Patient Assessment, Clinical Data Interpretation, and Cardiopulmonary Monitoring
Assessment sequence
Start with ABCs, work of breathing, mental status, and hemodynamics. Inspect for cyanosis, accessory muscle use, tripod position, and secretion burden. Auscultate for wheeze, crackles, pleural rub, and diminished sounds. Monitor SpO2, capnography, ABG, ECG, and vital signs. Capnography shows ventilation and confirms ETT placement; waveform changes can signal obstruction, leak, or low output.
Data interpretation
Use ABG systematically: pH, PaCO2, HCO3, and compensation. Acute respiratory acidosis: HCO3 rises 1 mEq/L per 10 mm Hg PaCO2 above 40. Chronic: 3–4 mEq/L per 10 mm Hg. Metabolic acidosis: expected PaCO2 = 1.5 × HCO3 + 8 ± 2 (Winter's formula). Pulse oximetry can be falsely normal in carbon monoxide poisoning, methemoglobinemia, and poor perfusion.
Worked example
pH 7.28, PaCO2 60, HCO3 27. The low pH and high CO2 indicate respiratory acidosis. Expected acute HCO3 = 24 + (20/10 × 1) = 26; measured 27 is close, so this is acute or acute-on-chronic, not fully compensated. Oxygenate, treat the cause (bronchospasm, hypoventilation, fatigue), and reassess.
Traps
Do not interpret PaO2 without knowing FiO2. Do not assume a normal SpO2 excludes hypercapnia. Do not ignore trend data in favor of a single value.