What perfusion actually requires
The Fick principle gives you three conditions that all have to hold. Break any one and the patient is in shock, even with a textbook blood pressure.
The three conditions
- Loading — adequate oxygen on to hemoglobin at the alveolus. Fails in respiratory failure, CO poisoning, severe anemia.
- Delivery — enough pump and pipes to move that blood. Fails in hypovolemic, cardiogenic, obstructive, distributive shock.
- Offloading — cells able to accept and use the oxygen. Fails in sepsis, cyanide toxicity.
What happens in the cell
- Aerobic metabolism stops; the cell switches to anaerobic glycolysis — roughly 2 ATP per glucose instead of ~36.
- Lactic acid accumulates → metabolic acidosis.
- Without ATP the sodium–potassium pump fails; sodium and water flood in, the cell swells.
- Lysosomes rupture, releasing enzymes that digest the cell. Damage becomes structural, not just functional.
Blood pressure is a product of perfusion, not a measure of it. A young trauma patient can clamp down hard enough to hold a normal systolic while their gut and kidneys are already starving. Treat the patient's skin, mentation, and pulses — not the cuff.
Three stages, one direction
Compensation is expensive. The body buys time by sacrificing skin, gut, and kidney perfusion to protect brain and heart. Your job is to recognize it during the purchase, not after the bill comes due.
Body is winning, briefly
Baroreceptors fire, catecholamines surge, RAAS and ADH retain fluid. Tachycardia, tachypnea, anxiety or restlessness, pale cool clammy skin, delayed capillary refill, narrowing pulse pressure, thirst, decreased urine output — with a systolic BP still in normal range.
Compensation exhausted
Vasoconstriction can no longer hold pressure. Frank hypotension, altered mental status, weak or absent peripheral pulses, mottling or cyanosis, marked tachypnea, oliguria or anuria. Still reversible with aggressive, correct treatment.
Structural cell death
Widespread cell and organ death; multiple organ dysfunction syndrome. Pressure may transiently respond to volume and vasopressors but the patient does not recover. Diagnosed in retrospect, never in the field.
Pulse pressure is systolic minus diastolic. In hypovolemic and cardiogenic shock, compensatory vasoconstriction raises diastolic while stroke volume drops systolic — the pressure narrows. In distributive shock, vessels dilate, diastolic falls, and the pulse pressure widens. It shifts before the systolic does.
Four categories
Map each back to the hero equation: something is wrong with the tank, the pipes, the pump, or the flow through them.
Preload failure · "The tank"
Hypovolemic
Not enough circulating volume. Hemorrhagic (trauma, GI bleed, ruptured AAA, ectopic) or non-hemorrhagic (vomiting, diarrhea, burns, DKA, third-spacing).
- Class I — up to 15% (~750 mL). Near-normal vitals, maybe slight anxiety.
- Class II — 15–30% (750–1500 mL). Tachycardia, tachypnea, narrowed pulse pressure, normal systolic.
- Class III — 30–40% (1500–2000 mL). Hypotension, marked tachycardia, confusion.
- Class IV — over 40%. Profound hypotension, lethargy, thready or absent peripheral pulses.
SVR failure · "The pipes"
Distributive
Volume is adequate but the container dilated — relative hypovolemia. Skin is often warm, dry, and flushed rather than cool and clammy.
- Septic — infection-driven vasodilation, capillary leak, and impaired cellular oxygen use. Often fever, warm early then cold late.
- Anaphylactic — IgE-mediated mast cell degranulation. Urticaria, angioedema, bronchospasm, GI upset plus hypotension.
- Neurogenic — spinal cord injury above roughly T6 knocks out sympathetic tone. Look for the triad: hypotension with bradycardia and warm dry skin.
Contractility or rate failure · "The pump"
Cardiogenic
The heart cannot generate adequate output despite adequate volume. Classically follows infarction of a large portion of the left ventricle.
- Causes: acute MI, cardiomyopathy, myocarditis, acute valve failure, and arrhythmia — both extremes of rate.
- Findings: hypotension with signs of backup — crackles, JVD, peripheral edema, frothy sputum.
