Book of Medical Disorders in Pregnancy - Tintash

increases slightly.
Arterial blood gases: Physiological
hyperventilation results in respiratory
alkalosis with compensatory renal
excretion of bicarbonate.
Fig7. 2: Serial measurements of lung
volume compartments during
pregnancy. (From Prowse CM,
gaensler EA: Respiratory and acid
base changes during pregnancy.
Anesthesiology 26:381, 1965)
The arterial carbon dioxide pressure
reaches a plasma level of 28-32 mm Hg,
and bicarbonate is decreased to 18-21
mmol/L, maintaining an arterial pH in
the range of 7.40-7.47. Mild hypoxemia
might occur when the patient is in the
supine position. Oxygen consumption
increases at the beginning of the first
trimester and increases by 20-33% by
term because of fetal demands and
increased maternal metabolic processes.
In active labour, hyperventilation increases
and tachypnea caused by pain and
anxiety might result in marked hypocapnia
and respiratory alkalosis, adversely
affecting fetal oxygenation by
reducing uterine blood flow. In some
pati-ents, severe pain and anxiety can
lead to rapid shallow breathing with
101
alveolar hypoventilation, atelectasis, and
mild hypoxemia.
Dyspnea during pregnancy is quite
common, occurring by most estimates in
approximately 60% of women with
exertion and fewer than 20% at rest.
Physiologic dyspnea can occur early in
pregnancy and does not interfere with
daily activities.
Although mechanical impediment by the
gravid uterus is often blamed, hyperventilation
due to incre-ased progesterone
levels probably is the most important
mechanism. The pre-sence of
other symptoms and signs of cardiopulmonary
disease indicates a possible
pathologic nature of dyspnea.
Fig7.3: Time course of % changes in
minute ventilation, oxygen uptake,
and basal metabolism during
pregnancy. (From Prowse CM,
Gaensler EA: Respiratory and acidbase
changes during pregnancy.
Anesthesiology 26:381, 1965)
Safety of drugs used in pregnancy:
Methylxanthine:
Both theophylline and aminophylline
readily cross the placenta, but no fetal ill