2.1 Hypovolemic & Distributive Shock: Recognition, Pathophysiology & Fluid Resuscitation

Hypovolemic & Distributive Shock: Recognition, Pathophysiology & Fluid Resuscitation

Clinical Core: Recognize shock from inadequate tissue perfusion before hypotension develops. Restore circulation while identifying the cause. Volume loss, hemorrhage, sepsis, anaphylaxis, and spinal cord injury require different immediate actions.

Recognize compensated and hypotensive shock

Oxygen delivery ($DO_2$) depends on cardiac output ($CO$) and arterial oxygen content ($CaO_2$). Cardiac output is heart rate times stroke volume. With heart rate in beats/min and stroke volume in mL/beat, divide the product by 1,000 to express output in L/min. When arterial oxygen content is in mL/dL, convert flow from L/min to dL/min before multiplying:

$DO_2\;(\text{mL/min}) = 10\times CO\;(\text{L/min})\times CaO_2\;(\text{mL/dL})$

A child with a cardiac output of $2\text{ L/min}$ and arterial oxygen content of $15\text{ mL/dL}$ has a systemic oxygen delivery of $300\text{ mL/min}$. Falling stroke volume can reduce oxygen delivery even as tachycardia and peripheral vasoconstriction maintain an apparently acceptable blood pressure.

In compensated shock, look for tachycardia out of proportion to the clinical context, delayed capillary refill, cool or mottled skin, weak peripheral pulses, altered interaction, or falling urine output. Perfusion findings differ by cause. Some children with septic vasodilation have brisk refill and bounding pulses. Assess trends and use an appropriately sized cuff.

For a 4-year-old, $70 + (2\times4) = 78\text{ mmHg}$. A pressure of $74/42\text{ mmHg}$ together with lethargy, weak pulses, and delayed refill supports hypotensive shock. The threshold screens for low pressure, while the clinical findings establish impaired perfusion. Hypotension with impaired perfusion requires immediate action.

Replace nonhemorrhagic volume loss

Gastrointestinal losses, osmotic diuresis, and capillary leak can lower circulating volume. Use a cause-specific approach. For previously healthy children with severe dehydration from gastroenteritis, the CHOP emergency pathway gives an initial $20\text{ mL/kg}$ normal saline bolus. In hypotensive shock, establish rapid vascular access and deliver resuscitation promptly, with intraosseous access when IV access cannot be obtained rapidly. For DKA, extensive burns, or suspected cardiac dysfunction, follow the corresponding resuscitation pathway.

After each bolus, reassess heart rate, pressure, pulse quality, capillary refill, mental status, respiratory effort, and signs of congestion. New or worsening crackles, increasing respiratory support requirements, or new hepatomegaly raise concern for fluid intolerance. Pause additional boluses and reassess the mechanism and need for hemodynamic or respiratory support. Follow urine output over time.

Prioritize hemostatic resuscitation in traumatic hemorrhage

Obtain rapid hemorrhage control and activate the pediatric massive transfusion protocol early when indicated. The Pediatric Traumatic Hemorrhagic Shock Consensus Conference prioritizes blood products over crystalloids for children with traumatic hemorrhagic shock. Low-titer group O whole blood may be used if available under the local protocol. When using components, provide weight-based aliquots according to the blood bank protocol. The consensus suggests a high plasma-to-red-cell ratio near 1:1 and early attention to platelet replacement.

Track the response to resuscitation while arranging definitive bleeding control. Minimize heat loss and monitor temperature, acid–base status, coagulation, and ionized calcium. Citrate in blood products binds ionized calcium. Significant hypocalcemia can worsen myocardial performance and coagulation. Monitor and replace calcium using the pediatric massive transfusion protocol.

Worked distinction: A child with three days of diarrhea, no bleeding, and signs of hypotensive shock needs rapid isotonic fluid and reassessment. A child with similar perfusion findings after blunt abdominal trauma, a distended abdomen, and suspected ongoing hemorrhage needs early blood and a team able to control bleeding. Waiting for a preset crystalloid volume can delay definitive care.

