Dear Sir,
While Tsuji et al (1) have shown an association between impaired auto
-regulation and cerebral injury, the lack of early cerebral ultrasound
limits the definition of which comes first in the causal pathway.
Prospective data from ourselves (2) and others (3) would suggest that in
about a third of their babies, the IVH would have preceded the near infra-
red spectroscopy (NIRS) studies.
If impaired auto-regulation does come first, the question remains
whether it is a primary or intermediate factor in the causal pathway. In
animal models, cerebral autoregulation is quickly impaired by an hypoxic
or ischaemic insult (4). In a large study, we have shown that late IVH is
almost always preceded by a period of low blood flow to the upper body,
with flows less than 30% of that found in healthy babies (2). This low
flow usually occurs in the first 6 to 12 hours of life and is often not
reflected by low blood pressure. Like Wyatt and Meek (5), we would suggest
that this ischaemic period may be the primary event, causing damage to the
white matter, the watershed germinal matrix capillaries and
autoregulation. The latter two setting the baby up for a haemorrhage once
flow improves, as it almost always does after 24 hours (2). Thus the
possibility that impaired autoregulation is an intermediate factor in the
causal pathway needs to be recognised.
Presumably blood pressure was maintained medically during this study
(there are no details), yet despite this, cerebral injury still occurred.
This seems counter-intuitive if the cerebral circulation is truly pressure
passive, if you maintain pressure you should maintain cerebral blood flow
and prevent injury. This makes more sense if you consider cross sectional
data from ourselves and others which shows only a weak relationship
between blood pressure and upper body blood flow (2) and cerebral blood
flow (6). Low flows can occur in the presence of normal blood pressure,
with or without inotropes (2). These observations are not contradictory to
those of Tsuji et al (1), but suggest that in those babies with impaired
autoregulation, there is inter-individual variation in the level at which
blood pressure and cerebral blood flow correlate.
The title of this paper is somewhat misleading particularly as data
from the whole study group showed low correlation. Neonatology needs to
move beyond blood pressure as the gold standard of haemodynamic health. I
would suggest that only by recognising and preventing low flow, rather
than low pressure, will progress in preventing end organ injury be made.
NIRS and Doppler techniques are useful research techniques for measuring
blood flow but, in reality, both are a long way from being routine
clinical tools. Such a clinical tool is badly needed.
Yours Sincerely,
Dr Nick Evans
References.
1. Tsuji M, Saul JP, du Plessis A et al. Cerebral intravascular
oxygenation correlates with mean arterial pressure in critically ill
premature infants. Pediatrics 2000;106:625-632.
2. Kluckow M, Evans N. Low superior vena cava flow and intraventricular
haemorrhage. Arch Dis Child 2000;82: F188-F194.
3. Ment LR, Oh W, Philip AG, Ehrenkranz RA, Duncan CC, Allan W, Taylor KJ,
Schneider K, Katz KH, Makuch RW. Risk factors for early intraventricular
hemorrhage in low birth weight infants. J Pediatr 1992;121:776-83,
4. Tweed A, Cote J, Lou H, Gregory G, Wade J. Impairment of cerebral blood
flow autoregulation in the newborn lamb by hypoxia. Pediatr Res
1986;20:516-9.
5. Wyatt J, Meek J. Commentary on cerebral intravascular oxygenation
correlates with mean arterial pressure in critically ill premature
infants. Pediatrics 200;106:828
6. Tyszczuk L, Meek J, Elwell C, Wyatt J. Cerebral blood flow is
independent of mean arterial pressure in preterm infants undergoing
intensive care. Pediatrics 1998;102:337-341.
eLetters: ![[Read eLetters]](/icons/misc/sectionDOWN.gif){kind=icon}
