1.3 Cardiogenic Shock, Acute Heart Failure & Mechanical Circulatory Support

1.3 Cardiogenic Shock, Acute Heart Failure & Mechanical Circulatory Support

Clinical Scenario: A 68-year-old male who underwent primary PCI for an anterior STEMI develops confusion, tachypnea, and cool, mottled extremities 6 hours post-procedure. Urine output drops to 10 mL/hr. Vital signs: BP 76/46 mmHg (MAP 56 mmHg), HR 118 bpm, SpO2 89% on 4 L nasal cannula. ABG reveals pH 7.24, PaCO2 30 mmHg, PaO2 56 mmHg, HCO3 13 mEq/L, and lactate 5.8 mmol/L. Invasive PAC hemodynamics: CVP 18 mmHg, PAP 48/28 mmHg, PCWP 25 mmHg, CO 2.6 L/min, CI 1.5 L/min/m², and SVR 1169 dynes·sec/cm⁵.

Pathophysiology & Diagnostic Criteria of Cardiogenic Shock

Cardiogenic shock is sustained tissue hypoperfusion caused by cardiac dysfunction. Causes include acute MI, decompensated heart failure, myocarditis, acute valve failure and dysrhythmias. Determine whether dysfunction predominantly involves the LV, RV or both ventricles, and assess for accompanying vasoplegia or hypovolemia.

Pathophysiological Cascade

1. Myocardial injury sharply drops stroke volume, cardiac output, and stroke work.
2. Arterial hypotension reduces coronary perfusion pressure ($\text{CorPP} = \text{Diastolic BP} - \text{LVEDP}$), worsening ischemia and depressing contractility.
3. Reflex sympathetic activation triggers peripheral vasoconstriction, elevating systemic vascular resistance (SVR), LV afterload, wall stress, and myocardial oxygen demand ($MVO_2$).
4. Elevated LVEDP and PCWP cause pulmonary venous congestion, alveolar flooding, and hypoxemia.

Diagnostic Assessment

• Hypoperfusion: Altered mentation, cool skin, oliguria (<0.5 mL/kg/hr), rising lactate or organ injury in a patient with cardiac dysfunction.
• Blood Pressure: SBP $<90$ mmHg or a need for support is common; preserved blood pressure does not exclude shock.
• Cardiac Index: Often $<2.2$ L/min/m² when measured; interpret with clinical perfusion.
• Filling Pressures: LV-dominant congestion commonly raises PCWP $>15$ mmHg. RV-dominant shock raises right atrial pressure with relatively normal PCWP; biventricular failure raises both.
• Vascular Resistance: SVR often increases, but may be normal or low with accompanying vasoplegia.
• Cardiac Power Output: $[(\text{MAP} \times \text{CO})/451]$ Watts; a CPO $<0.6$ W is an adverse prognostic marker.

Cardiomyopathy Phenotypes and Obstructive HCM

A dilated phenotype combines ventricular enlargement with systolic dysfunction; restrictive physiology causes impaired filling, often with preserved ejection fraction. Hypertrophic cardiomyopathy (HCM) produces otherwise unexplained myocardial thickening and may cause dynamic left-ventricular outflow obstruction. Evaluate ECG, echocardiography, family history and acquired causes; cardiac MRI helps characterize scar or infiltration. In obstructive HCM, reduced preload/afterload or increased contractility can worsen obstruction and hypotension. Correct hypovolemia cautiously; if hypotension persists despite appropriate fluid administration, use phenylephrine or another vasoconstrictor without inotropic activity, with expert guidance. Escalating dobutamine for this obstructive mechanism may worsen shock. Nonvasodilating beta blockers are first-line treatment for symptomatic obstructive HCM; specialist assessment also addresses sudden-death risk and family screening.

SCAI Clinical Staging of Cardiogenic Shock

The Society for Cardiovascular Angiography and Interventions (SCAI) classification standardizes shock severity and guides clinical escalation:

Assign stage from the overall clinical state and reassess serially. A medication or support device alone does not establish severity. Use the arrest modifier when cardiac arrest raises concern for anoxic brain injury.

Vasoactive Pharmacotherapy Protocols

Pharmacotherapy balances restoring perfusion pressure against exacerbating myocardial oxygen demand:

Vasopressors

• Norepinephrine: A reasonable first vasopressor for hypotensive cardiogenic shock. SOAP II found fewer arrhythmias overall with norepinephrine than dopamine and a mortality signal favoring norepinephrine in the cardiogenic-shock subgroup. Alpha-1 agonism restores SVR and coronary perfusion; mild beta-1 provides inotropic support with minimal chronotropy. Titrate 0.02–0.5 mcg/kg/min (target MAP $\ge 65$ mmHg).
• Vasopressin: Second-line non-adrenergic V1 agonist (0.03–0.04 units/min) for refractory vasoplegia without increasing PVR.
• Epinephrine: Potent alpha/beta agonist reserved for refractory shock or post-arrest collapse. Risks: tachycardia, heightened $MVO_2$, arrhythmias, and Type B lactic acidosis.

