12.2 Hypovolemic, Cardiogenic & Anaphylactic Shock in Children

Hypovolemic, Cardiogenic & Anaphylactic Shock in Children

Circulatory shock in pediatric patients stems from diverse pathophysiological mechanisms requiring vastly different transport resuscitation strategies. While hypovolemic shock demands rapid volume expansion, cardiogenic shock can be dramatically exacerbated by fluid boluses, and anaphylactic shock requires immediate intramuscular catecholamines to arrest immune mast cell degranulation. Critical care transport clinicians must possess the diagnostic precision to differentiate these shock states rapidly at the bedside and initiate targeted, organ-sparing therapies.

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Hypovolemic Shock: Dehydration vs Hemorrhage

Hypovolemic shock is the most prevalent etiology of shock in children worldwide. The fundamental physiological defect is an absolute deficiency of intravascular blood volume, which leads to decreased venous return (preload) to the right atrium, reduced ventricular end-diastolic volume, diminished stroke volume, and a secondary drop in cardiac output ($CO = HR \times SV$).

Clinical Grading of Non-Hemorrhagic Dehydration

In gastroenteritis, diabetic ketoacidosis, or environmental heat illness, fluid loss depletes extracellular fluid compartments. Clinicians assess severity by calculating estimated percentage of body weight loss:

• Mild Dehydration (3%–5% in infants, 3%–4% in older children):
  • Physical findings: Alert mental status, slightly dry buccal mucosa, normal fontanelle, normal heart rate, normal blood pressure, capillary refill $<2\text{ seconds}$, and normal or mildly decreased urine output.
• Moderate Dehydration (6%–9% in infants, 6%–8% in older children):
  • Physical findings: Sunken anterior fontanelle, slightly sunken orbits, dry mucous membranes, marked resting tachycardia, decreased skin turgor (delayed skin pinch recoil), oliguria ($<1\text{ mL/kg/hr}$), cool extremities, and prolonged capillary refill (2 to 3 seconds).
• Severe Dehydration ($\ge 10\%$ in infants, $\ge 9\%$ in older children):
  • Physical findings: Deeply sunken fontanelle and orbits, dry parchment-like mucosa, absence of tears, profound lethargy or stupor, marked tachycardia with thready pulses, delayed capillary refill ($>3-4\text{ seconds}$), cool and mottled extremities, anuria, and progression to hypotension (decompensated shock).

Resuscitation Protocol for Dehydration

1. Emergent Volume Expansion: Administer 20 mL/kg of isotonic balanced crystalloid (Plasma-Lyte or Lactated Ringer's) via rapid push over 10 to 20 minutes. If shock signs persist, repeat 20 mL/kg boluses up to 40 to 60 mL/kg, reassessing heart rate, pulse quality, and capillary refill after each aliquot.
2. Deficit & Maintenance Calculation: Once systemic perfusion is restored, transition to continuous IV infusion. Calculate 24-hour maintenance fluids using the Holliday-Segar method:
   • $100\text{ mL/kg}$ for the first 10 kg of body weight
   • $50\text{ mL/kg}$ for the next 10 kg (11–20 kg)
   • $20\text{ mL/kg}$ for each additional kg above 20 kg
   • Add remaining deficit fluid (calculated as $\%\text{ dehydration} \times \text{weight (kg)} \times 10\text{ mL}$), replacing half over the first 8 hours and the remainder over the subsequent 16 hours.

Hemorrhagic Shock in Pediatric Trauma

Pediatric circulating blood volume averages 70 to 80 mL/kg. Consequently, acute loss of relatively small absolute blood volumes (e.g., 150 mL in an infant) represents massive hemorrhage (>20% blood volume):

• Class I Hemorrhage (<15% blood loss): Normal vital signs, mild anxiety.
• Class II Hemorrhage (15%–30% blood loss): Tachycardia, tachypnea, narrowed pulse pressure, delayed capillary refill, normal systolic blood pressure (compensated shock).
• Class III Hemorrhage (30%–40% blood loss): Marked tachycardia, tachypnea, oliguria, altered mental status, and hypotension (decompensated shock).
• Class IV Hemorrhage (>40% blood loss): Profound hypotension, barely palpable central pulses, obtundation, anuria, and imminent cardiac arrest.

