this article addresses effects from sampling with UACs, a more recent article16
demonstrates a similar decrease in cerebral oxygenation and CBV when sampling from an umbilical venous catheter.