22.1 Introduction to Cardiac Emergencies and Shock

Pathophysiology and Clinical Manifestations of Cardiac Emergencies and Shock

Cardiac emergencies and shock are life-threatening conditions that disrupt blood flow to the heart or throughout the body. Quick recognition and treatment directly affect patient survival and long-term outcomes. As a nurse, you need to understand the underlying pathophysiology so you can anticipate which drugs will be used and why.

This section covers the two major acute coronary syndromes (AMI and unstable angina), the clinical signs you'll see across cardiac emergencies and shock, and the four main categories of shock with their distinct causes.

Acute Myocardial Infarction and Unstable Angina

These two conditions fall on a spectrum. Both involve disrupted blood flow through the coronary arteries, but they differ in severity.

Acute myocardial infarction (AMI) happens when a coronary artery becomes completely blocked. The usual sequence looks like this:

1. An atherosclerotic plaque (fatty deposit in the artery wall) ruptures.
2. A thrombus (blood clot) forms at the rupture site.
3. The clot fully occludes the artery, cutting off blood supply downstream.
4. The affected myocardial tissue becomes ischemic (oxygen-starved) and begins to undergo necrosis (cell death).
5. Cardiac output drops, and if enough muscle is damaged, cardiogenic shock can develop.

Unstable angina involves partial occlusion of a coronary artery. The myocardium becomes ischemic, but because some blood flow continues, there is no necrosis. Causes include partial plaque rupture, vasospasm (sudden narrowing of the vessel), or a spike in myocardial oxygen demand. The critical point: unstable angina can progress to a full AMI if the blockage worsens, so it's treated as a medical emergency.

Clinical Manifestations of Cardiac Emergencies and Shock

Many of these signs overlap across different emergencies. You'll often see several at once, and the combination helps guide the differential diagnosis.

Cardiac-related signs:

• Chest pain or discomfort radiating to the jaw, neck, back, or left arm, often described as pressure, squeezing, or tightness
• Shortness of breath (dyspnea)
• Diaphoresis (excessive sweating)
• Nausea and vomiting
• Tachycardia (rapid heart rate) or bradycardia (slow heart rate)

Signs of inadequate perfusion (shock indicators):

• Hypotension (low blood pressure)
• Altered mental status or confusion
• Cool, clammy skin
• Lightheadedness or syncope (fainting)
• Decreased urine output (oliguria), reflecting reduced renal perfusion

Oliguria is a particularly useful marker because the kidneys are sensitive to drops in blood flow. If urine output falls below about 0.5 mL/kg/hr, that's a red flag for inadequate perfusion.

Four Main Types of Shock and Their Underlying Causes

All forms of shock share a common endpoint: tissues don't receive enough oxygen to meet metabolic demands. What differs is why perfusion fails. Understanding the mechanism matters because it determines which drugs and interventions you'll reach for.

Cardiogenic shock results from the heart failing to pump effectively. Cardiac output drops because the pump itself is damaged or dysfunctional. Common causes include AMI (the most frequent), cardiomyopathy (weakened heart muscle), and severe valvular disease.

Hypovolemic shock occurs when there isn't enough intravascular volume to maintain adequate perfusion. The heart is pumping fine, but there simply isn't enough fluid in the system. Causes include hemorrhage, severe dehydration, and extensive burns (which cause massive fluid shifts out of the vasculature).

Distributive shock is characterized by inappropriate vasodilation and a drop in systemic vascular resistance. The volume is technically still there, but the "container" has expanded so much that blood pressure plummets. There are three subtypes:

1. Septic shock results from a severe systemic infection triggering widespread inflammation and vasodilation. This is the most common type of distributive shock in hospital settings.
2. Anaphylactic shock is caused by a severe allergic reaction (to medications, foods, insect stings, etc.) that triggers massive histamine release and vasodilation.
3. Neurogenic shock occurs when a spinal cord injury disrupts sympathetic nervous system tone, removing the body's ability to maintain vascular constriction.

Obstructive shock happens when blood flow is physically blocked somewhere outside the heart itself. The heart can contract, but something prevents it from filling or ejecting properly. Causes include:

1. Pulmonary embolism blocks the pulmonary arteries with a blood clot, preventing blood from reaching the left side of the heart.
2. Tension pneumothorax occurs when air trapped in the pleural space collapses the lung and compresses the heart and great vessels.
3. Cardiac tamponade develops when fluid accumulates in the pericardial sac (the protective covering around the heart), compressing the chambers and preventing them from filling adequately.