Saturation
How full are the available oxygen binding sites? The remaining blood can stay well saturated when the airway and lungs still work.
ACM EXTENDED / SYSTEMS
Oxygen delivery
Why a casualty can have normal SpO₂ and still receive too little oxygen at the tissues. Hemorrhage affects filling and flow; dilution affects oxygen carrying content; altitude and lung disease affect saturation.
Read oxygenation and perfusion
02
SpO₂ during hypovolemia and poor perfusion
A normal saturation can coexist with severe shock. SpO₂ describes the oxygen saturation of the hemoglobin being sampled. It does not tell you how much blood remains, how much hemoglobin is circulating, or how much oxygen reaches the tissues each minute.
Oxygen content
How much oxygen can that blood carry? Less hemoglobin means less oxygen content at the same saturation. Losing red cells and plasma together can initially preserve concentration while total circulating volume falls.
Oxygen delivery
How much reaches tissue per minute? Hemorrhage reduces filling and flow. A high saturation does not replace that lost flow.
Sources and revision
Physiology background: OpenStax, oxygen transport and hemoglobin saturation. The model examples below show the separate effect of blood volume and flow.
Why the probe becomes less reliable
Check the signal quality
Pulse oximetry also needs a usable pulsatile signal at the probe. Poor circulation and cold skin can reduce reading reliability. During hypovolemia, peripheral vasoconstriction and reduced perfusion can therefore make the signal weak or unavailable. A displayed number may be misleading; there is no dependable rule that every poor perfusion error points downward.
| Observation | What it can mean | Read alongside it |
|---|---|---|
Normal SpO₂ with weak pulses | Saturation may be adequate while blood flow and tissue delivery are inadequate | Bleeding, pulse quality, pressure, mental state and circulation trend |
Erratic value or poor pulse signal | The probe may not have a reliable arterial signal | Probe placement, local perfusion and whether its pulse rate matches an assessed pulse |
Low SpO₂ | May reflect true impaired oxygenation, measurement error, or both | Airway, respiratory effort, chest findings, waveform quality and repeat measurements |
No useful reading | Does not establish an arterial saturation of zero | Patient assessment and other monitoring; do not wait for a reassuring number |
Other real world limitations include skin pigmentation, skin temperature and nail polish. These are background device limitations; the game specific effects are listed separately below. FDA pulse oximeter limitations.
SpO₂ also does not measure ventilation or CO₂ clearance. Compare it with breathing, delivered ventilator volumes and the capnography waveform explorer.
03
What the pulse oximeter actually simulates
In gameUnderlying value and displayed reading
The mod has an underlying oxygen value and a separate pulse oximeter reading. The displayed reading adds measurement variation and blood volume/pressure effects. The DO₂ calculation reads the underlying oxygen value, so copying a distorted monitor number into the calculator will not reproduce the patient’s hidden DO₂ exactly.
Systolic pressureAt least 80 mmHgBelow this, the reviewed device paths lose their reading
Peripheral pulseRequiredElectrical activity alone does not satisfy this check
Probe limbNo tourniquetA tourniquet on that limb blocks the reading
| Implemented effect | Current behavior | Practical implication |
|---|---|---|
Blood volume distortion | Subtracts a term rising from 0 to 25 points as the native blood volume readout falls from 5.5 to 3 L. That term is scaled by underlying oxygen ÷ target oxygen | The displayed SpO₂ can fall even when underlying oxygen is unchanged. This is a game measurement rule |
Low pressure term | Adds 0 to 15 points as systolic pressure falls from 90 to 80 mmHg, before the no reading cutoff below 80 | The arithmetic can partly offset the blood loss subtraction. A rising display does not necessarily show improving oxygenation |
Measurement variation | Random spread increases as underlying oxygen falls; the nominal spread is 1 point at 95% and 5 points at 75% | Read trends and reassess the patient. The displayed sample need not match the underlying value |
Very slow rate | The fingertip device suppresses its reading below 30 BPM. The shared SpO₂ function returns no reading below 20 BPM | The fingertip and AED paths have slightly different low rate gates |
Update timing | Pulse oximeter readings refresh approximately every 3–4 seconds in the reviewed paths | Allow for update delay after an intervention; it is not a breath by breath saturation measurement |
Same oxygen, different displayed numberGame calculation example
Hold native blood volume at 4 L, underlying oxygen at 97%, and target oxygen at 97%. Assume a detected peripheral pulse, no tourniquet and a sufficient heart rate. Removing random variation isolates the current measurement formula:
SBP 100 mmHg
82%97 − 15 + 0
SBP 85 mmHg
89.5%97 − 15 + 7.5, rounds to 90%
SBP 79 mmHg
No readingSignal gate fails
Underlying oxygen
97%Held constant in all three cases
This illustrates the game’s display arithmetic. It is not a correction formula for a real pulse oximeter or a prediction that human saturation rises when pressure falls.
