Cardiogenic Shock: Pathophysiology of Pump Failure
Cardiogenic shock occurs when the heart is unable to generate sufficient cardiac output to meet tissue oxygen demands despite adequate circulating volume. The primary failure lies in myocardial contractility, rhythm, or mechanical function. As cardiac output falls, systemic perfusion declines and cellular hypoxia develops rapidly. Unlike hypovolaemic shock, increasing volume alone cannot correct the underlying problem. Understanding cardiogenic shock as pump failure explains its severity and high mortality.
What You Need to Know
Cardiogenic shock occurs when the heart is unable to generate sufficient cardiac output despite adequate circulating volume. The primary problem is pump failure, most commonly due to impaired ventricular contractility, although severe arrhythmias or mechanical dysfunction may also contribute. As stroke volume falls, systemic blood flow becomes inadequate to meet tissue oxygen demands, leading to widespread hypoperfusion. Unlike hypovolaemic shock, preload may be normal or elevated, but the failing ventricle cannot effectively eject blood.
As cardiac output declines, arterial pressure falls and coronary perfusion is reduced, particularly during diastole. This further compromises myocardial oxygen delivery and worsens contractile function, reinforcing a cycle of progressive pump failure. At the same time, elevated ventricular filling pressures cause blood to back up into the pulmonary or systemic circulation, producing congestion. The coexistence of low forward flow and venous congestion is a defining feature of cardiogenic shock.
Cardiogenic shock is most commonly associated with conditions that directly impair myocardial function, including:
acute myocardial infarction with loss of contractile myocardium
severe cardiomyopathy or acute decompensated heart failure
mechanical complications such as papillary muscle rupture or ventricular septal defect
malignant arrhythmias that prevent effective ventricular filling or ejection
Regardless of the cause, the downstream physiology is similar.
Neurohormonal compensation initially attempts to maintain perfusion through tachycardia and systemic vasoconstriction. Reduced perfusion activates the sympathetic nervous system, releasing adrenaline and noradrenaline to increase heart rate and cause vasoconstriction. At the same time, the renin–angiotensin–aldosterone system (RAAS) and antidiuretic hormone (ADH) are activated, promoting vasoconstriction and fluid retention to support blood pressure.
While these responses may transiently support blood pressure, they significantly increase afterload and myocardial oxygen demand. Increased systemic vascular resistance makes ventricular ejection more difficult, while tachycardia shortens diastole and further reduces coronary filling time. Rather than restoring stability, these mechanisms accelerate deterioration by increasing cardiac workload and worsening ischaemia.
Without interruption of this cycle, cardiogenic shock progresses rapidly to severe hypoperfusion, metabolic acidosis, and multiorgan dysfunction. Early recognition is critical because interventions must support both myocardial performance and systemic perfusion, rather than focusing on volume replacement alone.
Beyond the Basics
Primary Myocardial Dysfunction
The defining abnormality in cardiogenic shock is failure of myocardial contraction. Damage to the myocardium, most commonly from acute ischaemia or infarction, reduces the ability of ventricular muscle fibres to generate force. Even when venous return is adequate, the ventricle cannot eject an appropriate volume of blood, so stroke volume falls. In early stages, ventricular dilation may occur as the heart attempts to use the Frank–Starling mechanism to preserve output, but this compensation is short-lived and energetically costly. As myocardial injury progresses, dilation increases wall stress and further impairs contractile efficiency.
Unlike hypovolaemic shock, the problem is not lack of preload but inability to convert preload into forward flow. This distinction is critical, as excessive fluid loading in cardiogenic shock may worsen congestion without improving cardiac output.
Afterload Mismatch and Worsening Pump Failure
As cardiac output falls, the body responds with systemic vasoconstriction to maintain arterial pressure. This increases afterload, meaning the ventricle must eject blood against higher resistance. In a healthy heart, this is tolerated, but in cardiogenic shock the failing ventricle cannot overcome the increased vascular resistance. Stroke volume declines further, despite rising myocardial workload.
This creates an afterload mismatch, where vascular resistance exceeds the mechanical capacity of the heart. The myocardium consumes more oxygen to generate less output, accelerating fatigue and worsening pump failure. Attempts to support blood pressure through vasoconstriction therefore compound the underlying problem rather than correcting it.
