Question 8

List the causes of a sudden acute fall in systolic blood pressure to 50 mmHg one hour after an uneventful  coronary artery bypass operation.  Outline  your principles of management for each cause.

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College Answer

Potential causes are many, and more than one may co-exist.  Could be divided according to causes of shock: artefactual, hypovolaemic, obstructive, cardiogenic, and distributive (with principles of management in brackets). Simple manoeuvres should be considered early (eg. raise legs to autotransfuse).

Artefactual: transducer error (check transducer: zero, level, calibration), damping of waveform
(assess damping coefficient), malfunction of NIBP.

Hypovolaemic: blood loss (observe drain tubes, CXR, dressings; give fluid ± blood products), massive diuresis (observe urine output; give fluid).

Obstructive: pericardial tamponade (observe chest drainage ± clots, high filling pressures: may need to open chest), tension pneumothorax (observe expanded hemi-thorax, listen to chest: check existing chest drains, may need needle thoracostomy and replace/insert ICC), elevated intrathoracic pressure (gas trapping: disconnect from ventilator; shivering/valsalva/fighting: sedate ± paralyse; ensure ETT not blocked).

Myocardial:  decreased contractility (ischaemia due to blockage/kinking/spasm: treat with GTN, inotropes &/or short term vasoconstrictor ± fix technical problem; sudden removal of inotropic drug: restart drug) or rhythm disturbance on monitor/ECG (brady-asystole: pace ± atropine/isopenaline/adrenaline; SVT: eg. K/adenosine; AF eg. K/Mg/amiodarone, VT eg. K/Mg/lignocaine).

Distributive: anaphylaxis (rash/bronchospasm: remove hapten, adrenaline, fluids); vasodilator excess (recent boluses/infusion too high: stop responsible drug, ± titrated dose vasoconstrictor); sympathetic block (recent bolus epidural LA: titrated dose vasoconstrictor).


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Other incarnations of it are:

Approach to the haemodynamically unstable cardiac surgical patient  and causes of shock in the cardiac surgical patient are available in the shape of brief point-form summaries.
I reproduce some of this material here, to simplify revision:

Causes of Shock in the Post-Bypass Patient
Type of shock Cause Diagnostic strategy Management
Artifact of measurement Arterial blood pressure measurement is inaccurate Compare with non-invasive measurement and physical examination
  • Re-zero and recalibrate the arterial line
  • Resite arterial line or change the transducer
Cardiogenic Post-operative stunning of the myocardium TTE, ECG, cardiac output measurement by PiCCO or PA catheter
  • Fluid resuscitation
  • Commence inotrope infusion
  • Correct rhythm if in AF
  • Return to theatre, recommence cardiopulmonary bypass
Myocardial infarction TTE, ECG, cardiac enzymes
  • Consider IABP
  • Thrombolysis or anticoagulation likely contraindicated given recent cardiac surgery
  • Return to theatre
Obstructive Cardiac tamponade TTE, CVP trace
  • Fluid resuscitation
  • Emergency pericardiocentesis
  • Return to theatre
Massive pulmonary embolism TTE, CVP trace, ECG, CTPA
  • Consider emergency embolectomy
  • Thrombolysis or anticoagulation likely contraindicated given recent cardiac surgery
Tension pneumothorax

Physical examination;


  • Emergency decompression
  • Chest drain
Neurogenic Infarction of spinal cord due to ischaemia or embolic events Physical examination features, CT, MRI
  • Thrombolysis or anticoagulation likely contraindicated given recent cardiac surgery
  • commence vasopressor infusion
Hypovolemic Blood loss post operatively Examination of drains, FBC,
  • Replace blood products and red cells
  • Fluid resusiciation
  • Maintain normal acid-base balance and normothermia
  • return to theatre
Massive diuresis Observation of fluid balance charts
  • Replace lost fluid volume
  • Rewarm patient to normal temperature
Distributive Vasoplegia SVRI measurements by PiCCO
  • commence vasopressor infusion; consider vasopressin or methylene blue
Anaphylaxis Physical examination findings suggestive of angioedema
  • Adrenaline IM or as infusion
  • Withdrawal of the trigger substance
  • Corticosteroids and antihistamines