The reviewed pulse oximeter code does not separately model optical errors from skin pigmentation, nail polish or probe motion. Cold can affect the casualty through other systems, but the shared reading formula has no direct skin temperature input.
Sources and revision
Checked against ACM Extended 21f8694: reading calculation · fingertip device · AED monitoring · underlying oxygen used by DO₂.
Understand oxygen delivery
01
Content × flow
DO₂Oxygen delivery=Cardiac outputBlood flow×Arterial oxygen contentOxygen carried in the blood
Physiology: cardiac output is HR × stroke volume. Arterial oxygen content depends mainly on hemoglobin concentration and the fraction saturated with oxygen, plus a small dissolved component. SpO₂ estimates saturation; it does not measure circulating hemoglobin mass or blood flow.
In gameWhat the DO₂ percentage represents
In ACM Extended: delivery is a fraction of a healthy reference. It combines a blood content estimate, SpO₂ and a flow estimate based on volume and heart rate. A value of 100% means the default model reference, not a clinical oxygen delivery measurement in mL/min.
For the class definitions and the circulation terms, see hemorrhage classes and stroke volume and SVR.
04
Hemorrhage and resuscitation examples
Every row below lists its assumptions. These are snapshots calculated from the current model, not fixed outcomes of a hemorrhage class. Infused amounts represent fluid already admitted to the corresponding circulating compartment.
| Scenario | Blood | Saline | HR | SpO₂ | PPV / PEEP | SV estimate | DO₂ |
|---|---|---|---|---|---|---|---|
Baseline | 6.0 L | 0.0 L | 75 /min | 97% | Off | 100.0% | 100.0% |
20% loss | 4.8 L | 0.0 L | 110 /min | 97% | Off | 70.0% | 83.0% |
35% loss | 3.9 L | 0.0 L | 130 /min | 97% | Off | 50.2% | 64.9% |
45% loss | 3.3 L | 0.0 L | 140 /min | 97% | Off | 38.4% | 51.7% |
35% loss + PPV | 3.9 L | 0.0 L | 130 /min | 97% | 5 cmH₂O | 44.4% | 57.5% |
35% loss + PPV, higher PEEP | 3.9 L | 0.0 L | 130 /min | 97% | 15 cmH₂O | 41.5% | 53.7% |
35% loss + 1 L saline | 3.9 L | 1.0 L | 130 /min | 97% | Off | 72.3% | 74.4% |
35% loss + 1 L blood | 4.9 L | 0.0 L | 130 /min | 97% | Off | 72.3% | 93.5% |
35% loss + lower saturation | 3.9 L | 0.0 L | 130 /min | 85% | Off | 50.2% | 56.9% |
Compare the replacement examples
At 35% blood loss, the example rate of 130 supports DO₂ to about 65% while estimated SV is about 50%. A higher rate does not restore filling. Adding one liter of saline improves the example to about 74%; adding one liter of blood improves it to about 94%. The saline example includes its immediate dilution effect. All examples hold other inputs constant.
The lower saturation row represents the same hemorrhaged casualty with an additional oxygenation problem. It is not tied to a particular altitude. See Flight Physiology for the altitude model.
05
Explore the game calculation
Calculation and limits
Circulating volume is blood + saline + plasma. The preload ratio is circulating volume ÷ 6 L, capped at 1.1. Estimated SV is that ratio raised to 1.6, with the positive pressure penalty applied when relevant.
The rate multiplier starts at 1 + 0.4 × (HR ÷ 75 − 1), limited to 0.3 through 1.35. Above 150 BPM, an additional filling penalty increases toward 220 BPM and scales with the volume deficit. These are game approximations, not a universal human Frank–Starling curve.
Relative hemoglobin concentration is blood ÷ (blood + effective saline dilution + plasma), capped at 1. Delivery is this fraction × SV estimate × rate multiplier × (SpO₂ ÷ 97). The calculator uses freshly added saline, so effective dilution equals the saline input.
In the running game, saline’s modeled dilution contribution has a 20-minute half-life and follows changes in the saline compartment. That decay does not itself remove the saline volume. It is separate from fluid volume conversion. Plasma does not use this saline dilution decay.
The native blood pressure calculation has a separate cardiac output approximation. This calculator reproduces the DO₂ estimate and cannot predict exact cuff readings, rhythm transitions, drug responses or arrest.
