Cardiovascular Physiology
in Acute Care

• Calculate mean arterial pressure (MAP) and interpret it against the perfusion threshold for organ viability
• Distinguish the four shock states — hypovolemic, cardiogenic, distributive, obstructive — from bedside findings alone
• Explain preload, afterload, and contractility and how each is manipulated at the bedside
• Relate stroke volume and heart rate to cardiac output and to oxygen delivery (DO₂)
• Integrate the human exam — capillary refill, mentation, urine output, skin temperature — with monitored numbers
• Recognize when a monitored value is artifactual or misleading and resuscitate the patient instead of the waveform
• Appreciate how upstream cardiometabolic burden shapes the acute presentations the ward treats downstream

This module was developed from acute-care workflow analysis across North American emergency departments and intensive-care units — not from textbook theory alone. The modern monitor is a triumph: continuous arterial pressure, pulse oximetry, capnography, cardiac-output estimation, all on one glowing screen. WestNet HealthOS integrates every one of those feeds. And yet the most common failure we observe at the bedside is not a missing number. It is a clinician who has stopped looking at the patient.

Module 02 is not anti-monitoring. It is anti-monitor-worship — the habit of treating the displayed value as the patient, charting the trend without touching the skin, and chasing a pressure target while the human beneath the cuff slips quietly into shock the numbers have not yet caught.

Hemodynamic monitoring exists to extend the senses, not to replace them. A blood-pressure cuff reports a pressure; it cannot tell you whether the kidney is making urine, whether the brain is adequately perfused, or whether the skin over the knee is warm or mottled. Those answers live in the patient. On a busy unit at 3 AM, a normal-looking number on a screen can lull a tired clinician into missing a body that is already compensating its way toward collapse.

Mean arterial pressure (MAP) is the average pressure driving blood through the organs across a full cardiac cycle. Because the heart spends roughly two-thirds of each cycle in diastole, MAP is weighted toward the diastolic pressure rather than the systolic peak. The standard bedside estimate is:

A MAP of roughly 65 mmHg is the conventional floor for perfusing the brain, kidneys, and gut in most adults^[4]; the kidney in particular begins to suffer below this. But 65 is a population threshold, not a personal one. A chronically hypertensive patient may need a higher MAP to perfuse organs accustomed to high pressure, while the threshold means little if the cardiac output behind it is collapsing. The number frames the question; the patient answers it.

Enter a systolic and diastolic pressure to compute the mean arterial pressure and see where it falls against the perfusion bands. The tool also flags the pulse pressure, because a normal MAP built on a dangerously narrow pulse pressure is a trap for the unwary. This is a teaching aid — it never replaces clinical assessment of the patient in front of you.

These are the perfusion signs that no monitor reports and no clinician should skip. Each is fast, repeatable, and free. Together they form a continuous bedside cardiac-output monitor that runs whether or not the screen is plugged in — and they often turn before the numbers do.

At the Bedside — Talking to the Patient While You Assess

An acutely unwell, hypotensive patient is frightened, and fear drives tachycardia and oxygen demand. Calm, honest narration is itself a hemodynamic intervention. Examine the human and address the human — the two are the same act.

Shock is a state of inadequate tissue perfusion, not a number. Hypotension may be absent early and present late. The clinically useful question is which mechanism is failing, because the four states demand opposite first moves — fluid for one is fatal for another.^[1]

At-a-Glance — Differentiating the Four Shock States

One screen, five reads. Use this as a bedside memory aid, not a substitute for the full exam — mixed and evolving shock is common, and the patient always overrides the row.^[2]

Escalate from any row when MAP <65 mmHg, refill >3 s, new confusion, rising lactate, or a narrowing pulse pressure — and confirm management against current local critical-care guidelines, which may have changed since publication.

Three levers set the stroke volume of every heartbeat. Understanding them turns hemodynamic management from guesswork into reasoning — and each lever maps to a different bedside intervention.

Cardiac output is the engine of perfusion, and it reduces to two factors any clinician can reason about at the bedside:

This is why blood pressure can mislead. The body defends pressure first — clamping down the arterioles (raising afterload) to hold MAP even as cardiac output falls. The cuff looks reassuring while flow to the gut, kidney, and skin quietly drops. The bedside perfusion signs of §06 detect this failing flow long before the pressure surrenders.

Much of the cardiovascular emergency the acute-care ward treats so skilfully has a long upstream history. The myocardial infarction, the decompensated heart failure, the hypertensive crisis — these acute events are frequently the late expression of years of cardiometabolic load: insulin resistance, central adiposity, chronic inflammation, and a modern diet and sedentary pattern the body was never designed for. Acute care is, in many cases, treating the last act of a play written decades earlier.

