1.1 Pathophysiology of Shock and Sepsis

Pathophysiology of Shock

Shock is a life-threatening, generalized form of acute circulatory failure associated with inadequate oxygen utilization by the cells. It is a state in which cellular energy production fails to meet demand, leading to cellular dysfunction, cell death, and eventually multiorgan failure.

Cellular Hypoxia and Energy Crisis

At the cellular level, the lack of sufficient oxygen delivery (DO2) relative to consumption (VO2) forces cells to shift from aerobic metabolism (oxidative phosphorylation) to anaerobic metabolism (glycolysis). Anaerobic metabolism is highly inefficient, producing only 2 ATP molecules per glucose molecule compared to the 36-38 ATP produced aerobically. This shift leads to the accumulation of pyruvate, which is subsequently converted into lactate. The resulting lactic acidosis contributes to a lowered intracellular pH, impairing enzyme function and leading to the failure of energy-dependent ion pumps (like the Na+/K+ ATPase). As these pumps fail, cellular edema occurs, intracellular calcium levels rise, and apoptotic and necrotic pathways are triggered.

The Four Categories of Shock

Shock is traditionally classified into four distinct categories based on the primary physiologic derangement. Understanding the hemodynamic profile of each is crucial for appropriate clinical management.

Hypovolemic Shock

Hypovolemic shock results from a loss of intravascular volume, either through hemorrhage or non-hemorrhagic fluid loss (e.g., severe vomiting, diarrhea, burns). The reduction in circulating volume leads to decreased venous return (preload), which in turn reduces ventricular end-diastolic volume and stroke volume. The compensatory response is an increase in sympathetic tone, leading to tachycardia and severe peripheral vasoconstriction (increased SVR) to maintain blood pressure.

Cardiogenic Shock

Cardiogenic shock is characterized by the failure of the cardiac pump to deliver sufficient blood flow to the tissues, despite adequate intravascular volume. The most common cause is acute myocardial infarction. The primary insult is a profound decrease in cardiac output, which leads to increased left ventricular end-diastolic pressure (and consequently elevated PCWP). The body attempts to compensate by increasing SVR, but this often exacerbates the failing heart's workload.

Obstructive Shock

Obstructive shock occurs when there is an extracardiac physical barrier to blood flow. Common causes include massive pulmonary embolism (obstructing right ventricular outflow), tension pneumothorax, and cardiac tamponade (impeding ventricular filling). The hemodynamic profile often mimics cardiogenic shock, with low CO and high SVR, but the specific pressures (e.g., elevated CVP with normal/low PCWP in pulmonary embolism) depend on the site of obstruction.

Distributive Shock

Distributive shock is characterized by severe peripheral vasodilation and an abnormal distribution of blood flow. Sepsis is the most common cause, but anaphylaxis and neurogenic shock also fall into this category. In distributive shock, SVR is profoundly reduced. Cardiac output is typically normal or even elevated initially as a compensatory mechanism, although myocardial depression can occur in late stages.

The Pathophysiology of Sepsis

Sepsis is formally defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. When an invading pathogen (or its products, such as endotoxins or lipopolysaccharides) enters the bloodstream, it interacts with pattern recognition receptors (e.g., Toll-like receptors) on immune cells.

The Inflammatory Cascade

This interaction triggers a massive release of pro-inflammatory cytokines, including Tumor Necrosis Factor-alpha (TNF-α), Interleukin-1 (IL-1), and Interleukin-6 (IL-6). These cytokines initiate a complex cascade that leads to:

1. Endothelial Dysfunction: The endothelium becomes leaky, allowing fluid and proteins to escape into the interstitial space. This exacerbates hypovolemia and impairs oxygen diffusion.
2. Vasodilation: Nitric oxide synthase is upregulated, producing excessive amounts of nitric oxide, a potent vasodilator. This is the primary driver of the profound drop in SVR characteristic of septic shock.
3. Coagulopathy: Sepsis activates the coagulation cascade while simultaneously suppressing fibrinolysis. This leads to the formation of microthrombi in small vessels (disseminated intravascular coagulation or DIC), which further obstructs blood flow and contributes to tissue ischemia and organ failure.

Organ Dysfunction

The culmination of hypoperfusion, microvascular thrombosis, and direct cellular toxicity leads to multiorgan dysfunction syndrome (MODS). The lungs are frequently affected, manifesting as Acute Respiratory Distress Syndrome (ARDS). Acute kidney injury (AKI) is common due to renal hypoperfusion and tubular injury. Hepatic dysfunction, altered mental status, and myocardial depression (septic cardiomyopathy) frequently follow.

Clinical Recognition and Initial Management

Early recognition of sepsis is paramount. While SIRS (Systemic Inflammatory Response Syndrome) criteria were previously used, the qSOFA (quick Sequential Organ Failure Assessment) score is now often utilized as a rapid bedside tool to identify patients outside the ICU at high risk for poor outcomes. A qSOFA score incorporates:

• Respiratory rate $\ge$ 22 breaths/min
• Altered mentation
• Systolic blood pressure $\le$ 100 mmHg

The cornerstone of septic shock management is the immediate initiation of the 1-hour bundle:

1. Measure lactate level.
2. Obtain blood cultures before administering antibiotics.
3. Administer broad-spectrum antibiotics.
4. Begin rapid administration of 30 mL/kg crystalloid for hypotension or lactate $\ge$ 4 mmol/L.
5. Apply vasopressors if hypotensive during or after fluid resuscitation to maintain a mean arterial pressure (MAP) $\ge$ 65 mmHg.

Norepinephrine is the first-line vasopressor for septic shock. If the MAP target is not achieved with norepinephrine alone, vasopressin or epinephrine can be added.