2009B09 Classify the causes of hypotension in the early post-operative period, giving relevant examples.

Definition

<70mmHg or >20% below baseline

Equation

mAP = (CO x SVR) + CVP

Cause

·   ↓Preload

·   ↓Heart rate (or severe tachyarrhythmia impairing LV filling)

·   ↓Contractility

·   ↓ SVR (note may ↑CO hence partial offset)

Problem

·   ↓mAP -> ↓perfusion pressure -> ↓blood flow -> ↓O₂ supply

Especially if:

·   Organ tolerates ischaemia poorly (e.g. brain)

·   Organ unable to autoregulate (e.g. placenta “pressure-passive”)

·   mAP below autoregulatory ranges (e.g. mAP 70-170mmHg for kidney)

Residual general anaesthetic

e.g. propofol, sevoflurane

·   Venodilatation (Ca²⁺ channel block, ↓SNS outflow)

·   Immobility -> ↓skeletal muscle pump

·   ↓minute ventilation or apnoea -> ↓diaphragm pump

Residual neuraxial anaesthetic

e.g. bupivacaine

·   Venodilation (↓thoracolumbar SNS output)

·   Immobility -> ↓skeletal muscle pump

Hypovolaemia

·   Dehydration: fasting, evaporation, diuretics

·   Blood loss

·   Third spacing: sepsis, anaphylaxis

Obstruction to LV filling

·   Big PE

·   Tension pneumothorax

·   Tamponade

Residual GA

·   ↓medullary SNS output -> ↓β₁

Residual NA

·   If affecting cardiac SNS T1-4 -> ↓ β₁

Drug

·   β₁ blocker

·   Ca²⁺ channel blocker

·   AChE reversal i.e. neostigmine

Pathology

·   Arrhythmia

·   Ischaemia/infarction of SA node or AV node

Residual GA

·   ↓medullary SNS output -> ↓β₁

·   ↓L-Ca²⁺ activity

Residual NA

·   If affecting levels T1-4

Drugs

·   β-blocker

Pathology

·   Ischaemia, infarction

·   Septic cardiomyopathy

Residual GA

·   ↓Medullary SNS output

·   ↓L-Ca²⁺ activity

·   ↑Nitric oxide release

Residual NA

·   ↓Thoracolumbar SNS output

Drugs

·   ACE inhibitor or ARB commonly implicated

·   Others: α₁ antagonist, nitrates etc

Pathology

·   Sepsis

·   Anaphylaxis

·   Post-cardiopulmonary bypass vasoplegia

·   SIRS, ARDS, pancreatitis

·   Neurogenic: i.e. spinal cord injury