Framed respectfully, this is not a critique of acute medicine — it is its natural partner. A clinician who understands the upstream burden treats the patient before them with the same vigour and recognises the discharge conversation, the referral, and the metabolic follow-up as part of the same cardiovascular care. Making humans human again means seeing the whole arc, not only the monitored hour.

Myth vs. Evidence — Reading the Cardiovascular Story

Common framings, set beside what the literature actually supports. The aim is not to dismiss medication or acute care — both save lives — but to keep the cause in view alongside the rescue.^[6]

For the medication-burden side of this story — how well-intentioned prescribing accumulates and when optimisation is appropriate — see Module 06 — Polypharmacy & Iatrogenic Harm, and for the metabolic roots, Module 10 — Diabetes & Endocrine.

The following patterns recur across acute-care monitoring. Each is a composite drawn from recurring artifacts and pitfalls — not any single patient or institution. The lesson is the same: the number is only ever as good as its agreement with the patient.

Cardiac output and blood pressure rest on a handful of determinants, each tied to a bedside finding and a specific lever. Tap through the six to see what each contributes, how it fails, and what you actually do about it at the bedside.

The reflexive reading of a monitor is often the one that misleads. Tap any card to flip the screen-first interpretation into the perfusion-first one — and see why the body wins the tie.

Select the findings you observe at the bedside — blood pressure, heart rate, jugular venous pressure, skin, and lung sounds. The tool weighs the pattern toward the most likely shock state and names the first-line action. It is a teaching aid that reasons from classic patterns; the real patient always overrides it.

One of the most useful bedside splits in undifferentiated shock is warm (vasodilated, low resistance — classically distributive/septic) versus cold (vasoconstricted, low output — classically hypovolemic or cardiogenic). They demand different first moves. Mark what you actually observe; the tool weighs the picture.

Acute coronary syndrome (ACS) spans unstable angina, NSTEMI, and STEMI — a continuum of myocardial ischaemia from a threatened or occluded coronary artery. It is the single most common engine behind cardiogenic shock and lethal arrhythmia, so recognising it early is core cardiovascular physiology, not a separate skill. The hard part at the bedside is that the textbook presentation — crushing central chest pain radiating to the left arm — is only one of many. Women, older adults, and people with diabetes frequently present atypically: with breathlessness, fatigue, nausea, syncope, or simply “not feeling right.”

The bedside response is fast and protocol-driven: obtain a 12-lead ECG early (a STEMI is a time-critical reperfusion decision), gain IV access, attach continuous monitoring, and escalate. The physiology of §06–§09 still governs — a large infarct steals contractility, and the patient can slide from pain into cardiogenic shock while you watch.

The electrocardiogram is the cardiovascular system speaking in electricity. You do not need to be a cardiologist to extract enormous value at the bedside — a disciplined, repeatable read of rate, rhythm, axis, intervals, and morphology catches most of what threatens a patient acutely. The goal here is fluency with the building blocks, not formal interpretation, which remains a physician responsibility.

A rhythm disturbance matters at the bedside only insofar as it threatens perfusion. The single most important triage question is not “what is the rhythm called?” but “is the patient stable or unstable?” — because an unstable patient with a malignant rhythm needs treatment before a precise diagnosis. Instability means the rhythm is producing serious adverse signs: shock, syncope, chest pain of ischaemia, or acute heart failure.

Broadly, sort by rate and width. Too slow (symptomatic bradycardia) reduces output when stroke volume cannot compensate. Too fast shortens diastolic filling and can itself drop output — and a wide-complex tachycardia must be treated as ventricular in origin until proven otherwise. The detailed drug and energy choices belong to ACLS and your local protocol; the bedside skill is recognising danger and escalating.

Acute decompensated heart failure is best reasoned about at the bedside with a simple two-axis map that flows directly from this module’s physiology. One axis is congestion — “wet” (fluid backed up: crackles, raised JVP, oedema) versus “dry.” The other is perfusion — “warm” (adequate output) versus “cold” (low output: cool, narrow pulse pressure, sluggish refill). Four quadrants, four very different management emphases.

A valve that fails acutely can collapse a circulation in minutes, and the bedside picture often does not match the chronic textbook murmur. Acute severe regurgitation (a ruptured chord, a torn papillary muscle after MI, infective endocarditis destroying a leaflet) gives the ventricle no time to adapt — the result is flash pulmonary oedema and cardiogenic shock, sometimes with a surprisingly soft or absent murmur because flow equalises so fast. Critical aortic stenosis, by contrast, is a fixed obstruction to ejection that tolerates neither tachycardia nor vasodilation well.

You are not expected to make the valvular diagnosis at the bedside — that is an echo and a cardiology decision. The skill is recognising the pattern (sudden severe failure, a new murmur, a known valve lesion that has destabilised) and understanding why the usual reflexes can backfire. Tap the cards to flip a tempting first reflex into the safer consideration.

