6.3 Cardiogenic Shock and Systemic Vascular Resistance (SVR)

Cardiogenic Shock and Systemic Vascular Resistance (SVR)

Cardiogenic shock represents a state of critical end-organ hypoperfusion resulting from primary cardiac pump failure. It is clinically defined by sustained hypotension (systolic blood pressure [SBP] < 90 mmHg for > 30 minutes, or a mean arterial pressure [MAP] < 65 mmHg) and a cardiac index (CI) < 2.2 L/min/m² in the presence of adequate or elevated intravascular volume (pulmonary capillary wedge pressure [PCWP/PAOP] > 15 mmHg).

Pathophysiology and Compensatory Mechanisms

The most common cause of cardiogenic shock is acute myocardial infarction (AMI), typically involving more than 40% of the left ventricular myocardium, most often associated with proximal Left Anterior Descending (LAD) coronary artery occlusion. Other etiologies include acute mechanical complications of AMI (such as papillary muscle rupture causing acute mitral regurgitation or ventricular septal rupture), severe myocarditis, end-stage cardiomyopathy, and severe right ventricular infarction.

Pathophysiologically, the primary insult is a loss of myocardial contractility, which leads to a severe decrease in stroke volume (SV) and cardiac output (CO). To maintain blood pressure and vital organ perfusion in the face of falling cardiac output, the body activates compensatory neurohormonal pathways: the sympathetic nervous system (releasing norepinephrine and epinephrine) and the renin-angiotensin-aldosterone system (RAAS). This results in profound systemic vasoconstriction, elevating the systemic vascular resistance (SVR), and fluid retention, which increases preload.

While these compensatory mechanisms are designed to maintain perfusion pressure, they are highly maladaptive in cardiogenic shock. The elevated SVR increases the afterload that the failing left ventricle must overcome, which increases myocardial wall tension and oxygen demand, worsening ischemia. Concurrently, fluid retention increases preload beyond the heart's functional capacity, leading to pulmonary venous congestion, alveolar flooding, and severe hypoxia. This forms a vicious downward spiral: myocardial ischemia leads to dysfunction, which triggers compensation, which increases myocardial workload and worsens ischemia.

Invasive Hemodynamic Profiling

Flight paramedics must understand the invasive hemodynamic profiles of different shock states to guide targeted therapies.

Hemodynamic Formulas

The following hemodynamic equations are essential for critical care transport examinations:

• Mean Arterial Pressure (MAP): MAP = (2 * DBP + SBP) / 3
• Systemic Vascular Resistance (SVR): SVR = ((MAP - CVP) / CO) * 80 The multiplication factor of 80 is required to convert the pressure-flow ratio (mmHg/L/min) into dynes·s/cm⁵.
• Cardiac Index (CI): CI = CO / BSA Where BSA is the Body Surface Area (m²).
• Cardiac Output (CO): CO = HR * SV

Management Strategies

The primary goals of management in cardiogenic shock are restoring organ perfusion, reducing myocardial oxygen demand, and minimizing pulmonary congestion.

Fluid Management

Aggressive fluid resuscitation is strictly contraindicated in cardiogenic shock. Because the left ventricle is operating on the flat portion of the Frank-Starling curve, volume expansion will not increase stroke volume but will instead cause or worsen pulmonary edema. Cautious fluid challenges (100–250 mL of crystalloid) should only be administered if right ventricular failure is suspected (presenting with elevated CVP, clear lung sounds, and low PAOP) and there is no evidence of left-sided pulmonary congestion.

Pharmacological Support

Pharmacotherapy is directed at improving contractility (inotropes) and maintaining coronary perfusion pressure (vasopressors):

• Norepinephrine: Norepinephrine is the first-line vasopressor for cardiogenic shock with profound hypotension (MAP < 65 mmHg). It acts primarily as an Alpha-1 adrenergic agonist to cause vasoconstriction, along with mild Beta-1 agonist activity to support contractility. Norepinephrine increases MAP and coronary perfusion pressure with less tachycardia and lower arrhythmic potential than dopamine. The landmark Sepsis Occurrence in Acutely Ill Patients (SOAP) II trial demonstrated that dopamine was associated with a significantly higher rate of dysrhythmias and an increased risk of death in patients with cardiogenic shock compared to norepinephrine.
• Dobutamine: Dobutamine is the first-line inotrope. It acts primarily as a Beta-1 agonist, which increases myocardial contractility, stroke volume, and cardiac index. However, it also has mild Beta-2 effects that cause systemic vasodilation, which can precipitate or worsen hypotension. Therefore, dobutamine should only be used in patients with a stable blood pressure (SBP > 90 mmHg) or in combination with a vasopressor (such as norepinephrine) in patients with severe hypotension. Dosing starts at 2–5 mcg/kg/min and is titrated to effect up to 20 mcg/kg/min.
• Milrinone: A phosphodiesterase-3 (PDE3) inhibitor that prevents cAMP breakdown in cardiac muscle and vascular smooth muscle, acting as a positive inotrope and systemic/pulmonary vasodilator (an 'inodilator'). It is highly effective in patients on chronic beta-blocker therapy or those with secondary pulmonary hypertension. However, it has a long half-life (2–3 hours) and can cause profound, prolonged hypotension; it must be avoided in patients with inadequate blood pressure.

Mechanical Circulatory Support (MCS)

In patients refractory to pharmacological management, mechanical circulatory support is indicated.

• Intra-Aortic Balloon Pump (IABP):
  • Mechanism of Action: A balloon is placed in the descending thoracic aorta. It is programmed to inflate during diastole (triggered by the dicrotic notch on the arterial waveform), which increases diastolic pressure and retrogradely displaces blood toward the coronary arteries, enhancing myocardial oxygen supply. The balloon deflates rapidly during systole (triggered by the R-wave of the ECG), which creates a vacuum effect that lowers aortic pressure, reducing left ventricular afterload, myocardial workload, and oxygen demand.
  • Contraindications: IABP is strictly contraindicated in patients with moderate-to-severe aortic regurgitation (inflation would worsen regurgitant flow into the left ventricle) and aortic dissection (inflation could propagate the dissection).
• Impella: A microaxial flow pump placed across the aortic valve that actively pulls blood from the left ventricle and ejects it into the aorta, providing up to 5.0 L/min of forward flow, directly unloading the left ventricle.
• Veno-Arterial Extracorporeal Membrane Oxygenation (VA-ECMO): Provides complete cardiopulmonary support by draining venous blood, oxygenating it, and pumping it back into the arterial system.