What is the role of pulmonary function studies before heart surgery?

Pulmonary Function Studies Before Heart Surgery

Pulmonary function testing before cardiac surgery provides critical prognostic information that reclassifies risk in approximately one-third of patients and should be performed routinely, particularly measuring FEV1, FVC, and DLCO to identify those at substantially elevated mortality risk. 1

Risk Stratification Framework

Who Requires PFT Before Cardiac Surgery

All patients undergoing cardiac surgery should undergo preoperative pulmonary function assessment, as clinical history alone misclassifies COPD status in 31% of patients 1. The American College of Physicians identifies the following as significant risk factors requiring formal evaluation and perioperative interventions 2:

• Age >60 years (strongest patient-related risk factor) 2
• COPD or chronic lung disease 2, 1
• ASA class II or greater 2
• Functionally dependent status 2
• Congestive heart failure 2
• Low serum albumin (<35 g/L) - one of the most powerful predictors 2

Key Measurements and Thresholds

Primary tests to obtain:

• FEV1 and FVC ratio: Airway obstruction defined as FEV1/FVC <0.7 1
• FEV1 percent predicted: Moderate-severe obstruction when FEV1 <80% predicted 1
• DLCO (diffusing capacity): Critical threshold at <50% predicted 1

Critical risk stratification findings:

• Patients with moderate or severe airway obstruction (FEV1/FVC <0.7 AND FEV1 <80% predicted) have 3.2-fold higher operative mortality (6.7% vs 2%) 1
• Patients with DLCO <50% predicted have 4.9-fold higher mortality risk 1
• Combined abnormality (moderate-severe obstruction PLUS DLCO <50%) carries a 10-fold mortality risk 1

Clinical Impact on Outcomes

Postoperative Morbidity Patterns

PFT results predict specific postoperative complications 3:

• Combined obstructive-restrictive disease: Significantly longer mechanical ventilation duration and hospital stay compared to all other groups 3
• Restrictive disease alone: Longer postoperative ventilation and hospitalization than normal patients 3
• Percent predicted FEV1: Negative correlation with both mechanical ventilation duration (R² = 0.052) and postoperative hospitalization (R² = 0.042) 3

Temporal Pattern of Dysfunction

Pulmonary function deteriorates predictably after cardiac surgery 4, 5:

• First 2 postoperative days: Severe reductions in FVC, FEV1, and gas exchange 4
• Two weeks post-surgery: Significant reductions in volumes, diffusion, and PaO2 persist 5
• 3-4 months post-surgery: Partial recovery occurs, but volumes and diffusion remain significantly reduced compared to preoperative baseline 5

The mechanism appears primarily related to chest wall alteration rather than parenchymal injury, as diffusion corrected for volume shows no change 5.

Practical Algorithm for Cardiac Surgery

Step 1: Obtain baseline PFTs on all patients

• Measure FEV1, FVC, and DLCO 1
• Do NOT rely on clinical history alone for COPD diagnosis 1

Step 2: Risk stratify based on results

• Low risk (proceed with standard perioperative care):

  • FEV1/FVC ≥0.7 OR
  • FEV1 ≥80% predicted with FEV1/FVC <0.7 (mild obstruction)
  • DLCO ≥50% predicted
  • Expected mortality ~2% 1

• Moderate risk (implement enhanced pulmonary hygiene):

  • FEV1/FVC <0.7 AND FEV1 <80% predicted (moderate-severe obstruction) OR
  • DLCO <50% predicted (but not both)
  • Expected mortality 3-7% 1

• High risk (consider alternative strategies, intensive perioperative management):

  • FEV1/FVC <0.7 AND FEV1 <80% predicted PLUS DLCO <50% predicted
  • Expected mortality >10% 1

Step 3: Implement risk-appropriate interventions

For moderate-high risk patients, the American College of Physicians recommends 2:

• Deep breathing exercises or incentive spirometry postoperatively 2
• Early mobilization 2
• Selective (not routine) nasogastric tube use 2
• Optimize pain control to facilitate effective breathing 6

Common Pitfalls to Avoid

Do not skip PFTs based on clinical assessment alone - History of COPD has poor predictive accuracy, with 37% of patients with reported COPD having no obstruction on testing, and 27% without COPD history having obstruction 1.

Do not use preoperative spirometry solely for risk prediction without acting on results - The American College of Physicians states routine spirometry should not be used for prediction alone, but when obtained, results must guide perioperative management 2.

Do not assume normal preoperative values - Even asymptomatic cardiac surgery patients frequently have mild-moderate obstruction (58% in one series), reduced diffusing capacity (47%), and mild hypoxemia (53%), often smoking-related 5.

Recognize that restrictive patterns matter - Restrictive disease independently predicts prolonged ventilation and hospitalization, not just obstructive disease 3.