Arch reconstruction without circulatory arrest: scientific basis for continued use and application to patients with arch anomalies

Aortic arch hypoplasia is a common constituent of congenital heart disease. While repair of these lesions has been performed routinely during deep hypothermia and circulatory arrest, new approaches are emerging. One such approach, regional low-flow perfusion, will be described here. This technique exploits the anticipated modified Blalock-Taussig shunt as a perfusion conduit. With control of the brachiocephalic vessels and the descending thoracic aorta, circulatory support can be provided to the neonate with exposure identical to that obtained by circulatory arrest. While first applied to children undergoing the Norwood operation for hypoplastic left heart syndrome, this technique has recently been applied to children requiring complex arch surgery in the setting of biventricular repair. To date, 36 neonates requiring arch reconstruction (27 Norwood operations, 9 biventricular repairs) have been supported with regional low-flow perfusion. Thirty-day and hospital discharge survival has been 74% (20/27) for neonates undergoing Norwood operation, and 88% (8/9) for those undergoing biventricular repair. We will review the operative technique, methodologies, and clinical studies that led us to conclude that regional low-flow perfusion provides cerebral, as well as somatic, circulatory support to the neonate undergoing arch reconstruction.

Cerebral intravascular oxygenation correlates with mean arterial pressure in critically ill premature infants

OBJECTIVES

Premature infants experience brain injury, ie, germinal matrix-intraventricular hemorrhage (GMH-IVH) and periventricular leukomalacia (PVL), in considerable part because of disturbances in cerebral blood flow (CBF). Because such infants are susceptible to major fluctuations in mean arterial blood pressure (MAP), impaired cerebrovascular autoregulation would increase the likelihood for the changes in CBF that could result in GMH-IVH and PVL. The objectives of this study were to determine whether a state of impaired cerebrovascular autoregulation could be identified reliably and conveniently at the bedside, the frequency of any such impairment, and the relation of the impairment to the subsequent occurrence of severe GMH-IVH and PVL.

PATIENTS AND METHODS

To monitor the cerebral circulation continuously and noninvasively, we used near-infrared spectroscopy (NIRS) to determine quantitative changes in cerebral concentrations of oxygenated hemoglobin (HbO(2)) and deoxygenated hemoglobin (Hb) from the first hours of life. Our previous experimental study showed a strong correlation between a measure of cerebral intravascular oxygenation (HbD), ie, HbD = HbO(2) - Hb, determined by NIRS, and volemic CBF, determined by radioactive microspheres. We studied 32 very low birth weight premature infants (gestational age: 23-31 weeks; birth weight: 605-1870 g) requiring mechanical ventilation, supplemental oxygen, and invasive blood pressure monitoring by NIRS from 1 to 3 days of age. MAP measured by arterial catheter pressure transducer and arterial oxygen saturation measured by pulse oximetry were recorded simultaneously. The relationship of MAP to HbD was quantitated by coherence analysis.

RESULTS

Concordant changes (coherence scores >. 5) in HbD and MAP, consistent with impaired cerebrovascular autoregulation, were observed in 17 of the 32 infants (53%). Eight of the 17 infants (47%) developed severe GMH-IVH or PVL or both. Of the 15 infants with apparently intact autoregulation, ie, coherence scores <.5, only 2 (13%) developed severe ultrasonographic lesions. Thus, for the entire study population of 32 infants, 8 of the 10 with severe lesions exhibited coherence scores >.5.

CONCLUSIONS

We conclude that NIRS can be used in a noninvasive manner at the bedside to identify premature infants with impaired cerebrovascular autoregulation, that this impairment is relatively common in such infants, and that the presence of this impairment is associated with a high likelihood of occurrence of severe GMH-IVH/PVL.

Near infrared spectroscopy detects cerebral ischemia during hypotension in piglets

We have previously reported concordant changes in cerebral intravascular oxygenation measured by near infrared spectroscopy (NIRS) and mean arterial blood pressure (MAP) in premature infants. We hypothesized that the cerebral oxygenation changes are caused by MAP-induced alterations in cerebral blood flow (CBF) and studied these parameters in neonatal piglets (n = 6). Changes in cerebral intravascular oxygenation were measured by NIRS from the hemoglobin difference (HbD) signal (oxyhemoglobin-deoxyhemoglobin). CBF was measured by the radioactive microsphere technique. The cerebral circulation was also monitored by Doppler determinations of CBF velocity (time average mean velocity) in the anterior cerebral artery. Hypotension to <50% of baseline MAP was achieved by a ligature around the ascending aorta. Arterial oxygenation was maintained constant by mechanical ventilation. As observed in our studies of premature infants, cerebral HbD and MAP showed concordant changes. Hypotension was accompanied by significant decreases both in CBF (42.8 +/- 12.5% of baseline p < 0.01) and HbD (-65.0 +/- 22.0 micromol/L x dpf, p < 0.01). HbD was significantly correlated with MAP (p < 0.05) and time average mean velocity (p = 0.01). Importantly, decreases in cerebral total hemoglobin (HbT), a measure of cerebral blood volume, did not correlate significantly with decreases in MAP. We conclude that 1) decreases in cerebral intravascular oxygenation, as assessed by NIRS, observed with decreases in MAP reflect a decline in CBF, and hence oxygen delivery, 2) the HbD signal is more sensitive to changes in CBF than the HbT signal, and 3) NIRS recordings may have clinical utility in detecting cerebral ischemia.