A very high blood pressure number is, by itself, not the emergency. The decisive question — entirely consistent with this module’s “treat the patient, not the number” thesis — is whether the pressure is damaging an organ right now. Hypertensive emergency is severe hypertension with acute end-organ injury: the brain (encephalopathy, stroke), the heart (ischaemia, pulmonary oedema, aortic dissection), the kidneys (acute injury), or the eyes (retinal changes). Hypertensive urgency is severely elevated pressure without acute end-organ damage.

The management pace differs sharply and is genuinely dangerous to get wrong. A true emergency is treated with controlled, monitored pressure reduction — but lowering pressure too fast can itself infarct a brain or kidney accustomed to high pressure (the autoregulation lesson from §04 and the MAP calculator). Urgency is generally managed with gradual oral therapy and close follow-up, not rapid drops. Specific agents and targets vary by the organ involved and belong to current local protocol.

Vasoactive drugs are the pharmacological levers for the determinants of §08 — preload, afterload, and contractility. Used well, they buy time and restore perfusion; used reflexively, they paper over an unaddressed mechanism. This section teaches principles and categories, deliberately and explicitly without doses: dosing, selection, and titration are physician- and protocol-governed and must be verified against current local critical-care guidelines and pharmacy resources.

The categories below are best understood by what determinant they move. The recurring WestNet caution applies: a vasopressor that holds a number while a tank bleeds, or an inotrope flogging an ischaemic heart, treats the monitor and harms the patient. Pick the agent that fixes the mechanism you named.

“Hypotensive, so give fluid” is the reflex this module exists to discipline. Only about half of haemodynamically unstable patients are actually fluid responsive — meaning a bolus will meaningfully raise their stroke volume. The rest are already on the flat part of the Frank–Starling curve (§08), where more fluid buys no output and instead floods the lungs, worsens oedema, and harms. The modern skill is to predict responsiveness before committing, not to bolus and hope.

Static measures (a single central venous pressure, a snapshot of the IVC) predict responsiveness poorly. Dynamic measures are better because they actively challenge the circulation and watch the response. The most elegant is the passive leg raise — tilting the legs up gives a reversible, self-donated “auto-bolus” of the patient’s own blood; if stroke volume rises, real fluid will likely help, and if it does not, you have your answer with nothing infused.

Of all the bedside forks in §07, the most consequential is recognising cardiogenic shock — because it inverts the most common reflex. For hypovolemic and distributive shock, early fluid is usually right. For cardiogenic shock, a large fluid flood into an already-overfilled, failing ventricle is exactly the wrong first move. Getting this fork right is, bluntly, life or death.

The discriminating findings are the ones you already own from §06. Cardiogenic shock classically shows a congested picture — raised JVP, pulmonary crackles, sometimes a gallop — with cold, poorly-perfused peripheries: the ventricle is full but cannot eject. Hypovolemic shock shows the opposite filling picture: flat neck veins, clear lungs, a history of loss. Distributive shock is often warm early with low resistance. Bedside echo (§27) resolves the fork quickly where available.

Return of spontaneous circulation (ROSC) is the start of a new and fragile phase, not the finish line. The post-arrest patient has a stunned myocardium, a body that has just suffered a whole-body ischaemia–reperfusion injury, and a brain whose recovery hinges on the quality of the hours that follow. The cardiovascular physiology of this module is now operating in a system that has been to the edge and back — perfusion, oxygenation, and the avoidance of a second hit are everything.

Point-of-care ultrasound (POCUS) has become the bedside extension of the perfusion exam — a way to see the physiology this module reasons about. It does not replace formal echocardiography, and it demands training and credentialing; but in skilled hands a focused cardiac and volume look can resolve the shock fork of §25 in minutes. The philosophy is unchanged: the image is another window onto the patient, integrated with the exam — not a new number to worship.

Focused goal-directed echo answers a small set of binary, life-relevant questions rather than producing a full study. Tap through the core views to see what each is asking and how it changes the bedside decision.

This section consolidates the hands-on cardiovascular examination — the original, never-artifactual monitor of §06, expanded into a disciplined sequence. None of it requires equipment beyond your senses, and every finding ties to a physiological lever you now understand. Mastery is not knowing more signs; it is performing the same few signs reliably, every time, and trusting them when the screen disagrees.

The clinical content of this module is drawn from peer-reviewed literature indexed by the U.S. National Library of Medicine (PubMed / PMC) and from current statements of major clinical-guideline bodies. Each entry below links to the source: journal articles open a PubMed search for the article title; guideline organisations link to their official site.

Sixteen questions. Pass threshold: 11/16 for CE credit (upon accreditation approval).

Course Director: WestNet Medical Clinical Education Division
Publication: WestNet Medical Publications • WestNet Catalog 731985456574 • ISBN 978-0-XXXXX-XXX-X (Pending)
Platform: WestNet Unified Health Platform / HealthOS v3.6