Page 735 - ACCCN's Critical Care Nursing

712 S P E C I A LT Y P R A C T I C E I N C R I T I C A L C A R E

Heart Rate, Stroke Volume and
TABLE 26.2 Key physiological changes in pregnancy Cardiac Output
Maternal heart rate increases by 10–15 beats per minute
Change during during pregnancy with an increase noted as early as 5
Parameter pregnancy weeks’ gestation. 16,22 The increase in heart rate may be a
Cardiovascular system: compensatory response related to the generalised vasodi-
Heart rate ↑ 10–15 beats/min latation, although a hormone-related effect cannot be
Blood pressure ruled out. Tachycardia (>100 beats/min) is an abnormal
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Systolic ↓ 5–9 mmHg 24
Diastolic ↓ 6–17 mmHg sign and warrants further investigation. The stroke
Cardiac output ↑ 30–50% volume is noted to increase between 18 and 32%, begin-
Systemic vascular resistance ↓ up to 35% ning as early as 8 weeks’ gestation. 25,26 An increase in
Central arterial and venous pressures Unchanged cardiac output is detectable from 5 weeks gestation and
Blood and associated components: continues to be 30–50% higher by 32 weeks gestation. 17,26
Blood volume ↑ 40–50% Hence, a normal cardiac output in pregnancy may be as
Plasma volume ↑ 40–50% high as 8 L/min. The increased cardiac output is achieved
Red blood cells ↑ 20–40% by a combination of the increases in heart rate and stroke
White blood cells ↑ 100–300% volume.
Platelets Unchanged
Fibrinogen ↑ 100%
Serum albumin level ↓ 10–15% Systemic Vascular Resistance
Respiratory system The generalised vasodilatation observed in early preg-
Respiratory rate Unchanged nancy reduces systemic vascular resistance by up to 35%,
Tidal volume ↑ 25–40% with some reduction already detectable by 8 weeks’ gesta-
Minute volume ↑ 40–50% 27
Oxygen consumption ↑ 15–20% tion. The development of the low-resistance utero-
Arterial blood gas analysis values placental junction was thought to act as an arteriovenous
80–110 mmHg shunt and contribute to the lowered SVR seen in preg-
PaO 2
28–32 mmHg
PaCO 2 nancy. However, the very-early-observed decrease in SVR
pH 7.40–7.45
− argues against this theory and perhaps circulating sub-
HCO 3 18–21 stances that exert a vasodilatory effect on the vasculature
SaO 2 ≥95%
Vital capacity Unchanged is a more likely proposition.
Functional reserve capacity ↓ 17–20%
Airway compliance and resistance Unchanged Effect of Posture on Maternal
Renal system Haemodynamics
Glomerular filtration rate ↑ 40–50%
Serum urea and creatinine ↓ It is evident that from as early as 5–8 weeks’ gestation,
Urine output Unknown pregnancy is characterised by general vasodilatation,
Proteinuria <300mg/day increased blood volume, increased cardiac output and
is generally a hyperdynamic state. As the pregnancy
advances, the bulk of the uterus begins to have an impact
on maternal haemodynamics. After 20 weeks’ gestation,
a woman lying flat on her back may experience supine
a stable level until delivery. 17,20 Women who do not expe- hypotension, secondary to compression of the inferior
rience this normal increase in blood volume are more vena cava and aorta with subsequent reduction in venous
prone to adverse outcomes such as preeclampsia or small- return, cardiac output and placental flow. A reduction in
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for-gestational-age infant. The additional blood volume placental flow may occur even without a recorded drop
is also thought to accommodate the normal blood loss in blood pressure. Consequently, it is inadvisable to nurse
associated with birth (<500 mL). Pregnant women are a pregnant woman more than 20 weeks’ gestation, flat on
renowned for being able to maintain stable vital signs, her back. A left lateral lying position results in the best
with blood losses as much as 1500 mL, before acutely cardiac output, although manually displacing the uterus
deteriorating. to the left whilst the woman remains supine is also effec-
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tive in relieving the aorto-caval compression. Otherwise,
Blood Pressure the use of a wedge or pillows to maintain a left lateral tilt
Blood pressure reduces in pregnancy, with the lowest of at least 15 degrees is recommended to minimise aorto-
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normal blood pressure recorded during the second tri- caval compression.
mester (16–28 weeks), and returns to pre-pregnancy
levels near term (see Table 26.2). Blood pressure begins Postpartum Cardiovascular Changes
dropping as early as 8 weeks’ gestation, in association Heart rate returns to pre-pregnancy levels by 10 days post-
with the generalised vasodilatation occurring at this time. partum; blood pressure has normally returned to pre-
If a woman does not experience the characteristic lower- pregnancy levels by term and does not change during the
ing of blood pressure, particularly during the second tri- puerperium. 23,27 The first few days of the puerperium are
mester, it is viewed with suspicion and as a potentially associated with a diuresis which reduces the circulating
abnormal sign. volume and results in haemoconcentration of blood.