Understanding Shock: Definitions and Fluid Dynamics

Definitions of Shock

• Oxford defines shock as a sudden upsetting or surprising event.
• Wikipedia describes shock as insufficient blood flow to bodily tissues due to circulatory system problems. (Circulatory Shock)
• ATLS defines shock as an abnormality in the circulatory system leading to inadequate organ perfusion and tissue oxygenation
• At a cellular level, shock happens when oxygen demand is greater than the supply, resulting in anaerobic metabolism, lactic acid production, and metabolic acidosis.
• Circulatory shock can be reversed in its early stages but delays can cause irreversible changes, multi-organ failure (MOF), & death.
• Undifferentiated shock is diagnosed when shock is identified, but the underlying cause remains unknown.

Intravascular Fluid and Third Spacing

• In an average 70 kg man, there are 42 liters of total body water
• The intracellular fluid volume is around 28 liters.
• The extracellular fluid volume is around 14 liters.
• 9.5 liters make up the interstitial fluid volume..
• 3.5 liters make up the plasma volume.
• 1 liter is the transcellular fluid volume.
• Fluid shifts occur when fluids move between body compartments because of combined hydrostatic and osmotic pressure gradients.
• The first (intravascular) and second (interstitial) spaces are constituents of the ECF, normal physiological compartments

Blood Volume Parameters

• Adults have a blood volume of about 7% of their body weight.
• A 70 kg person typically has 5 liters of blood.
• Obese adults' blood volume is estimated based on their ideal body weight.
• Pediatrics patients’ blood volume is 8-9% of their body weight, approximately 70-80 mL / kg.
• About 70% of blood resides in the venous system.
• "Third space" refers to an area other than ICF and ECF e.g. peritoneal cavity (ascites), pleural cavity (pleural effusion), lumen of the gastrointestinal tract, tissues swelling after surgery/trauma.
• Fluid shifting from intravascular space to the interstitial or third space can cause hypotension, reduced CO2, and edema.

Hypovolemic Shock

• Hypovolemic shock is defined as a decrease in intravascular volume, and has two broad subtypes:
  • Hemorrhagic
    • Trauma
    • GIT bleeding
    • Vascular etiologies include aortoenteric fistula or ruptured aortic aneurysm.
    • Spontaneous bleeding results from bleeding tendencies.
  • Non-hemorrhagic
    • GIT losses due to vomiting, diarrhea, NG suction, or drains.
    • Renal losses occurs from medication-induced diuresis or endocrine disorders like hypoaldosteronism.
    • Skin/insensible losses from burns, heatstroke, pyrexia.
    • Third-space loss is possible from pancreatitis, cirrhosis, intestinal obstruction, or trauma.

Hemorrhagic Shock Management

• Management includes assessing airway, breathing, circulation, disability, exposure and stopping bleeding.
• Methods to control bleeding include local pressure application, usage of pneumatic garments, tourniquets, angio-embolization, surgical or endoscopic intervention.
• Damage control resuscitation includes "Permissive hypotensive protocols" and "Massive blood transfusion protocol" and to pay attention to response and lethal triad, and be ready for "?Undifferentiated shock".

Clinical Signs of Hypovolemic Shock

• Rapid, weak pulse alongside tachycardia arises from reduced blood flow.
• Vasoconstriction causes sweaty and cool skin, especially in the extremities.
• Rapid and shallow breathing results from sympathetic nervous system stimulation and acidosis.
• Hypothermia stems from decreased perfusion and sweat evaporation.
• Fluid depletion leads to a thirst and dry mouth.
• Additional signs include narrowed pulse pressure, pallor, and flattened jugular venous pulsations.
• Decreased urine output, with dark and concentrated urine.
• Hypotension is a later sign, with the other signs coming earlier.
• Mental status changes

Classifications of Hemorrhagic Shock

• Class I hemorrhage involves less than 15% bloodloss, normal blood pressure and pulse pressure, and no change to urine output or mental status.
• Class II (mild) hemorrhage shows approximate blood volume loss of 15-30% which is a slightly increased heart rate and respiratory rate.
• Class III (moderate) hemorrhage shows a blood loss of 31-40%, a decrease in blood pressure, urine output and Glasgow Coma Scale score.
• Class IV (severe) hemorrhage means over 40% blood loss and includes signs such as marked tachycardia (↑↑), significantly decreased blood pressure (↓), urine output (↓↓) and Glasgow Coma Scale score (↓)

Cardiogenic Shock

• Cardiogenic shock is due to cardiac causes that reduce cardiac output and systemic hypo-perfusion.
• Cardiomyopathies
• Acute myocardial infarction
• Congestive heart failure
• Severe bradycardia or heart block
• Arrhythmias
• Cardiac arrest
• Myocarditis.
• Valvular diseases
• Myocardial contusion
• The clinical signs of cardiogenic shock are different from those of hypovolemic shock.
• Signs include elevated jugular venous pulsations.
• Narrow pulse pressure
• Indicative cardiac disease signs: chest pain, lung crackles (pulmonary congestion), arrhythmias, or weak/absent pulse.