Damage Control Resuscitation & Massive Transfusion Protocol

• Crystalloid Restriction: Clear crystalloid resuscitation in hemorrhagic trauma must be strictly limited to 20 to 40 mL/kg maximum. Infusing large volumes of crystalloid causes hemodilution of clotting factors and platelets, lowers core body temperature (inducing hypothermia), disrupts endothelial integrity, and dislodges early hemostatic clots ("popping the clot").
• Balanced Massive Transfusion (1:1:1 Ratio): If hypotension persists after an initial 20 mL/kg crystalloid bolus, or immediately in catastrophic hemorrhage, initiate balanced blood product resuscitation. Transfuse uncrossed Type O-negative (or type-specific) Packed Red Blood Cells (PRBCs), Fresh Frozen Plasma (FFP), and Platelets in a 1:1:1 ratio (pediatric protocols use balanced ratios, commonly between 1:1:1 and 2:1:1) (administering aliquots of 10 to 15 mL/kg each).
• Tranexamic Acid (TXA): Administer TXA—a synthetic lysine analogue that competitively inhibits plasminogen activation to prevent hyperfibrinolysis—to all pediatric trauma patients with severe hemorrhagic shock within 3 hours of injury:
  • Loading dose: 15 to 20 mg/kg IV (maximum 1,000 mg) infused over 10 minutes.
  • Maintenance infusion: 2 to 5 mg/kg/hr IV infused over the subsequent 8 hours.

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Cardiogenic Shock: Recognition, Pathophysiology & Management

Cardiogenic shock is characterized by primary intrinsic myocardial pump failure, resulting in inadequate cardiac output and cellular hypoperfusion despite normal or elevated intravascular volume (elevated preload).

Underlying Etiologies

• Acute viral myocarditis (Coxsackievirus B, Enterovirus, Adenovirus, Parvovirus B19)
• Dilated or hypertrophic cardiomyopathy
• Congenital heart disease (post-operative repair, critical aortic stenosis, ALCAPA)
• Severe prolonged dysrhythmias (sustained SVT or complete heart block)
• Severe myocardial contusion following blunt chest trauma

Pathophysiology & The "Fluid Trap"

In cardiogenic shock, depressed myocardial contractility prevents effective ventricular emptying. End-systolic and end-diastolic ventricular volumes rise dramatically, generating dangerously elevated ventricular filling pressures. These elevated pressures transmit backward into the pulmonary veins and right atrium, creating pulmonary capillary hypertension (pulmonary edema) and systemic venous congestion (hepatomegaly and jugular venous distension).

CRITICAL TRANSPORT HAZARD: The Fluid Trap in Cardiogenic Shock

In hypovolemic shock, volume expansion restores preload and cardiac output. In cardiogenic shock, however, the failing ventricle is already functioning at or beyond the peak of the Frank-Starling curve.

Administering a standard $20\text{ mL/kg}$ fluid bolus to a child with cardiogenic shock precipitously elevates pulmonary capillary wedge pressure above oncotic pressure, triggering acute alveolar pulmonary edema, severe hypoxemic respiratory failure, and rapid cardiovascular collapse.

Differentiating Clinical Profile

• Hepatomegaly: A firm, tender liver edge displaced $>2\text{ to }4\text{ cm}$ below the right costal margin.
• Auscultation: S3 or S4 gallop rhythm, holosystolic regurgitant murmurs, and bilateral pulmonary crackles (rales).
• Respiratory: Tachypnea, grunting, intercostal retractions, and orthopnea.
• Radiograph: Cardiomegaly (cardiothoracic ratio $>0.55$) and prominent pulmonary vascular congestion ("bat-wing" appearance).

