Critical care

The A-a gradient is the difference between the calculated alveolar oxygen tension and the measured arterial oxygen tension. A gradient that is normal for age suggests hypoventilation or a low inspired oxygen tension (such as altitude), whereas a widened gradient points to V/Q mismatch, right-to-left shunt, or impaired diffusion. The expected upper limit rises with age, roughly (age / 4) + 4 mmHg on room air at sea level.

The anion gap (sodium minus chloride and bicarbonate) screens for and characterises metabolic acidosis. Because albumin is the main unmeasured anion, a low albumin masks a real gap, so this tool also reports an albumin-corrected value.

A simple 5-point score (0 to 5) for early risk stratification in acute pancreatitis, predicting in-hospital mortality within the first 24 hours. Each criterion scores 1 point. A score of 3 or more identifies substantially higher mortality and organ-failure risk.

High glucose pulls water out of cells into plasma, diluting the measured sodium so it reads lower than the patient's true sodium status. Correcting for glucose estimates the sodium that would be seen at a normal glucose. This tool reports the correction with both the original Katz factor (1.6 per 100 mg/dL above 100) and the later Hillier factor (2.4 per 100 mg/dL above 100); the headline value uses 2.4.

FENa is the percentage of filtered sodium that ends up in the urine. In oliguric acute kidney injury a value below 1% points to prerenal azotemia (kidneys avidly reabsorbing sodium in response to hypoperfusion), while a value above 2% suggests intrinsic tubular injury such as acute tubular necrosis. It was derived in selected oliguric patients and is unreliable in several common situations, most notably diuretic use.

The Glasgow Coma Scale grades consciousness across best eye opening (1 to 4), verbal response (1 to 5), and motor response (1 to 6) for a total of 3 to 15. It is the standard way to describe and track impaired consciousness; a total of 8 or less is severe and should prompt consideration of airway protection.

The Devine formula estimates ideal body weight from sex and height, the basis many drugs use for weight-based dosing instead of actual weight. Adjusted body weight blends ideal and actual weight (correction factor 0.4) for drugs that partly distribute into excess tissue.

Mean arterial pressure estimates the average pressure driving blood to the organs across the cardiac cycle. Because diastole lasts longer than systole, it is weighted toward the diastolic pressure. A MAP of around 65 mmHg is a commonly cited floor for adequate organ perfusion in resuscitation.

Winters formula predicts the PaCO2 the lungs should reach to compensate for a primary metabolic acidosis. Comparing the measured PaCO2 against the expected range tells you whether compensation is appropriate or a second respiratory disorder is present.