Inotropes

• Dobutamine: Synthetic catecholamine with potent beta-1 and mild beta-2 agonism. Augments stroke volume and lowers PCWP via cAMP generation (2.5–20 mcg/kg/min). Risks: tachyarrhythmias, increased $MVO_2$, and hypotension in hypovolemia.
• Milrinone: PDE-3 inhibitor preventing cAMP breakdown in myocytes and vascular smooth muscle ("inodilator"). Reduces PCWP, SVR, and PVR without adrenergic stimulation. An option in chronic beta-blockade or RV failure with pulmonary hypertension if blood pressure and renal function permit. Dosed 0.125–0.75 mcg/kg/min (omit boluses in shock; adjust for renal clearance in AKI).

Infiltrative Disease: Amyloidosis and Sarcoidosis

Consider cardiac amyloidosis with unexplained ventricular wall thickening, restrictive heart failure and extracardiac clues such as neuropathy or proteinuria. Screen for AL amyloidosis with serum free light chains plus serum and urine immunofixation; ordinary protein electrophoresis alone is insufficient. If AL is excluded, appropriate bone-tracer scintigraphy can establish ATTR cardiac amyloidosis in the correct clinical setting. An abnormal monoclonal-protein screen requires specialist evaluation and often biopsy with amyloid typing; a positive scan alone does not settle the type. Sarcoidosis may involve lungs, lymph nodes, heart or nervous system. Compatible findings, nonnecrotizing granulomas when tissue confirmation is needed, and exclusion of mimics support diagnosis. New AV block, ventricular arrhythmia or cardiomyopathy warrants cardiac evaluation. Organ-threatening sarcoidosis requires specialist-directed anti-inflammatory treatment, commonly glucocorticoids.

Mechanical Circulatory Support (MCS) Devices

When pharmacotherapy fails, urgently consult the shock team for individualized temporary MCS selection:

Intra-Aortic Balloon Pump (IABP)

• Mechanism: Counterpulsation balloon placed in descending thoracic aorta (tip 1–2 cm distal to left subclavian origin).
• Counterpulsation Timing:
  • Inflation: Initiated at the dicrotic notch of arterial waveform (aortic valve closure, early diastole). Displaces blood retrogradely, augmenting coronary and cerebral perfusion.
  • Deflation: Initiated during presystole immediately before aortic valve opening (timed with the console’s ECG or arterial-pressure signal). Creates a vacuum effect, dramatically reducing LV afterload, wall stress, and $MVO_2$.
• Contraindications: Moderate-to-severe aortic regurgitation (diastolic inflation forces retrograde volume into LV, worsening acute failure), aortic dissection, and severe peripheral arterial disease.

Impella Percutaneous Ventricular Assist Device

• Mechanism: Transvalvular microaxial pump inserted across aortic valve into left ventricle (Impella 2.5, CP, 5.0, 5.5). Continuously expels blood from LV into ascending aorta (2.5–5.5 L/min).
• Hemodynamics: Directly unloads LV, reducing end-diastolic volume, LVEDP, PCWP, and wall tension while augmenting MAP and cardiac output.
• Management: A continuous pressurized purge prevents blood from entering the motor. Heparin in dextrose is standard. A bicarbonate-based purge is an option when heparin cannot be used. Complications include shear-stress hemolysis (plasma free hemoglobin $> 40$ mg/dL, elevated LDH, hemoglobinuria), thrombocytopenia, limb ischemia, and catheter malposition.

Extracorporeal Membrane Oxygenation (ECMO)

• Veno-Arterial (VA) ECMO: Drains venous blood, oxygenates it, and pumps pressurized blood into arterial system (femoral artery), providing biventricular and respiratory support.
• Afterload Challenge: Retrograde arterial flow increases LV afterload. In severe LV failure, this triggers LV distension, elevated PCWP, pulmonary edema, and intracardiac stasis. Assess pulsatility, aortic valve opening and LV size. Persistent LV distension with pulmonary edema may require active unloading, such as an Impella, surgical vent or atrial septostomy.
• Veno-Venous (VV) ECMO: Drains and returns blood within the venous system. Supports gas exchange in selected patients with severe respiratory failure and provides no direct circulatory support.