Cardiopulmonary Diagnostics (Blood Gas Analysis, Pulmonary Function Testing, Sleep Studies)
Blood gas and PFT rules
Preanalytical error: air bubbles raise PaO2 and lower PaCO2; delay lowers glucose and raises lactate. PFT interpretation: obstruction = FEV1/FVC below the lower limit of normal (often <0.70); severity by FEV1 % predicted. Restriction requires reduced TLC, not just FEV1/FVC. Significant bronchodilator response = ≥12% and ≥200 mL increase in FEV1 or FVC. DLCO is reduced in emphysema, interstitial disease, anemia, and pulmonary vascular disease.
Sleep studies
Polysomnography measures EEG, EOG, EMG, airflow, effort, SpO2, and ECG. Apnea = ≥90% airflow reduction for ≥10 seconds; hypopnea = ≥30% reduction with ≥3% desaturation or arousal. AHI 5–15 mild, 15–30 moderate, >30 severe OSA. CPAP titration identifies pressure eliminating events in all sleep positions and REM.
Worked example
FEV1 55% predicted, FVC 80%, FEV1/FVC 0.55: obstruction. After bronchodilator, FEV1 rises to 63% and 250 mL: this meets ≥12% and ≥200 mL, so reversible airflow limitation. A low DLCO would suggest emphysema rather than pure asthma.
Traps
Do not diagnose restriction from spirometry alone. Do not call a 10% FEV1 rise significant without the 200-mL criterion. Do not use AHI from a home device without confirming artifacts.