Disorders of Low Blood Pressure

Overview

Low blood pressure (hypotension) can result from a variety of physiological disturbances. 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 change.

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 thus decreasing 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 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 occur upon standing and are more prominent with hypovolemia (low blood volume).

• Central nervous system (CNS):

  • Lightheadedness or 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) due to decreased cardiac output

• Musculoskeletal: Neck pain (secondary to muscle hypoperfusion)

• Symptoms are more common in the morning (due to nocturnal diuresis and lower 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 (diarrhea, vomiting)

• Inadequate fluid intake

• Excessive blood loss (internal or external bleeding)

• Chronic conditions (e.g., heart failure, 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 Orthostatic Hypotension

• Results from autonomic nervous system (ANS) dysfunction, leading to impaired baroreflex response.

• Reduced sympathetic nervous system (SNS) output (norepinephrine, epinephrine) is a key feature.

• Common causes include aging (impaired baroreflexes), Parkinson's disease, diabetic peripheral neuropathy, spinal cord injury, and stroke.

Populations Affected

• More common in the elderly, often due to systolic blood pressure changes and inadequate compensatory mechanisms.

• Greater medication use and prolonged illnesses increase risk.

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

Treatment and Prevention of Orthostatic Hypotension

• Prevention:

  • Prevent dehydration 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, blood pressure is < 90 mm Hg.

• Shock is characterized by circulatory failure with insufficient compensatory mechanisms, leading to a shift from aerobic to anaerobic metabolism and potential 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: Proinflammatory cytokine-induced vasodilation and decreased systemic vascular resistance (SVR).

• Anaphylactic shock: Histamine-induced vasodilation and decreased SVR.

• Neurogenic shock: Loss of sympathetic outflow, leading to vasodilation and decreased SVR.

• Hypovolemic shock: Insufficient stroke volume (SV) due to decreased blood volume.

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

Shock Syndrome Characteristics

Key Equations

• Mean Arterial Pressure (MAP):

• Cardiac Output (CO):

Summary Table: Comparison of Orthostatic Hypotension and Shock

Additional info: The above notes expand on the provided slides and text, adding definitions, mechanisms, and clinical context for clarity and completeness.