Patient Data: Gathering, Interpreting, and Avoiding Interpretation Traps
Building the database
Data collection starts with chart review - history, medications, allergies, code status, prior pulmonary function - then interview and physical exam: inspection (accessory muscle use, cyanosis, clubbing), palpation (tactile fremitus, tracheal position), percussion, and auscultation (crackles, wheeze, absent sounds). Add vital signs and diagnostics: ABG, SpO2, capnography, CBC, chest radiograph, ECG, spirometry, and hemodynamics (CVP, PA pressure, cardiac output, SvO2).
Interpreting ABGs in a fixed order
Check pH, then PaCO2, then HCO3 to name the primary disorder, then assess compensation. Acute respiratory acidosis raises HCO3 about 1 mEq/L per 10 mm Hg rise in CO2; chronic raises 3–4. For metabolic acidosis, Winter's formula gives expected PaCO2 = 1.5 × HCO3 + 8 ± 2. Finally judge oxygenation with PaO2, P/F ratio, and A-a gradient (normal ≈ age/4 + 4).
Traps
Pulse oximetry measures saturation, not ventilation or oxygen content; carboxyhemoglobin reads falsely high and methemoglobin plateaus near 85%. Capnography normally reads 2–5 mm Hg below PaCO2 and diverges further with V/Q mismatch. Never label a value abnormal without clinical context - an SpO2 of 92% on room air may be baseline for a COPD patient.
Worked example
CaO2 = (1.34 × Hb × SaO2) + (0.003 × PaO2). With Hb 10 g/dL, SaO2 100%, PaO2 100 mm Hg: 13.4 + 0.30 = 13.7 vol%. Raise Hb to 15 and the same saturation yields 20.4 vol%. The deficit is anemia, not hypoxemia.