- The distinguishing feature from hypovolemia is pulmonary congestion. Fluid makes this patient worse.
Mechanical blockage · "The flow"
Obstructive
Pump and volume are fine; something physically blocks filling or ejection. These are procedure-responsive — you fix them, not medicate them.
- Tension pneumothorax — absent breath sounds, hyperresonance, JVD, hypotension, tracheal deviation late. Needle decompression.
- Cardiac tamponade — Beck's triad: hypotension, JVD, muffled heart tones. Also pulsus paradoxus, narrow pulse pressure.
- Massive PE — sudden dyspnea, hypoxia, clear lungs, JVD, right heart strain.
Assessment that finds it early
Signs that beat the cuff
- Mentation — anxiety, restlessness, and combativeness are cerebral hypoperfusion until proven otherwise.
- Skin — colour, temperature, moisture; capillary refill over 2 seconds; mottling at the knees.
- Pulse quality and location — a thready radial with a strong carotid means the patient is clamping down.
- Respiratory rate — rises early to blow off CO₂ and buffer acidosis.
- Waveform capnography — ETCO₂ falls as pulmonary blood flow drops. A trending-down ETCO₂ in a normally ventilated patient is a perfusion alarm.
Numbers worth carrying
- Shock index = heart rate ÷ systolic BP. Normal roughly 0.5–0.7; above 0.9 suggests significant occult blood loss and predicts transfusion need better than either number alone.
- Pulse pressure — narrowing under about 25% of the systolic is a compensated-shock flag.
- Lactate — where available, over 4 mmol/L marks serious hypoperfusion regardless of pressure.
- MAP ≈ diastolic + ⅓ pulse pressure. Under 65 mmHg, organ perfusion is generally failing.
- Pediatric hypotension — systolic below 70 + (2 × age in years) for ages 1–10.
Absent tachycardia does not rule out shock. Beta blockers and calcium channel blockers blunt the heart rate response; pacemakers fix it; neurogenic shock produces bradycardia by mechanism; and well-conditioned athletes and some late-stage patients simply do not mount it. Judge perfusion, not rate.
Management by category
Universal first steps: airway and oxygenation to correct hypoxia, keep the patient supine, aggressively prevent hypothermia, obtain access, and move early. Beyond that, the category dictates the plan — and the wrong plan actively harms.
Hypovolemic / hemorrhagic
- Stop the bleeding first. Direct pressure, wound packing, tourniquet for extremity hemorrhage, pelvic binder for suspected unstable pelvis. No amount of fluid outruns an open artery.
- Blood products over crystalloid where carried. Large-volume crystalloid dilutes clotting factors, drops oxygen-carrying capacity, and cools the patient.
- Permissive hypotension in uncontrolled hemorrhage — titrate to a radial pulse or roughly 90 mmHg systolic, enough to perfuse without popping formed clot. Do not use this in traumatic brain injury, where a single hypotensive episode markedly worsens outcome; target a higher pressure there.
- TXA per protocol, ideally within 3 hours of injury.
- Fight the lethal triad: hypothermia, acidosis, coagulopathy — with hypocalcemia often named as a fourth point. Blankets on early.
Distributive
- Anaphylaxis — epinephrine 1 mg/mL, 0.3–0.5 mg IM into the lateral thigh is the first and most important intervention, repeatable every 5–15 minutes. Add fluids for hypotension, then an epinephrine infusion if refractory. Antihistamines and steroids are adjuncts and never delay epi. Consider glucagon if the patient takes beta blockers and does not respond.
- Sepsis — fluid resuscitation commonly 30 mL/kg of balanced crystalloid, reassessing for overload; early antibiotics if within scope; norepinephrine as first-line vasopressor targeting MAP ≥ 65.
- Neurogenic — spinal motion restriction, cautious fluids (this is a pipe problem, not a tank problem), a vasopressor for tone, and atropine for symptomatic bradycardia. Always rule out hemorrhage first: assume the trauma patient is bleeding until proven otherwise.
Cardiogenic
- Obtain a 12-lead early and transport to a PCI-capable facility. Reperfusion is the definitive treatment.
