CARDIOVASCULAR JOURNAL OF AFRICA • Vol 22, No 6, November/December 2011
AFRICA
331
plasma volume increases by 30–40%.
11
Red blood cell mass
increases by approximately 20%, but with the increased volume
there is a relative decrease in the haematocrit.
The cardiac output increases on average by approximately
35%, commencing early in the first trimester, reaching a peak at
14 to 16 weeks and remaining at a plateau until labour. In labour,
cardiac output increases moderately with each contraction and
more appreciably with each expulsive effort in the second stage
of labour. Most of the increase in cardiac output falls dramati-
cally very soon after delivery (Fig. 1).
The increase in cardiac output in pregnancy is the result of an
increase in pulse rate and stroke volume. The heart rate increases
on average by 15 to 20 beats per minute and the stroke volume
by 5–10 ml.
11
Cardiac output is also influenced by maternal position. In the
supine position (the patient lying on her back), venous return is
reduced owing to pressure exerted by the pregnant uterus on the
inferior vena cava. This reduced return leads to reduced output
and hypotension (supine hypotension syndrome). This phenom-
enon is most often seen in late pregnancy.
Arterial blood pressure (Fig. 2)
In the lateral recumbent position, the blood pressure is higher
in the upper arm than the lower (10–12 mmHg). While sitting,
the blood pressure is slightly higher than in the supine position.
Peripheral vascular resistance decreases during pregnancy due
to the relaxing effect of progesterone on the smooth muscles.
The subsequent decrease in blood pressure reaches a nadir in the
second trimester compared with the early third trimester – the
well-known drop in blood pressure.
The average decrease in systolic blood pressure is 5–10
mmHg and the decrease in diastolic is 10–15 mmHg. If this
decrease fails to occur, it is reported that such women are more
likely to develop hypertension in the third trimester of preg-
nancy.
12
Definition of hypertension in pregnancy
Hypertension in pregnancy is defined as systolic blood pres-
sure
≥
140 mmHg and/or diastolic blood pressure
≥
90 mmHg
(Korotkoff 5). It should be noted that because elevations of both
systolic and diastolic blood pressure have been associated with
adverse maternal and foetal outcomes, both are important. Also,
detecting a rise in blood pressure from ‘booking’ or pre-concep-
TABLE 1. ANTIHYPERTENSIVE DRUGS FOR USE DURING PREGNANCY
Drug
Route Dose
Time
Action
Side effects
Methyldopa
po 0.25–1.5 g twice/day
3–5 days
False neurotransmitter
Orthostasis, sleepiness, depression
Labetalol
po
iv
200–1200 mg/d two or three times/
day in divided doses
20–40 mg iv every 30 min as needed
2–4 h
acts within
5 min
Non-selective
b
-blockade Tremulousness, headache
Nifedipine
po 30–120 mg/day
30 min
Calcium channel blocker Oedema, orthostasis, dizziness
Monohydralazine
po 50–300 mg/d two or three times/day 1–2 h/
20–30 min
Direct vasodilator
Lupus-like syndrome with chronic use
Dihydralazine
iv
po
10 mg every 2 h as needed
12.5–25 mg daily
Hydrochlorothiazide
po 12.5–25 mg daily
3–5 d
Diuretic
Emergency medications
Labetalol as noted
hydralazine as noted
Nifedipine as noted
Diazoxide
iv
iv
po
iv 30–50 mg every 5–15 min
2–4 min
Direct vasodilator
Hypotension, hypoglycaemia
Nitroprusside
iv 0.25
m
g/kg/min
1–2 min
Direct vasodilator
Hypotension, cyanide toxicity if used
>
4 h
po
=
per os; iv
=
intravenous
Fig. 1. Maternal cardiac output during pregnancy.
100
75
50
25
0
Cardiac output increase (%)
12
28
40
Duration of pregnancy (weeks)
Increase
during
labour and
delivery
Increase
during
pregnancy
Increase in cardiac output
In some studies a decrease was
shown in the late second and
early third trimesters
Fig. 2. Variation in arterial blood pressure during preg-
nancy.
150
125
100
75
50
0
Blood pressure
First
trimester
Second
trimester
Third
trimester
Labour and
delivery
Immediate
Blood pressure in mmHg
120
110
120
130
120
80
60
80
80
85
100
90
80
70
Pulse rate
72
87
65
Pulse rate in beats per minute
