Cardiovascular changes in pregnancy
[Ref: PK1:p346-368]
Haemodynamics
Heart rate
- Occurs as early as 4 weeks after conception
- Increases by 17% by end of 1st trimester
- Increases to 25% at the middle of 3rd trimester
Stroke volume
- Increased by 20-30%
- Most of the increase occurs in 1st trimester
Total peripheral vascular resistance
- Decreases by 30% at 12th week
- By 35% by 20th week
- Due to vasodilation mediated by progesterone, prostaglandins, and down-regulation of alpha-adrenergic receptors
- Also due to placental blood flow, which acts as an AV-shunt
* [KB2:p248]
Cardiac output
- Increase progressively throughout pregnancy
- 40-45% above non-pregnant values at 12th to 28th week
- Reach peak of 50% during 32-36th week
- Then decrease slightly to 47% above non-pregnant level at term
NB:
According to [KB2:p248], cardiac output increases by 30% only
Causes of cardiac output increase
- Venodilation
- Increased vascular volume
* Caused by oestrogen
Blood pressure
- Decreased in mean arterial blood pressure
- Systolic BP and diastolic BP decrease by 10%
- ????Stable after 20weeks
Others
Central venous pressure and pulmonary capillary wedge pressure
--> Unchanged
Oncotic pressure falls by 14%
--> Predisposition to oedema
Blood flow and volume
Blood flow changes
- Uteroplacental blood flow increase to 750mL/min at term
* 85% goes to the placenta
- Renal blood flow increase by 80% in the first trimester
- Also increased blood flow to skin, breast, and GIT
- Cerebral and hepatic blood flow unchanged
Aortocaval compression syndrome
- Occurs in about 15% of women near term
- When supine
--> Compression of inferior vena cava
--> Decreased venous return and cardiac output
--> Hypotension, pallor, nausea, and vomiting
- May develop as early as the 20th week
- May also be associated with uterine arterial hypotension and reduced uteroplacenta perfusion
- May be prevented by lying on the left side
Maternal blood volume
Near term
- Maternal blood volume is increased by 35-40% (about 1000-1500mLs)
- Plasma volume increases by 45%
* Due to sodium and water retention by oestrogen stimulation of renin-angiotensin system.
- RBC volume increases by 20%
* Due to increased renal erythropoietin synthesis
Thus
- The increase in RBC volume is relatively less than the increase in plasma volume
--> Haematocrit falls to 33%
--> Physiologic anaemia of pregnancy
NB:
According to [KB2:p248],
* Blood volume increase by 40-45%
* Plasma volume increase by 50%
* RBC volume increase by 30%
* Most of the increase occurs in the first 2 trimester
Labour
During labour, each uterine contraction squeezes about 300mL of blood into the central maternal circulation
During labour, cardiac output:
- Increase by 15% during latent phase of labour
- Increase by 30% during the active phase
- Increase by 45% during the expulsive stage
Maternal systolic and diastolic arterial BP increase by 10-20mmHg during uterine contraction
After delivery
Cardiac output and BP returns to non-pregnant level by 2 weeks after delivery
Other notes
Placenta blood flow is passive and pressure-dependent (not autoregulated)