- If the lungs are clear, a small fluid challenge (about 250 mL) with reassessment is reasonable. If crackles are present, stop.
- Norepinephrine is generally preferred over dopamine, which carries a higher arrhythmia burden. Dobutamine improves contractility but can drop blood pressure through vasodilation, so it suits the patient with a marginal pressure and poor output.
- Correct the reversible: fix rate and rhythm, treat hypoxia, and be cautious with nitrates and morphine in a hypotensive patient. Avoid nitroglycerin in suspected right ventricular infarct.
Obstructive
- Tension pneumothorax — immediate needle decompression; do not wait for tracheal deviation, which is a late and unreliable sign.
- Tamponade — fluids may temporarily improve filling; definitive care is pericardial drainage. Move.
- Massive PE — oxygenation, judicious fluids, vasopressor support, and rapid transport for thrombolysis or thrombectomy.
Vasoactive quick reference
Confirm doses against your own protocols — ranges vary by system. Fix volume before reaching for a pressor: squeezing an empty tank does not create perfusion.
| Agent | Receptors | Effect | Best used in |
|---|---|---|---|
| Norepinephrine0.05–0.5 mcg/kg/min | α₁ ≫ β₁ | Strong vasoconstriction, modest inotropy, minimal rate increase | First-line for septic, neurogenic, cardiogenic, and most undifferentiated shock |
| Epinephrine0.05–0.5 mcg/kg/min | α₁, β₁, β₂ | Vasoconstriction plus inotropy, chronotropy, bronchodilation | Anaphylaxis, cardiac arrest, shock with bradycardia or bronchospasm |
| Push-dose epi10 mcg/mL; 0.5–2 mL q1–5 min | α₁, β₁, β₂ | Brief pressure bridge, roughly 5–10 minutes per dose | Peri-intubation hypotension; bridging to an infusion |
| Phenylephrine0.5–2 mcg/kg/min | Pure α₁ | Vasoconstriction only; may cause reflex bradycardia | Neurogenic shock; when tachyarrhythmia must be avoided |
| Dopamine5–20 mcg/kg/min | Dose-dependent β₁ then α₁ | Inotropy at moderate doses, vasoconstriction at higher | Symptomatic bradycardia; alternative where norepinephrine is unavailable |
| Dobutamine2–20 mcg/kg/min | β₁ > β₂ | Increases contractility; vasodilates and can lower BP | Cardiogenic shock with an acceptable pressure but poor output |
Populations that lie to you
Pediatric
- Compensate ferociously with heart rate and SVR, then crash abruptly. Hypotension is a pre-arrest sign.
- Watch for tachycardia, delayed capillary refill, weak peripheral pulses, and decreased responsiveness before pressure changes.
- Fluids at 20 mL/kg boluses, reassessing between; smaller 10 mL/kg volumes for neonates, DKA, and suspected cardiogenic causes.
- Large surface-area-to-mass ratio means they lose heat fast — hypothermia worsens everything.
Geriatric
- Reduced cardiac reserve and stiff vessels blunt compensation; medications blunt it further.
- A "normal" 120/80 may be relative hypotension for a chronically hypertensive patient.
- Anticoagulants turn modest trauma into significant hemorrhage.
- Sepsis often presents as confusion, weakness, or a fall without fever.
Pregnancy
- Plasma volume rises substantially, so a patient may lose roughly 30–35% of blood volume before showing hypotension — while the fetus is already being shunted away from.
- Resting heart rate is higher and blood pressure lower at baseline.
- After about 20 weeks, displace the uterus to the left or tilt the board to relieve caval compression.
- Resuscitate the mother to resuscitate the fetus.
Occult and mimicking presentations
- Adrenal crisis — hypotension refractory to fluids and pressors; think steroid-dependent patients.
- Toxicologic — beta blocker, calcium channel blocker, and clonidine overdoses produce bradycardic hypotension.
- Mixed shock is common. The septic patient with an MI, the trauma patient with a cord injury. Reassess after every intervention rather than committing to a first impression.
Practice questions
Twenty questions with rationales. Answer before you read the explanation — recognizing an answer is not the same as retrieving it.