Transport Management Protocol

1. Cautious Fluid Challenge: If intravascular volume status is uncertain, administer a test challenge of only 5 to 10 mL/kg of balanced crystalloid over 10 to 20 minutes (PALS), maintaining continuous auscultation of breath sounds and liver palpation. Abort immediately if respiratory status deteriorates or the liver edge descends.
2. Inotropic & Lusitropic Support:
   • Milrinone: A phosphodiesterase-3 (PDE-3) inhibitor that prevents degradation of intracellular cAMP in cardiomyocytes and vascular smooth muscle. It provides potent inotropy (contractility) and lusitropy (diastolic relaxation) while reducing systemic and pulmonary vascular resistance (afterload reduction). Initiate at 0.25 to 0.75 mcg/kg/min. Avoid IV bolus loading during transport to prevent sudden profound hypotension.
   • Dobutamine: Synthetic catecholamine with strong $\beta_1$-adrenergic inotropic activity and mild $\beta_2$ vasodilation. Infuse at 2 to 10 mcg/kg/min.
   • Low-Dose Epinephrine: (0.02 to 0.05 mcg/kg/min) indicated if cardiogenic shock is accompanied by severe hypotension refractory to milrinone, maintaining coronary perfusion.

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Anaphylactic Shock: Pathophysiology & Transport Resuscitation

Anaphylactic shock is an acute, life-threatening Type I IgE-mediated hypersensitivity reaction triggered by exposure to foreign antigens, such as foods (peanuts, tree nuts, cow's milk), insect venoms (Hymenoptera stings), medications (antibiotics, muscle relaxants), or natural rubber latex.

Allergen Exposure
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Cross-linking of Specific IgE on Mast Cells & Basophils
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Explosive Degranulation & Mediator Release (Histamine, Tryptase, Leukotrienes)
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  ┌────┴──────────────────────────────┬──────────────────────────────┐
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Massive Distributive Vasodilation   Endothelial Hyperpermeability   Bronchoconstriction & Angioedema
  │                                   │                              │
Profound Drop in SVR                Extravasation of 35% Blood Vol  Laryngeal Edema & Stridor
Severe Arterial Hypotension         Severe Distributive Hypovolemia  Severe Intractable Wheezing

Degranulation Cascade & Multi-System Collapse

When an allergen cross-links antigen-specific IgE bound to high-affinity $Fc\epsilon RI$ receptors on tissue mast cells and circulating basophils, explosive degranulation occurs. This releases preformed mediators (histamine, tryptase, chymase) and stimulates rapid de novo synthesis of leukotrienes ($LTC_4, LTD_4, LTE_4$), prostaglandin $D_2$, and platelet-activating factor:

• Cardiovascular Collapse: Histamine stimulates vascular $H_1$ and $H_2$ receptors, triggering nitric oxide release that induces massive systemic arterial vasodilation. Simultaneously, severe endothelial hyperpermeability allows up to 35% of intravascular blood volume to extravasate into extravascular tissues within 10 minutes, generating profound combined distributive and hypovolemic shock.
• Airway & Respiratory Failure: Leukotrienes and histamine stimulate bronchial smooth muscle contraction, inducing severe bronchospasm, mucous plugging, and lower airway wheezing. Mucosal edema of the pharynx, larynx, and vocal cords causes acute upper airway obstruction manifested by stridor, hoarseness, and asphyxiation.
• Cutaneous & Gastrointestinal Signs: Diffuse urticaria, intense pruritus, periorbital/lip angioedema, nausea, colicky abdominal cramping, vomiting, and diarrhea.

