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 -> ↑atrial stretch -> Bainbridge effect (?)

·   ↓SVR -> ↓mAP -> baroreceptor response (?)

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

Stroke volume

·   ↑Uteroplacental demand -> ↑preload

·   ↑RR and VT -> ↑respiratory pump activity (?)

(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

 

 

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