A 63 year old male smoker undergoes routine coronary artery bypass surgery. He remains ventilated on 10 cmH2O PEEP and FiO2  60% the following morning with a PaO2 of 63 mmHg.  Outline your diagnostic and therapeutic approach

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College Answer

3 - 4 key features
•    Hypoxaemia post-cardiac surgery is common, most commonly due to basal atelectasis, but need to consider other possibilities such as pneumothorax, haemothorax, acute lung injury, airflow obstruction, pneumonia. Intra-cardiac shunt and PE unlikely unless specific history.
•    Diagnosis by history (?smoker, chronic lung disease), examination (asymmetry of movement, air entry), search for posterior basal bronchial breath sounds (common) and chest radiograph (basal atelectasis may not be obvious on CXR – but can be on clinical exam), CT scan shows much more atelectasis but rarely justified
•    Drain pneumothorax/haemothorax, bronchodilators for airflow obstruction, may need antibiotics for infection, consider ‘marginal’ extubation – often well tolerated in CABG patients but this patient probably too hypoxaemic. Basal atelectasis need time and try recruitment with specific manoeuvres and or higher level PEEP if tolerated (likely to depress BP so need careful supervision)

Discussion

Causes of hypoxia in the post-bypass patient are explored in a brief summary hidden among the "Required Reading" notes for the topic of cardiothoracic ICU. They are separated into four major groups:

Mechanical problems

  • Pneumothorax
  • Haemothorax
  • Endotracheal tube migration
  • Mucus plugging the bronchus
  • Blood clot plugging the bronchus
  • Phrenic nerve palsy

Pulmonary oedema

  • Post-bypass ARDS (as a circuit-related SIRS complication)
  • Poor LV function
  • Anaphylaxis-associated increase in pulmonary vascular permeability

Intrapulmonary shunt

  • Atelectasis (due to surgical handling, or incomplete reinflation of the left lung)
    • There is a 70% chance of atelectasis, particularly of the left lower lobe.
    • Chest wall compliance will be poor
    • Pain will be an issue limiting tidal volumes
    • Again, phrenic nerve palsy
  • Inhibition of hypoxic pulmonary vasoconstriction( by GTN or nitroprusside)

Intracardiac shunt

  • Unveiling of a previously silent septal defect by an increase in right sided pressures

A systematic approach is called for:

  • Increase FiO2 to ensure satisfactory oxygenation
  • examine patient and ventilator, looking for evidence of decreased lung compliance or increased resistance to airflow
  • auscultate the chest, looking for evidence of atelectasis, consolidation, pneumothorax or unequal air entry suggestive of ETT malposition
    • address these by increasing PEEP, changing I:E ratio, paralysing the patient or administering bronchodilators
  • ECG looking for evidence of PE or right ventricular infarction
  • CXR, looking for
    • pneumothorax/hemothorax
    • atelectasis
    • ETT dislodgement into the right main bronchus
    • acute lung injury/ARDS
  • Search of history for evidence of chronic lung disease eg. COPD or pulmonary fibrosis

References

References

Ng, Calvin SH, et al. "Pulmonary dysfunction after cardiac surgery." CHEST Journal121.4 (2002): 1269-1277.

Tenling, Arne, et al. "Atelectasis and gas exchange after cardiac surgery."Anesthesiology 89.2 (1998): 371-378.