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Pathophysiology

Pathology

Myocardium

The myocardial infarction, more than an isolated event, is a process that develops over several hours. Initially there is a central area of infarction surrounded by a border of ischemic myocardium. As time goes by, and in the absence of reperfusion measures, this area of ischemia will enlarge outwards from the central area. As this area of infarcted myocardium grows, so does the chances of developing cardiogenic shock, leading to the conclusion that pump failure follows severe loss of myocardial mass. This loss usually develops over several hours, to days. This conclusion is supported by the fact that most patients develop cardiogenic shock in hospital, several hours after being admitted for acute MI.

These patients who develop shock after admission usually suffer of what is known as infarct extension, which may result from:

• propagation of an intracoronary thrombus;
• reocclusion of a transiently patent infarct artery;
• decreased perfusion pressure of coronary arteries simultaneously with an increased myocardial oxygen consumption.

• INFARCT EXTENSION AND EXPANSION
• REMOTE ISCHEMIA
• DIASTOLIC DYSFUNCTION
• VALVULAR ABNORMALITIES - (page 155 article #2)

Cellular

• ENERGY METABOLISM
• ION PUMPS
• NECROSIS
• APOPTOSIS

Myocardial dysfunction

• STUNNING
• HIBERNATING

Reperfusion Injury

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Cardiogenic shock and Inflammatory Mediators

The Pathophysiologic "Spiral" of Cardiogenic shock

Among patients with acute MI, there is often a downward spiral of hypoperfusion leading to further ischemia which leads to a further reduction in cardiac output and further hypoperfusion. The lactic acidosis that develops as a result of poor systemic perfusion can further reduce cardiac contractility. Reduced cardiac output leads to activation of the sympathetic nervous system, and the ensuing tachycardia that develops further exacerbates the myocardial ischemia. The increased left ventricular end diastolic pressures is associated with a rise in wall stress which results in further myocardial ischemia. Hypotension reduces epicardial perfusion pressure which in turn further increases myocardial ischemia.

Patients with cardiogenic shock in the setting of STEMI more often have multivessel disease, and myocardial ischemia may be present in multiple territories. It is for this reason that multivessel angioplasty may be of benefit in the patient with cardiogenic shock. Non-culprit or remote territories may also exhibit myocardial stunning in response to an ischemic insult which further reduces myocardial function. The pathophysiology of myocardial stunning is multifactorial and involves calcium overload in the sarcolemma and "stone heart" or diastolic dysfunction as well as the release of myocardial depressant substances. Areas of stunned myocardium may remain stunned after revascularization, but these regions do respond to inotropic stimulation. In contrast to stunned myocardium, hibernating myocardium does respond earlier to revascularization.

The multifactorial nature of cardiogenic shock can also be operative in the patient with critical aortic stenosis who has "spiraled": There is impairment of left ventricular outflow, with a drop in cardiac output there is greater subendocardial ischemia and poorer flow in the coronary arteries, this leads to further left ventricular systolic dysfunction, given the subendocardial ischemia, the left ventricle develops diastolic dysfunction and becomes harder to fill. Inadvertent administration of vasodilators and venodilators may further reduce cardiac output and accelerate or trigger such a spiral.

Pathophysiologic Mechanisms to Compensate for Cardiogenic shock

Cardiac output is the product of stroke volume and heart rate. In order to compensate for a reduction in stroke volume, there is a rise in the heart rate in patients with cardiogenic shock. As a result of the reduction in cardiac output, peripheral tissues extract more oxygen from the limited blood that does flow to them, and this leaves the blood deoxygenated when it returns to the right heart resulting in a fall in the mixed venous oxygen saturation.

Pathophysiology of Multiorgan Failure

The poor perfusion of organs results in hypoxia and metabolic acidosis. Inadequate perfusion to meet the metabolic demands of the brain, kidneys and heart leads to multiorgan failure.

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Differential Diagnosis

Classification of shock based on hemodynamic parameters. (CO, cardiac output; CVP; central venous pressure; PAD, pulmonary artery diastolic pressure; PAS, pulmonary artery systolic pressure; RVD, right ventricular diastolic pressure; RVS, right ventricular systolic pressure; SVO2, systemic venous oxygen saturation; SVR, systemic vascular resistance.)

Type of Shock	Etiology	CO	SVR	PCWP	CVP	SVO2	RVS	RVD	PAS	PAD
Cardiogenic	Acute Ventricular Septal Defect	↓↓	↑	N — ↑	↑↑	↑ — ↑↑	N — ↑	↑	N — ↑	N — ↑
	Acute Mitral Regurgitation	↓↓	↑	↑↑	↑ — ↑↑	↓	↑	N — ↑	↑	↑
	Myocardial Dysfunction	↓↓	↑	↑↑	↑↑	↓	N — ↑	N — ↑	N — ↑	↑
	Right Ventricular Infarction	↓↓	↑	N — ↓	↑↑	↓	↓ — ↑	↑	↓ — ↑	↓ — ↑
Obstructive	Pulmonary Embolism	↓↓	↑	N — ↓	↑↑	↓	↓ — ↑	↑	↓ — ↑	↓ — ↑
	Cardiac Tamponade	↓ — ↓↓	↑	↑↑	↑↑	↓	N — ↑	↑	N — ↑	N — ↑
Distributive	Septic Shock	N — ↑↑	↓ — ↓↓	N — ↓	N — ↓	↑ — ↑↑	N — ↓	N — ↓	↓	↓
	Anaphylactic Shock	N — ↑↑	↓ — ↓↓	N — ↓	N — ↓	↑ — ↑↑	N — ↓	N — ↓	↓	↓
Hypovolemic	Volume Depletion	↓↓	↑	↓↓	↓↓	↓	N — ↓	N — ↓	↓	↓