Distributive Shock

• Distributive shock is caused by dilatation of blood vessels and pooling of blood.
• Results in reduced venous return, cardiac output, and circulatory failure.
• Potential causes include:
  • Systemic infection (septic shock)
  • Severe allergic reaction (anaphylaxis)
  • Neurogenic shock
  • Endocrine shock

Septic Shock

• Septic shock, a common cause of distributive shock, presents with an abnormal body temperature, warm skin, and a response to vasopressors
• It involves severe circulatory, cellular, and metabolic abnormalities which lead to tissue hypoperfusion.
• Septic shock can lead to Systemic Inflammatory Response Syndrome (SIRS) and multi-organ failure.

Anaphylactic Shock

• Anaphylactic shock is a severe hypersensitivity reaction mediated by immunoglobulin E (Ig-E) and histamine releases.
• This causes widespread vasodilation, leading to hypotension and increased capillary permeability, cardiovascular collapse, and respiratory distress from bronchospasm.
• Immediate hypersensitivity reactions occur within seconds to minutes after exposure to an inciting antigen.
• Common allergens are drugs (e.g., antibiotics, NSAIDs), food, insect stings, and latex.
• Clinical signs of anaphylactic shock are a history of exposure to common allergens.
• Skin changes like hives, itching, and urticaria.
• Wheezing, inspiratory stridor, and shortness of breath
• Additional signs include abdominal pain, diarrhea, and vomiting.
• Lightheadedness, confusion, headaches, and loss of consciousness also appear.

Neurogenic Shock

• Neurogenic shock is a shock state caused by imbalance between sympathetic and parasympathetic regulation of cardiac/vascular smooth muscle action.
• Profound vasodilation and drop in heart rate arises from the imbalance.
• Causes include direct injury to circulatory regulation centers (brainstem trauma) or altered afferents (dysregulated vagal reflexes).
• Interruption of descending connections to the spinal cord can also trigger neurogenic shock.
• Clinical indicators are a sudden drop of SAP below 100 mmHg and heart rate below 60/min, a slow heart rate with a loss of cardiac sympathetic tone and warm skin from the peripheral blood vessels.
• Tachycardia/cutaneous vasoconstriction/narrowed pulse pressure are usually absent.

Endocrine Shock

• Endocrine shock is not officially a shock subcategory but severe endocrine disturbances can result in shock.
• Common causes of endocrine shock:
  • Hypothyroidism (myxedema)
  • Thyrotoxic crisis
  • Acute adrenal insufficiency (Addisonian crisis)
  • Relative adrenal insufficiency in critically ill patients with insufficient hormone levels.

Obstructive Shock

• Obstructive shock stems from extracardiac causes.
• Causes include cardiac tamponade, constrictive pericarditis, and tension pneumothorax..
• Other causes are pulmonary embolism, severe pulmonary hypertension, and aortic stenosis.
• An additional cause is hypertrophic sub-aortic stenosis, where ventricular muscle occludes the ventricular outflow tract.
• Abdominal compartment syndrome (intra-abdominal pressure > 20 mmHg) can causes organ dysfunction due to increase pressure reducing blood flow back to the heart.

Clinical Stages of Shock

• Pre-shock (compensated shock) is marked by compensatory mechanisms to counter reduced tissue perfusion.
  • Mechanisms include tachycardia, peripheral vasoconstriction, and blood pressure changes.
• Manifested shock sees classic signs and symptoms appear as early organ dysfunction emerges with compensatory mechanisms becoming insufficient.
• End-organ dysfunction represents the final stage and leads to multi-organ failure and death.

Commonality and Statistics of various shocks

- Distributive shock is the most common type of shock.
- Followed by hypovolemic and cardiogenic shock.
- Obstructive shock is relatively less common.
- Septic shock is the most common type of distributive shock and it has mortality rate between 40-50%.
- In the United States 1.2 million cases of shock each year in the emergency room, and their risk of death is between 20 and 50%.