Select vasoactive support in septic shock

Sepsis can cause vasodilation, capillary leak, and myocardial dysfunction in varying combinations. Warm extremities, bounding pulses, and a wide pulse pressure can suggest vasodilation. Cool, mottled extremities, weak pulses, and delayed refill can reflect poor peripheral perfusion. These are clues. The examination alone cannot establish the cardiac output or systemic vascular resistance, and the pattern can change during resuscitation. Cardiac and lung point-of-care ultrasound can help guide treatment where trained staff and resources are available.

In systems with intensive care availability, the 2026 pediatric Surviving Sepsis Campaign suggests up to $40–60\text{ mL/kg}$ of crystalloid in the first hour, in $10–20\text{ mL/kg}$ aliquots, for septic shock. Reassess after each bolus and stop when shock resolves or overload develops. Balanced or buffered crystalloid is conditionally preferred. Normal saline is suitable if balanced fluid is unavailable and can be preferred for some electrolyte or neurologic concerns. The Phoenix section covers the separate recommendations for systems without intensive care.

Start vasoactive support when indicated by the child's hemodynamics and response. The 2026 guideline found insufficient evidence to prefer epinephrine or norepinephrine universally, or to recommend starting before or after $40\text{ mL/kg}$. Choose and titrate based on suspected physiology, available assessments, and the local protocol. Start through suitable peripheral venous access when indicated while arranging central access if needed. Monitor the infusion site. Persistent shock on high-dose catecholamines may warrant further titration or vasopressin. Evidence is insufficient to routinely add an inodilator for persistent cardiac dysfunction.

Continue infection treatment and reassess perfusion, ventilation, fluid status, and other causes of shock. Track lactate in context when elevated. When central access is available, the guideline conditionally suggests targeting $ScvO_2\ge70\%$ in septic shock. Hydrocortisone is discouraged when fluids and vasoactive agents restore stability. For persistent instability despite adequate therapy, the guideline found insufficient evidence for or against hydrocortisone. Give stress-dose corticosteroids for suspected or documented adrenal insufficiency.

Treat anaphylactic shock immediately

Suspect anaphylaxis when acute respiratory or circulatory compromise follows a likely allergen, particularly with urticaria or angioedema. Epinephrine is the first-line treatment. Give $0.01\text{ mg/kg}$ IM using the $1\text{ mg/mL}$ solution into the anterolateral thigh. The U.S. allergy practice parameter describes a usual maximum of $0.3\text{ mg}$ in children and teenagers and $0.5\text{ mg}$ in adults. Repeat IM dosing every 5–15 minutes as needed. For a child weighing $24\text{ kg}$, the calculated dose is $0.24\text{ mg}$ or $0.24\text{ mL}$ of the $1\text{ mg/mL}$ solution.

Provide oxygen and airway support as needed and obtain vascular access for isotonic crystalloid if circulation is compromised. Inhaled bronchodilator can be added for persistent wheeze. Antihistamines can relieve itching or hives after epinephrine. Corticosteroids and antihistamines are unreliable for preventing biphasic anaphylaxis. For shock persisting after repeated IM epinephrine and fluids, obtain critical care help for a monitored IV epinephrine infusion per protocol. The concentrated IM preparation is dangerous as an IV push.

Recognize neurogenic shock after spinal cord injury

Cervical or high thoracic cord injury can interrupt sympathetic outflow. Loss of vascular tone leads to hypotension, with relative bradycardia or lack of the expected tachycardia and sometimes warm skin. The pattern supports neurogenic shock, but trauma patients can have both autonomic dysfunction and occult blood loss. Maintain spinal motion restriction, evaluate for hemorrhage, and support oxygenation and circulation.

If hypotension persists after addressing volume loss, use vasoactive support with the pediatric trauma and critical care team. Set an age- and injury-specific pressure goal with spine specialists. A pediatric multidisciplinary consensus proposed a mean arterial pressure of 80–90 mmHg for children aged six and older with moderate or severe acute spinal cord injury. This is expert consensus for that group. Younger children require individualized targets.