Transport Resuscitation Protocol for Anaphylaxis

1. First-Line Drug — Intramuscular Epinephrine:
   • Formulation: 1:1,000 concentration ($1\text{ mg/mL}$).
   • Dosing: 0.01 mg/kg ($0.01\text{ mL/kg}$ of 1:1,000). Maximum single dose is 0.3 mg in prepubertal children and 0.5 mg in adolescents.
   • Anatomical Site: Administer intramuscularly into the mid-anterolateral thigh (vastus lateralis). The superior vascularity of the vastus lateralis achieves peak plasma epinephrine concentrations within 8 minutes—significantly faster and higher than deltoid or subcutaneous routes.
   • Frequency: Repeat every 5 to 15 minutes as clinically indicated for persistent symptoms or refractory hypotension.
2. Aggressive Volume Expansion: Rapidly infuse 20 mL/kg balanced crystalloid boluses under pressure to replace massive third-space extravasation and fill the dilated vascular bed. Repeat as needed up to 40 to 60 mL/kg.
3. Continuous Epinephrine Infusion for Refractory Anaphylaxis: If shock or bronchospasm persists despite repeated IM injections and fluid boluses, initiate continuous intravenous Epinephrine at 0.05 to 0.3 mcg/kg/min titrated to blood pressure and respiratory ease.
4. Second-Line Adjunctive Medications (Never Delay or Substitute for Epinephrine):
   • $H_1$-Antihistamine: Diphenhydramine $1\text{ mg/kg}$ IV/IM (max 50 mg) for urticaria and pruritus relief.
   • $H_2$-Antihistamine: Famotidine $0.25\text{ to }0.5\text{ mg/kg}$ IV (max 20 mg) to block cardiac histamine receptors.
   • Systemic Corticosteroids: Methylprednisolone $1\text{ to }2\text{ mg/kg}$ IV (max 60–125 mg) or Dexamethasone $0.3\text{ to }0.6\text{ mg/kg}$ IV. Corticosteroids take hours to act. They are often given, but evidence that they prevent biphasic reactions (recurrence after apparent resolution, reported in roughly 5% of cases and usually within about 12 hours) is weak.
   • Nebulized Bronchodilators: Albuterol ($2.5\text{ to }5\text{ mg}$) and Ipratropium for refractory bronchospasm.
5. Proactive Airway Management: If stridor, severe lingual swelling, or voice alteration develops, perform early endotracheal intubation before progressive angioedema obliterates the glottic opening. Prepare an endotracheal tube $0.5\text{ to }1.0\text{ mm}$ smaller than calculated for age, have video laryngoscopy ready, and prepare a needle cricothyrotomy setup at the bedside.

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Comparison Table: Differentiating Pediatric Shock Types

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Realistic Transport Scenario: Pediatric Anaphylaxis in Transit

A transport team is dispatched to a summer camp to retrieve an 8-year-old male (weight 26 kg) who was stung by multiple yellow jackets. Within 10 minutes, the child developed generalized urticaria, facial swelling, hoarseness, and inspiratory stridor, collapsing onto the ground. On crew arrival, the patient is lethargic, displaying prominent intercostal retractions, loud stridor, and diffuse wheezing. Vital signs: heart rate 168 bpm, respiratory rate 42 breaths/min, blood pressure 72/40 mmHg (mean arterial pressure 51 mmHg; minimum acceptable systolic blood pressure is $70 + [2 \times 8] = 86\text{ mmHg}$, confirming decompensated anaphylactic shock), and SpO2 86% on room air.

The transport nurse immediately injects 0.26 mg of Epinephrine 1:1,000 (0.26 mL) intramuscularly into the right mid-anterolateral thigh and applies 100% FiO2 via non-rebreather mask. While the flight paramedic establishes a proximal tibial IO line, 520 mL (20 mL/kg) of Plasma-Lyte is rapidly infused under pressure. Five minutes post-injection, the child remains hypotensive (BP 76/44 mmHg) with persistent inspiratory stridor. A second dose of 0.26 mg Epinephrine 1:1,000 IM is injected into the left anterolateral thigh, and a continuous IV Epinephrine infusion is initiated at 0.1 mcg/kg/min. Methylprednisolone (50 mg IV) and Famotidine (13 mg IV) are administered via the IO line.

Within 4 minutes of the second epinephrine dose, the stridor completely resolves, wheezing diminishes, heart rate slows to 124 bpm, and blood pressure rebounds to 96/58 mmHg (MAP 71 mmHg). The child becomes fully alert and speaks in full sentences, successfully averting emergency surgical airway placement during subsequent rotor transport to the pediatric trauma center.

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Clinical Pearls for Pediatric Shock Forms