A classic four-quadrant ABG teaching chart — anion gap, A-a gradient, and the oxyhemoglobin dissociation curve, checked line by line.
pH on the vertical axis, PCO2 on the horizontal — the standard first-pass ABG orientation, and it's drawn correctly.
The chart's core logic holds: low pH + high PCO2 = respiratory acidosis (hypoventilation); low pH + low HCO3 = metabolic acidosis; high pH + low PCO2 = respiratory alkalosis (hyperventilation); high pH + high HCO3 = metabolic alkalosis. The listed causes for each quadrant — asthma/PE/hypoxia/aspirin overdose/anxiety for respiratory alkalosis; vomiting/potassium depletion/volume depletion for metabolic alkalosis — are standard teaching examples and accurate.
| Chart's formula | Anion gap = (Na⁺ + K⁺) − (HCO3⁻ + Cl⁻), normally about 8 mmol/L |
| Standard teaching | Correct formula. Normal range is more commonly cited as 8–12 mEq/L with modern ion-selective electrode analyzers (older methods gave higher baseline values, up to ~16) — the chart's "about 8" sits at the lower end of the accepted range, not wrong, just worth knowing your own lab's reference range can run a few points higher. |
| Causes listed | Raised gap: DKA, lactic acidosis, renal failure — all correct, standard "MUDPILES"-type causes. Normal gap: GI bicarb loss (diarrhea, fistula) — correct; renal tubular acidosis is the other classic normal-gap cause, not listed here but not wrong to omit from a compact chart. |
Both the derivation and the bedside rule-of-thumb check out.
The alveolar gas equation as drawn — PAO2 = PIO2 − PACO2/R — and the worked example (21% oxygen, sea level, PCO2 40 mmHg → expected alveolar O2 ≈100 mmHg/13.3 kPa) are both correct, standard respiratory physiology. The simplified bedside approximation, PAO2 ≈ 150 − (5/4)×PaCO2, is a widely taught shortcut and matches the full equation closely at sea level. The "normal A-a gradient < Age/4 + 4 mmHg" rule is a standard, commonly cited clinical estimate for the upper limit of normal by age — accurately stated.
This is the one correction worth making before you use this chart to teach.
The dashed lines on the oxyhemoglobin dissociation curve appear to mark the point where saturation is 90%, at a PO2 around 50 mmHg — and label that intersection "P50."
P50 is, by definition, the PO2 at which hemoglobin is 50% saturated with oxygen — not 90%. The normal adult P50 is 26.6–26.8 mmHg at 37°C and pH 7.4 (Kerry Brandis's widely cited reference value is 26.6 mmHg). It's the standard single-number way to describe where the whole curve sits: a lower P50 means a left-shifted, higher-affinity curve (fetal hemoglobin, P50 ≈18–19 mmHg); a higher P50 means a right-shifted, lower-affinity curve that favors oxygen unloading at the tissues.
The chart's underlying clinical point — that saturation holds up reasonably well until PO2 drops into the 50–60 mmHg range, then falls off a cliff — is itself correct and clinically important (it's the same physiology behind the "SpO2 88% means you've already lost half your oxygen tension reserve" warning box elsewhere on the same chart). The dashed line is marking a real, useful point on the curve. It's just not P50 — if anything, it's closer to marking where saturation crosses roughly 90%, which happens around a PO2 of 55–60 mmHg on a normal curve. If you're adapting this chart, either relabel that point (e.g. "critical desaturation threshold") or redraw the dashed lines to actually cross at 50% saturation / ~27 mmHg if you want to keep the P50 label.
This is a solid, standard teaching chart — the four-quadrant acid-base framework, anion gap formula, and A-a gradient math are all correctly presented and safe to use as-is. The one fix worth making: whatever point the dashed lines mark on the oxyhemoglobin curve, it isn't P50 unless it's drawn at 50% saturation (≈27 mmHg). Everything else on the chart, including the genuinely important "small SpO2 drop = large PaO2 drop" warning box, holds up.