Disorders of Low Blood Pressure

Overview

Low blood pressure, or hypotension, can result from a variety of physiological disturbances. Two major clinical entities associated with low blood pressure are orthostatic hypotension and shock syndromes. Understanding their mechanisms, causes, and clinical presentations is essential for diagnosis and management in clinical practice.

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)

• 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 from a lying position, gravity causes blood to pool in the lower extremities, reducing venous return to the heart. The body compensates through several mechanisms:

• Immediate response: Baroreceptors in the carotid sinus and aortic arch detect the drop in blood pressure and signal the brainstem to increase sympathetic nervous system (SNS) output.

• Physiological effects:

  • Increased peripheral vascular resistance (vasoconstriction)

  • Increased heart rate (HR)

  • Increased cardiac contractility

• Prolonged response: Hormonal mechanisms such as vasopressin release and activation of the renin-angiotensin-aldosterone system (RAAS) help maintain blood pressure.

Clinical Presentation of Orthostatic Hypotension

• Signs: Decrease in blood pressure upon standing.

• Symptoms:

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

  • Pulmonary: Dyspnea (shortness of breath)

  • Cardiac: Angina (chest pain)

  • Musculoskeletal: Neck pain (secondary to muscle hypoperfusion)

• Symptoms are more prominent when due to hypovolemia (low blood volume).

• Commonly occurs in the morning (due to nocturnal diuresis) or post-prandially (after meals, due to increased splanchnic blood capacity).

Main Causes of Orthostatic Hypotension

• Functional (Non-neurogenic):

  • Reduction in absolute or relative blood volume

  • 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 be associated with:

• Excessive urinary losses due to diuretic medications (e.g., 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 or cirrhosis (causing fluid redistribution)

Medications Associated with Orthostatic Hypotension

• Diuretics (loop, thiazide, potassium-sparing, spironolactone)

• Vasodilators (nitrates, phosphodiesterase-5 inhibitors such as sildenafil, hydralazine)

• Tricyclic antidepressants (e.g., amitriptyline)

• Alpha-1 receptor antagonists (e.g., prazosin)

• Alpha-2 receptor agonists (e.g., clonidine)

Neurogenic Causes

• Dysfunction of the autonomic nervous system (ANS), leading to impaired baroreflex response.

• Reduced sympathetic nervous system output (norepinephrine, epinephrine).

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

Populations Affected

• More common in the elderly due to impaired compensatory mechanisms and higher medication use.

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

Treatment of Orthostatic Hypotension

• Prevention: Maintain hydration, ensure access to fluids, limit heat exposure, avoid excessive alcohol, manage diuretic medications.

• Education: Advise patients to rise slowly, start from a sitting position, and move legs to promote 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 (hypoperfusion) insufficient to meet the metabolic needs of the body, resulting in cellular hypoxia.

• Typically, blood pressure is < 90 mm Hg.

• There is regulatory failure with insufficient compensatory mechanisms, leading to a shift from aerobic to anaerobic metabolism and potential organ failure.

Types of Shock

• Cardiogenic shock: Impaired cardiac contractility leads to decreased cardiac output.

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

• Septic shock: Proinflammatory cytokine-induced vasodilation.

• Anaphylactic shock: Histamine-induced vasodilation.

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

• Hypovolemic shock: Insufficient stroke volume due to low blood volume.

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

Shock Syndrome Characteristics

Key Terms and Formulas

• Cardiac Output (CO): The volume of blood the heart pumps per minute.

• 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.

Example: Clinical Application

A patient presents with dizziness and syncope upon standing. Blood pressure drops by 25 mm Hg systolic and 12 mm Hg diastolic. The patient is elderly, on diuretics, and has a history of diabetes. This presentation is consistent with orthostatic hypotension due to a combination of decreased intravascular volume and autonomic dysfunction.

Additional info: Some details, such as the full list of medications and the pathophysiology of each shock type, have been expanded for academic completeness.