Patient Assessment and Monitoring in Adult Critical Care
What to know cold
Assessment in the ICU is a synthesis of gas exchange, respiratory mechanics, and hemodynamics. Interpret ABGs in a fixed order: pH, then PaCO2 and HCO3, then check whether compensation is appropriate, then calculate the anion gap. Oxygenation is quantified with the PaO2/FiO2 ratio and the alveolar gas equation; ventilation with PaCO2. Ventilator graphics are commonly tested: peak vs plateau pressure, driving pressure (Pplat minus total PEEP), flow-time evidence of auto-PEEP, and pressure-time evidence of patient effort. Monitoring also includes arterial line waveform quality (overdamping underestimates systolic, underdamping overestimates), cuff pressure (20-30 cmH2O), sedation with RASS, delirium with CAM-ICU, and respiratory muscle strength with NIF/MIP, vital capacity, and the rapid shallow breathing index.
Traps
- A normal pH with abnormal PaCO2 and HCO3 signals a mixed disorder, not compensation.
- SpO2 lags and misleads with poor perfusion, motion, or dyshemoglobinemia.
- Auto-PEEP is measured with an end-expiratory hold in a passive patient; total PEEP differs from set PEEP.
- Rising peak with stable plateau points to resistance (secretions, bronchospasm, kinked tube); rising plateau points to compliance loss (edema, pneumothorax, abdominal distension).
- Do not label hypoxemia as hypoventilation when the A-a gradient is wide.
Worked example
A-a gradient: PAO2 = FiO2 x (Patm - 47) - (PaCO2 / 0.8). On FiO2 0.60 with PaCO2 40 and PaO2 70: PAO2 = 0.60 x 713 - 50 = 378 mmHg, so A-a = 308 mmHg. That wide gradient means shunt or V/Q mismatch, not simple hypoventilation. The P/F ratio is 117, indicating moderate-to-severe impairment.