Coronary Hypoperfusion and the Ischaemic Spiral
Myocardial perfusion depends on adequate diastolic pressure and sufficient time for coronary filling. The delivery of oxygenated blood to the myocardium occurs primarily during diastole (relaxation) because the contracting heart muscle compresses coronary vessels during systole, reducing blood flow. In cardiogenic shock, hypotension reduces coronary perfusion pressure, while tachycardia shortens diastole. The result is reduced oxygen delivery to an already compromised myocardium.
This initiates an ischaemic spiral: impaired contractility lowers cardiac output, which reduces coronary perfusion, leading to worsening ischaemia and further loss of contractile function. Without interruption, this cycle progresses rapidly and becomes increasingly resistant to treatment.
Pulmonary Congestion and Impaired Gas Exchange
Elevated left ventricular end-diastolic pressure is transmitted backward into the pulmonary circulation. This means blood begins to back up behind the left ventricle, increasing pressure in the pulmonary veins and capillaries. As pulmonary capillary hydrostatic pressure rises, fluid is pushed out of the vessels into the interstitial space and eventually into the alveoli, producing pulmonary oedema. The presence of fluid in and around the alveoli increases the distance oxygen has to diffuse and reduces the available surface area for gas exchange, lowering arterial oxygen content.
Hypoxaemia further limits myocardial oxygen supply and worsens systemic hypoxia, meaning both the heart and other tissues receive less oxygen than they need. At the same time, fluid-filled and stiff lungs are harder to ventilate, so the work of breathing increases, raising overall oxygen demand. Pulmonary congestion therefore creates a mismatch where oxygen delivery is reduced while demand is increased, amplifying both cardiac and peripheral dysfunction in cardiogenic shock.
Progression to Multiorgan Failure
Sustained low cardiac output results in global hypoperfusion. Organs with high metabolic demand are affected early. Reduced renal perfusion leads to acute kidney injury, impaired fluid clearance and worsening metabolic acidosis. Cerebral hypoperfusion causes altered consciousness and loss of protective reflexes. Hepatic congestion and hypoxia impair drug metabolism and coagulation.
If myocardial function is not restored or mechanically supported, cellular injury becomes irreversible. Multiorgan failure develops as oxygen debt accumulates, marking advanced cardiogenic shock with a high risk of mortality.
Clinical Connections
Cardiogenic shock typically presents with evidence of low cardiac output alongside signs of congestion. Hypotension and tachycardia are common, but unlike hypovolaemic shock, patients may appear fluid overloaded due to elevated filling pressures. Pulmonary oedema, hypoxaemia, and increasing work of breathing reflect backward failure of the left ventricle, while cool peripheries, delayed capillary refill, and rising lactate indicate inadequate forward flow. Altered mental status often signals worsening cerebral hypoperfusion and should be treated as a late and concerning finding.
A key clinical challenge is recognising that poor perfusion can coexist with volume overload. In cardiogenic shock, additional fluid does not improve preload utilisation and may worsen pulmonary congestion and gas exchange. Deterioration can occur rapidly as falling cardiac output further compromises coronary perfusion, reinforcing the cycle of pump failure.
Clinical features that support cardiogenic shock include:
hypotension with signs of pulmonary congestion
tachycardia with narrow or widened pulse pressure depending on severity
hypoxaemia related to pulmonary oedema rather than primary lung pathology
cool, clammy skin and reduced capillary refill indicating low forward flow
rising lactate and worsening metabolic acidosis despite adequate volume status
These findings should prompt consideration of primary pump failure rather than distributive or volume-related causes of shock.
Management of cardiogenic shock focuses on restoring perfusion while addressing the underlying cardiac cause. Initial care includes oxygen therapy and careful fluid management, as excess fluid can worsen pulmonary oedema. Medications such as inotropes are used to improve myocardial contractility, while vasopressors may be required to maintain blood pressure. At the same time, treatment targets the cause, such as revascularisation in myocardial infarction or management of arrhythmias, with mechanical support (for example, intra-aortic balloon pump) considered if instability persists. Due to the high acuity care required, patients with cardiogenic shock are typically managed in critical care areas, such as intensive care.
Concept Check
Why does impaired contractility reduce cardiac output in cardiogenic shock?
How does increased afterload worsen pump failure?
Why does reduced coronary perfusion accelerate myocardial dysfunction?
How does pulmonary oedema contribute to systemic hypoxia?
Why is fluid loading ineffective in cardiogenic shock?