Disorders of Low Blood Pressure: Orthostatic Hypotension and Shock Syndromes

Disorders of Low Blood Pressure

Overview

Low blood pressure (hypotension) can result from various physiological and pathological mechanisms. Two major categories discussed here are orthostatic hypotension and shock syndromes. Both conditions can lead to insufficient tissue perfusion and organ dysfunction.

Orthostatic Hypotension

Definition

Orthostatic hypotension is an abnormal decrease in blood pressure upon moving from a supine (lying down) to an upright position. It is objectively defined as:

• A decrease of ≥ 20 mm Hg in systolic blood pressure (SBP)

• Or a decrease of ≥ 10 mm Hg in diastolic blood pressure (DBP)

The presence and severity of symptoms are often more clinically relevant than the absolute change in blood pressure.

Normal Blood Pressure Homeostasis: Supine to Upright

When a person stands up, gravity causes blood to pool in the lower extremities, reducing venous return to the heart. This leads to a transient decrease in cardiac output (CO) and blood pressure (BP). The body compensates through several mechanisms:

• Baroreceptor reflex: Baroreceptors in the carotid sinus and aortic arch detect the drop in BP and signal the brainstem to increase sympathetic nervous system (SNS) activity.

• Peripheral vascular resistance: SNS activation causes vasoconstriction, increasing resistance and helping restore BP.

• Heart rate: SNS increases heart rate (HR) and cardiac contractility.

• Prolonged response: Release of vasopressin and activation of the renin-angiotensin-aldosterone system (RAAS) help maintain BP over time.

Equation:

$BP = CO \times SVR$

Where CO is cardiac output and SVR is systemic vascular resistance.

Clinical Presentation of Orthostatic Hypotension

Symptoms typically occur upon assuming an upright position and are more prominent in cases of hypovolemia. Common clinical signs and symptoms include:

• CNS: Lightheadedness, dizziness, presyncope (feeling faint), syncope (loss of consciousness), blurred vision (due to retinal/occipital lobe ischemia)

• Pulmonary: Dyspnea

• Cardiac: Angina (chest pain)

• Musculoskeletal: Neck pain secondary to muscle hypoperfusion

• Symptoms often occur in the morning (due to nocturnal diuresis and reduced blood volume) or post-prandially (due to increased splanchnic blood capacity)

Main Causes of Orthostatic Hypotension

• Functional (Non-neurogenic): Reduction in absolute or relative blood volume, drug-induced

• Structural (Neurogenic): Neurodegenerative disorders (e.g., Parkinson's disease), diabetes

Decreased Intravascular Volume

Inadequate blood volume or venous return can result from:

• Excessive urinary losses due to diuretic medications (thiazide diuretics, loop diuretics)

• Excessive diaphoresis (sweating) or fever

• Excessive gastrointestinal losses (diarrhea, vomiting)

• Inadequate fluid intake

• Excessive blood loss (internal or external bleeding)

• Heart failure, cirrhosis

Medications Associated with Orthostatic Hypotension

• Diuretics: Cause dehydration and decreased blood volume

• Vasodilators: Nitrates, phosphodiesterase-5 inhibitors (e.g., sildenafil), hydralazine

• Anticholinergic agents: Tricyclic antidepressants (e.g., amitriptyline)

• Alpha-1 receptor antagonists: Prazosin

• Alpha-2 receptor agonists: Clonidine

Neurogenic Causes

Dysfunction of the autonomic nervous system (ANS) impairs baroreflex responses. SNS output (norepinephrine, epinephrine) is crucial for maintaining BP. Conditions associated with impaired baroreflexes include:

• Aging

• Parkinson's disease

• Peripheral neuropathies secondary to diabetes

• Spinal cord injury

• Stroke

Populations Affected

• More common in the elderly

• Often due to systolic blood pressure changes

• Inadequate compensatory mechanisms (ANS dysfunction)

• Greater medication use

• Prolonged illnesses more common

• Institutionalized individuals at higher risk than those living at home

Treatment and Prevention of Orthostatic Hypotension

• Prevent dehydration: Ensure access to fluids

• Avoid or minimize alcohol

• Manage diuretic medications

• Education: Rise slowly, start in sitting position, move legs to increase venous return

• Pharmacologic treatment: Fludrocortisone (promotes sodium/water retention), vasoconstrictors (midodrine, pseudoephedrine, phenylephrine), droxidopa (norepinephrine pro-drug)

Shock Syndromes

Definition

Shock is defined as sustained hypotension and tissue hypoperfusion insufficient to meet the metabolic needs of the body, resulting in cellular hypoxia. Diagnostic criteria include:

• Blood pressure < 90 mm Hg

• Circulatory failure with insufficient compensatory mechanisms

• Predominance of anaerobic metabolism due to lack of sufficient oxygen

• Commonly associated with acute organ failure and death

Types of Shock Syndromes

• Cardiogenic shock: Impaired contractility of the heart leads to decreased cardiac output

• Distributive (normovolemic) shock: Includes septic, anaphylactic, and neurogenic shock; characterized by vasodilation and normal or increased blood volume

• Septic shock: Proinflammatory cytokine-induced vasodilation

• Anaphylactic shock: Histamine-induced vasodilation

• Neurogenic shock: Loss of sympathetic outflow leads to vasodilation

• Hypovolemic shock: Insufficient stroke volume due to decreased blood volume

• Obstructive shock: (Not discussed in detail)

Shock Syndrome Characteristics

Type of Shock	Primary Derangement	Mechanism
Cardiogenic	CO (Cardiac Output)	Impaired contractility
Hypovolemic	CO (Stroke Volume)	Insufficient blood volume
Septic	SVR (Systemic Vascular Resistance)	Proinflammatory cytokine-induced vasodilation
Anaphylactic	SVR	Histamine-induced vasodilation
Neurogenic	SVR	Loss of sympathetic outflow

Key Equations

The relationship between blood pressure, cardiac output, and systemic vascular resistance is fundamental in understanding shock:

$BP = CO \times SVR$

Where:

• BP: Blood Pressure

• CO: Cardiac Output

• SVR: Systemic Vascular Resistance

Summary Table: Comparison of Shock Syndromes

Shock Type	CO	SVR	Key Feature
Cardiogenic	↓	↑/normal	Impaired heart contractility
Hypovolemic	↓	↑	Low blood volume
Septic	↑/normal	↓	Vasodilation from cytokines
Anaphylactic	↑/normal	↓	Vasodilation from histamine
Neurogenic	↓/normal	↓	Loss of sympathetic tone

Example: Septic Shock

In septic shock, bacterial infection leads to the release of proinflammatory cytokines, causing widespread vasodilation and a drop in SVR. Despite normal or increased cardiac output, tissue perfusion is inadequate, resulting in cellular hypoxia and organ dysfunction.

Additional info: Some details, such as the full mechanism of baroreceptor reflex and the pharmacologic agents for orthostatic hypotension, were expanded for academic completeness.