From: YTDP43A@prodigy.com (MS GAIL M HART)
Malcom griffiths<<
I remember reading of research long ago (on puppies I think?) done
perhaps in the fifties or early sixties. The researchers were studying
the effect of O2 in new born puppies. When they gave non-distressed
pups O2, they noted an almost immediate constriction in the blood
vessles near the retina. They concluded (as I remember) that there is a
set point of oxygen which the body tries to keep and if more oxygen
comes than is needed, the body tries to deal with it by constricting
the blood flow. I can't put my finger on the reference (but I'll
have fun looking
>From a gold mine of a book "The Physiology Of The Newborn Infant" --
Clement Smith, MD; published by Charles Thomas, Springfield Illinois,
1946 (and in Canada by the Ryerson Press, Toronto). The author looks at
now incredible research done on animals and human infants back in the
days before "ethics" was in issue - most of these experiements would
probably land you in jail today
>From pg 71 .."An intriguing feature of fetal physiology is the response
of placental vessels to the oxygenation of blood passing through them.
Several authors have observed the placental vascular dilation which
accompanies a fall of fetal blood oxygenation below a cetain level, and
the CONTRACTION WHEN THE OXYGEN CONTENT RISES (my emphasis); Schmitt
(37) describes numerous perfusion experiments probing this
phenomenon".>>>"a like adjustment of the ductus arteriosus has been
described above. That the umbilical as well as the placental vessels
and the ductus respond to the stimuli of increased or decreased
oxygenation has been shown by perfusion experiments (39) in which
buffered Ringers solution or defibrinated blood was propelled though
peices of human umbilical cord. The satuation of the perfusing
solutions with oxygen caused the vascular walls to contract with vigor,
while saturation whith carbon dioxide or nitrogen caused dilation of
the vesssels." The author gives four references for this research,
published between 1926 and 1941 -- in Italian, German and American
journals. (I'll post if anyone is deadly curious about them
Simple words.. increased carbon dioxide causes dilation of the
placental vessels (perhaps why we hear slowing of the heart beat),
increased O2 causes constriction -- squeezing -- of the placental
vessels (perhaps why we may hear an increased FHR, but....... is the
baby really getting more O2? Perhaps not -- according to the AM J
article by Thorp! -- Perhaps the baby just "sounds better", but is not
getting help -- and might actually be harmed).
It looks as if the baby's body tries hard to maintain "normal" O2
levels...
Back to Clement Smith --- While discussing the chemical triggers of the
newborn breathing reflex, the author cites work showing that the
<<"""respiratory center" in the medula is ordinarily sensitive to
changes in carbon dioxide tension in the blood"">> --- the fetus tries
to keep a normal level of blood gases -- and says that """it might be
possible to depress or even to stop the breathing by purveying a
considerable excess of oxygen""". Perhaps giving (un-needed) O2 in
labor may depress the breathing reflex at birth.
Babies need enough O2! We need to do what we can to avoid an anoxic
baby -- we should clearly give O2 to an anoxic mom. But giving O2 to
a mom who is not herself anoxic (or acidotic?), might not be the way to
get more O2 to the baby.
I know maternal O2 for low FHTs is expected almost everywhere -- I
think we need to examine why we do it. Like anything else we do -- we
need to ask "Is it effective?" and "Is it safe?" Especially in light
of the reference<<<<"Prolonged oxygen treatment during second stage of
normal labour resulted in a deterioration of cord blood gas values at
birth."<<<<<<<