What changes delivery
06
Positive pressure and preload
Raising pressure inside the chest opposes venous return. A casualty with reduced circulating volume has less reserve to tolerate that change. PEEP keeps pressure elevated between breaths and can increase the filling cost.
For the 35%-loss example, PPV at PEEP 5 reduces the remaining SV estimate by about 11.5%; PEEP 15 reduces it by about 17.2%. The reduction multiplies what is left of SV, rather than subtracting those points from a normal SV of 100%.
Correct hemorrhage and inadequate oxygenation together. The model includes the consequences of delayed oxygenation as well as the filling cost of ventilation. Reassess pulse, pressure, chest findings and oxygenation after starting support or changing PEEP.
07
Cardiology factors
| Factor | Physiological relationship | How to interpret it in the game |
|---|---|---|
Sinus tachycardia | Can partially compensate for low SV. Extreme rates shorten diastole and filling time. | Treat the cause of a compensatory rate. Slowing a hemorrhaging casualty’s sinus rate does not replenish volume. |
AFib, AFib-RVR and SVT | Irregular or rapid rhythms can reduce effective filling. Loss of coordinated atrial contraction can matter in susceptible hearts. | The rhythm system applies its own rate and pressure effects. The DO₂ approximation uses volume and rate; it does not contain a separate atrial kick variable. See the rhythm cards for actual treatments. |
Ventricular rhythms and PEA | Electrical activity alone cannot establish useful mechanical output. | Assess a pulse and arrest state. True PEA has no mechanical output in the native cardiac output path. Do not use a displayed electrical rate as proof of circulation. |
Coronary perfusion | Left ventricular coronary supply occurs largely during diastole. Low diastolic pressure and short diastole reduce supply while tachycardia increases demand. | Coronary flow and oxygen demand are physiological background here. The reviewed DO₂ and native output calculations have no coronary flow or coronary pressure input; rapid rate and shock follow their separately implemented rules. |
Acidosis and calcium | Poor tissue delivery promotes metabolic debt; acidosis and hypocalcemia can impair myocardial function. | Calcium deficit reduces vascular tone and pressor responsiveness. Acidosis blunts pressure support. These are indirect approximations; the DO₂ formula has no separate contractility factor. Blood pressure can remain abnormal after volume begins to recover. |
Vasoconstriction and pressors | Increased resistance can maintain MAP despite reduced flow. | A higher cuff pressure alone does not establish restored DO₂. Reassess volume, oxygenation and perfusion together. |
Sources and revision
Implementation checked on 13 September 2026: DO₂ and preload, native output, resistance composition, blood pressure, and circulation and calcium effects.
Circulatory terms and implementation distinguishes modeled effects from background physiology. Cardiac rhythms lists the available presentations, transitions and game specific treatment responses. OpenStax cardiac physiology provides the physiological background.
08
Tissue delivery and brain injury
A stable mild or moderate TBI can now lose its acute burden while structural injury remains in the history. Recovery still requires adequate oxygen delivery. Hemorrhage can keep the acute injury active even with a normal saturation and an acceptable cuff pressure.
Delivery deficit
Below 70% of the healthy game reference, TBI records a delivery deficit. Once present, the deficit clears at 78% or higher. Lower delivery increases secondary injury, reaching the severe weighting at 35%.
Pressure and ICP
The model’s effective MAP must meet the structural injury’s recovery requirement and ICP must be at or below 25 mmHg. Severe structural TBI also needs CPP at least 55 mmHg; critical injury needs 60 mmHg. Mild and moderate injury have no added high CPP floor.
Breathing and recovery
The default recovery check also requires underlying SpO₂ at least 90% and the TBI ventilation ratio at least 0.9. A cleared delivery deficit, adequate perfusion and adequate breathing must occur together.
Reperfusion and renewed deficit
After delivery recovers, a 45-second TBI reperfusion window suppresses renewed injury accrual from low delivery. A new deficit still prevents the recovery check from passing until it clears. The separate general delivery model uses a critical reference below 50% and a clearing reference of 58%.
In gameEffective brain MAP
Head elevation subtracts 6 mmHg from the MAP used by the TBI model, so its effective MAP can be lower than the cuff value. These are game model defaults, not monitor targets or a guaranteed recovery timer. See the severity table and recovery guide for the MAP requirement, retained injury floors and the distinction between cerebral autoregulation and systemic autonomic tone.
Sources and revision
TBI update checked at ACM Extended dev revision 98d18bb: delivery deficit and recovery gates · severity and recovery defaults · delivery defaults.
Sources and revision
Game behavior checked against ACM Extended on 13 September 2026. Physiological explanations and illustrative examples are labeled separately from game calculations.