Overview
The simulator distinguishes modeled arterial saturation from whether a pulse oximeter can obtain a useful pulsatile signal. Low saturation can coexist with a strong pulse; continuous mechanical support can maintain oxygen delivery with little or no native pleth pulsatility.
Without detailed ventilation, a bounded oxygenation function uses congestion, output, pressure, rhythm and oxygen support. With the ventilator active, gas estimates use delivered alveolar ventilation and a simplified oxygen-content mixing model.
Ventilator gas equations
Alveolar PO₂ = FiO₂ × 713 − PaCO₂/0.8, using a sea-level dry-gas pressure assumption and fixed respiratory quotient. Oxygen content uses 1.34 × Hb × saturation + 0.0031 × PO₂ with fixed Hb 15 g/dL in this gas module. A Hill saturation curve uses P50 26.8 mmHg and exponent 2.7.
A bounded admixture fraction represents congestion, recruitment, overdistension and selected pulmonary disease. Venous oxygen content is a simplified function of forward flow. Numerical inversion finds the PO₂ that yields the mixed arterial content. P/F is PaO₂ divided by FiO₂, not a measured blood gas.
Gas loading feeds back
Hypercapnia and low alveolar oxygen add a bounded pulmonary vascular load. Reduced alveolar ventilation can therefore raise PVR, increase RV burden and reduce flow. This feedback uses alveolar oxygen rather than treating every arterial desaturation from a cardiac right-to-left shunt as alveolar hypoxia.
The gas module does not resolve regional ventilation/perfusion matching, acid–base chemistry, pH/temperature shifts of the dissociation curve, changing hemoglobin, oxygen consumption kinetics or an ABG analyzer. The Fick teaching panel uses its own explicitly stated oxygen assumptions.
Pleth waveform and missing readings
Pleth pulses use the same rhythm events, pulse strength, respiration, perfusion and support state as the circulation. NSVT weakens the pulses during a run; pauses remove them; low pulse pressure is not automatically enlarged to a normal trace.
ECMO/Impella support distinguishes continuous from pulsatile delivery. A flat or unreliable pleth does not mean circuit flow is zero. Conversely, a plausible saturation is not proof of adequate systemic oxygen delivery. Shunt/circuit situations with unresolved gas mixing can withhold P/F or conventional measurement readouts.
Oxygen versus recruitment
Oxygen concentration changes the gas calculation; PEEP also changes recruitment and hemodynamic loading. Better oxygenation or less apparent opacity does not imply that an obstructive lesion, structural shunt or all lung fluid has disappeared.
References & further reading
