2018B05 Describe the maternal cardiovascular changes that occur during pregnancy.

 

List:

·       Summary

·       Haemodynamics

·       Blood

·       Regional circulation

·       Labour

·       Other

 

Summary:

Objectives

·  Support foetal gas exchange

·  Prepare for massive PPH

Most important changes

·  ↑Blood volume

·  ↑Interstitial volume

·  ↑Cardiac output

·  ↑Coagulability

Causes

·  Placental hormones

·  ↑Demand

 

Haemodynamics:

Heart rate

·  ↑Uteroplacental demand -> ↑venous return

(Note: ↑HR occurs from ~4/40, not conception)

Stroke volume

·  ↑Uteroplacental demand -> ↑venous return

·  ↑RR and VT -> ↑respiratory pump

(Note: ↑SV occurs a bit later than for heart rate)

Cardiac output

·  ↑Uteroplacental demand

·  Note increased risk of heart failure

Systemic vascular resistance

·  Progesterone, prostaglandins ->

·  Smooth muscle relaxation

·  Downregulation of α1 adrenoceptors

Blood pressure

·  ↓↓Diastolic, ↓systolic, ↑pulse pressure

·  Due to ↓SVR with ↑stroke volume

 

Blood:

Blood volume

 

·  Causes:

o Oestrogen -> ↑RAAS -> retain Na+, H2O

o Oestrogen -> ↑Epo

·  Implications:

o Iron supplementation aids erythropoiesis

o Hypervolaemic haemodilution improves tolerance of massive PPH

o Hypoviscosity aids increased cardiac output

Interstitial volume

·  Mucosal swelling, peripheral oedema

·  Causes:

o Oestrogen -> ↑RAAS

o Progesterone -> vasodilatation

·  Implication:

o ? Fluid reserve in case of blood loss (no evidence for this)

Plasma proteins

·  ↓[Albumin], ↔AAG

·  ↓Oncotic pressure 14%

·  Cause: progesterone -> volume expansion -> dilution

·  Implication: ↑free % acidic drugs e.g. ↑risk thiopentone toxicity if GA LUSCS

Hypercoagulability

·  ↑Factors I (2x), VII, VIII, IX, X, XII, vWF

·  ↓Protein S, acquired resistance to Prot C

·  Cause: oestrogen

·  Implication: ↓risk lethal bleeding, ↑risk thromboembolism

 

Regional circulations:

Uterine blood flow

·  150mL/min -> 750mL/min (80% uteroplacental, 20% other)

·  Cause: ↑demand

·  Implication: potential for rapid bleeding

Other

·  Renal blood flow: ↑80%, mostly in T1

o Hence ↑urine output

·  ↑Flow rate through skin, muscle, breast, GIT

o Hence faster onset of SC and IM drugs (e.g. opioid in labour)

 

Labour:

Escalating cardiac output

·  Intrapartum: effort + pain + distress (attenuated by epidural)

·  Post-partum: uterine involution -> autotransfusion (highest risk of heart failure)

 

Pathology:

Supine aortocaval compression

·  i.e. supine -> ↓↓mAP

·  Blood shunted to azygos systems

·  Affects 15% patients

·  From 20/40

·  Prevention: wedge under right hip during LUSCS or resuscitation

Pre-eclampsia

·  Cause: ? abnormal invasion or spiral arteries by trophoblast

·  Characterised by multi-system endothelial dysfunction

·  Risk of hypertensive emergency and seizure