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 processes. Two major categories include orthostatic hypotension and shock syndromes. Understanding these conditions is essential for recognizing clinical presentations, underlying mechanisms, and appropriate interventions.

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 ≥ 10 mm Hg in diastolic blood pressure (DBP) within three minutes of standing.

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

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 and potentially lowering cardiac output (CO) and blood pressure (BP). The body compensates through several mechanisms:

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

• Physiological responses:

  • Increased peripheral vascular resistance (vasoconstriction)

  • Increased heart rate (HR)

  • Increased cardiac contractility

• Prolonged responses: Vasopressin release and activation of the renin-angiotensin-aldosterone system (RAAS) help maintain BP over time.

Clinical Presentation of Orthostatic Hypotension

• Symptoms typically occur upon standing and are more prominent in cases of hypovolemia (low blood volume).

• Common symptoms:

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

  • Pulmonary: Dyspnea (shortness of breath)

  • Cardiac: Angina (chest pain)

  • Musculoskeletal: Neck pain (secondary to muscle hypoperfusion)

• Symptoms are often worse in the morning (due to nocturnal diuresis and decreased blood volume) and after meals (due to increased splanchnic blood capacity).

Main Causes of Orthostatic Hypotension

• Functional (Non-neurogenic):

  • Reduction in absolute or relative blood volume (hypovolemia)

  • Drug-induced hypotension

• Structural (Neurogenic):

  • Neurodegenerative disorders (e.g., Parkinson's disease)

  • Diabetes (autonomic neuropathy)

Decreased Intravascular Volume

Inadequate blood volume or venous return can result from:

• Excessive urinary losses (e.g., due to diuretic medications such as thiazide or loop diuretics)

• Excessive diaphoresis (sweating) or fever

• Excessive gastrointestinal losses (e.g., diarrhea, vomiting)

• Inadequate fluid intake

• Excessive blood loss (internal or external bleeding)

• Chronic conditions such as heart failure or cirrhosis

Medications Associated with Orthostatic Hypotension

• Diuretics: Cause dehydration and decreased blood volume (loop diuretics have a greater effect than thiazide or potassium-sparing diuretics such as spironolactone).

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

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

• Alpha-1 receptor antagonists: Prazosin.

• Alpha-2 receptor agonists: Clonidine.

Neurogenic Causes

• Autonomic nervous system (ANS) dysfunction impairs the baroreflex response, reducing sympathetic output (norepinephrine, epinephrine).

• Common in chronic hypertension, aging, Parkinson's disease, diabetic neuropathy, spinal cord injury, and stroke.

Populations Affected

• More common in the elderly, often due to impaired compensatory mechanisms and increased medication use.

• Institutionalized individuals are at higher risk than those living at home.

Treatment and Prevention of Orthostatic Hypotension

• Prevention:

  • Maintain adequate hydration and ensure access to fluids.

  • Avoid or minimize alcohol intake.

  • Manage diuretic medications appropriately.

  • Educate patients to rise slowly, starting from a sitting position, and to move legs before standing to increase venous return.

• Treatment:

  • Fludrocortisone (promotes sodium and water retention)

  • Vasoconstrictor agents (e.g., midodrine, pseudoephedrine, phenylephrine)

  • Droxidopa (a 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.

• Typically, shock is characterized by a systolic BP < 90 mm Hg and circulatory failure with insufficient compensatory mechanisms.

• Leads to a shift from aerobic to anaerobic metabolism due to lack of sufficient oxygen, often resulting in acute organ failure or death.

Types of Shock

• Cardiogenic shock: Caused by impaired cardiac contractility, leading to decreased cardiac output (CO).

• Distributive (normovolemic) shock: Includes septic, anaphylactic, and neurogenic shock, characterized by abnormal distribution of blood flow and vasodilation.

• Septic shock: Caused by proinflammatory cytokine-induced vasodilation and decreased systemic vascular resistance (SVR).

• Anaphylactic shock: Caused by histamine-induced vasodilation and decreased SVR.

• Neurogenic shock: Caused by loss of sympathetic outflow, resulting in vasodilation and decreased SVR.

• Hypovolemic shock: Caused by insufficient stroke volume (SV) due to loss of blood or fluids.

• Obstructive shock: (Not discussed in detail in these notes.)

Shock Syndrome Characteristics

Type of Shock	Primary Derangement	Mechanism
Cardiogenic	Decreased CO	Impaired contractility
Hypovolemic	Decreased CO	Insufficient stroke volume (SV)
Septic	Decreased SVR	Proinflammatory cytokine-induced vasodilation
Anaphylactic	Decreased SVR	Histamine-induced vasodilation
Neurogenic	Decreased SVR	Loss of sympathetic outflow

Key Terms and Formulas

• Cardiac Output (CO): The volume of blood the heart pumps per minute. $CO = HR \times SV$

• Systemic Vascular Resistance (SVR): The resistance to blood flow offered by all of the systemic vasculature, excluding the pulmonary circuit.

• Mean Arterial Pressure (MAP): The average pressure in a patient's arteries during one cardiac cycle. $MAP = CO \times SVR$

Summary Table: Types of Shock

Type	CO	SVR	Key Feature
Cardiogenic	↓	↑ or normal	Impaired heart function
Hypovolemic	↓	↑	Low blood volume
Septic	↑ or normal	↓	Vasodilation, infection
Anaphylactic	↑ or normal	↓	Allergic reaction
Neurogenic	↓	↓	Loss of sympathetic tone

Additional info:

• Obstructive shock (not detailed here) is caused by physical obstruction of the great vessels or the heart itself (e.g., pulmonary embolism, cardiac tamponade).

• Early recognition and management of shock are critical to prevent irreversible organ damage and death.