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)