Understanding Anesthesiology - The Global Regional Anesthesia ...

crease in PaCO2 (32 mmHg). The concomitant rightward shift in
the oxyhemoglobin dissociation curve allows increased fetal transfer
of O2.
Circulatory Changes
Blood volume increases by 40% during pregnancy in preparation
for the anticipated 500-1000 cc average blood loss during vaginal
or Caesarian delivery, respectively. This is significant for two reasons.
Firstly, the normal signs of hypovolemia may not be seen until
a relatively greater blood loss has occurred. Secondly, the expanded
intravascular volume may not be tolerated by parturients
with concomitant cardiovascular disease, such as mitral stenosis.
Due to the increasing uterine size, aortocaval compression (obstruction
of the inferior vena cava and aorta) becomes relevant in
the third trimester. When the pregnant patient is in the supine position,
the heavy gravid uterus compresses the major vessels in the
abdomen leading to maternal hypotension and fetal distress. Left
lateral tilt, usually achieved with a pillow under the woman’s right
hip, is an important positioning maneuver.
Labour Analgesia
There are many methods of relieving the pain and stress of labour.
The non-invasive methods, such as transcutaneous electrical
nerve stimulation (TENS), hypnosis and massage require a wellprepared
patient who is able to accept the incomplete relief that
such methods inevitably provide. Invasive methods, such as inhaled
(nitrous oxide), intravenous (opioids) or regional (epidural)
are associated with side effects and risks to both fetus and mother.
Epidural labour analgesia will be discussed briefly in this section.
The pain of the first stage of labour is referred to the T10-L1 somatic
areas. This extends to include sacral segments (S2-4) during
the second stage. Thus, the principle of epidural analgesia is to administer
local anesthetics (with or without opioids) into the
epidural space to block the aforementioned spinal segments.
The primary advantages of epidural analgesia are its high degree
of effectiveness and safety. The patient remains alert and cooperative.
In the absence of complications, there are no ill effects on the
fetus. Epidural analgesia can be therapeutic for patients with preeclampsia
or cardiac disease where a high catecholamine state is
detrimental. Finally, the level and intensity of an epidural block
can be extended to provide anesthesia for operative delivery (Caesarian
section).
As well as blocking sensory fibres, local anesthetics in the epidural
space interrupt transmission along sympathetic and motor neurons.
The hypotension associated with sympathetic blockade can
be minimized by a one litre bolus of crystalloid prior to institution
of the block, slow titration of the local anesthetic, the use of lower
concentrations of local anesthetic and vigilant guarding against aortocaval
compression.
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