Shock: The Physiologic Perspective

Bryan E. Bledsoe, DO, FACEPAdjunct Associate Professor of Emergency Medicine

The George Washington University Medical CenterWashington, DC

Shock

A “rude unhinging” of the machinery of life.

Samuel Gross (1862)

Shock

Shock is inadequate tissue perfusion.

Cellular Requirements

Oxygen Glucose

All cells need oxygen and glucose to survive. In shock, cells are not getting enough oxygen and/or glucose.

Cellular Requirements

Proteins Carbohydrates Lipids

Glucose

Cells can break down proteins, carbohydrates and lipids to create glucose

Glucose is then broken down in the mitochondria to generate ATP

and the energy required by the cell to function

The “Mighty Mitochondria” are the power houses of the cells.

Basically, what the diagram at the left shows is how glucose is broken

down into Pyruvate, which generates ATP

(Adenosine Triphosphate)

ATP is the universal energy packet cells use to

drive all metabolism.

The chemical reaction within the mitochondria

The Kreb’s Cycle

The electron transport chain within the mitochondria

ATP

Cellular Requirements

OxygenRequired for the majority of energy production derived from Kreb’s Cycle and Electron Transport Chain.Metabolism with Oxygen = Aerobic MetabolismMetabolism without Oxygen = Anaerobic Metabolism

Oxygen Transport

Oxygen Transport:Hemoglobin-bound (97%)Dissolved in plasma (3%)

Monitoring:Hemoglobin-bound (SpO2)Dissolved in plasma (pO2)

Oxygen Transport

Carbon Dioxide Transport

As oxygen is delivered to the cell, carbon dioxide (CO2) is transported away. Mostly by reacting with water to form

H2CO3.

Oxygen Delivery

DO2 = Normal Oxygen DeliveryDO2 = Q X CaO2

DO2 = Q X (1.34 X Hb X SpO2) X 10Normal DO2 is 520 to 570 mL/minute/m2

This is the normal amount of oxygen delivered to tissues in a patient not in shock

Clinical Correlation

DO2 = Q X (1.34 X Hb X SpO2) X 10What factors can affect oxygen delivery to

the tissues?Cardiac Output (Q)

Available Hemoglobin (Hb)Oxygen Saturation (SpO2)

The higher the cardiac output, hemoglobin, and oxygen saturation, the more oxygen delivered to the

tissues. A shortage of any of these three can result in shock.

Oxygen Uptake

VO2 = Q X 13.4 X Hb X (SpO2-SvO2)

Oxygen uptake is the amount of oxygen the body is absorbing from the lungs. It is affected by…

Q Cardiac output (the amount of blood the heart is pumping per minute)

Hb Hemoglobin Level

SpO2 Arterial Oxygen Saturation

SvO2 Venous Oxygen Saturation

Oxygen Extraction Ratio

O2ER = VO2 / DO2 X 100

Normal O2ER = 0.2-0.3 (20 to 30%)

Oxygen extraction ratio is the percent of oxygen the body is using, compared to the

amount delivered.

Metabolic Demand

MRO2 :1. The metabolic demand for oxygen at the tissue level.2. The rate at which oxygen is utilized in the conversion of glucose to energy and water through glycolysis and Kreb’s cycle.

Shock

VO2 ≥ MRO2 = Normal Metabolism

VO2 < MRO2 =

In other words, when the oxygen extracted is less than is needed to meet the metabolic demand of the body, Shock

results.

Shock

Causes of Shock:Inadequate oxygen delivery

Inadequate respiration and oxygenationInadequate hemoglobinInadequate fluid in the vascular systemInadequate blood movement

Impaired oxygen uptake

Shock

Causes of Shock:Inadequate nutrient delivery

Inadequate nutrient intakeInadequate nutrient deliveryInadequate fluid in the vascular systemInadequate blood movement

Impaired nutrient (glucose) uptake

Shock

Causes of Shock:Inadequate oxygen delivery

Inadequate respiration and oxygenation• Respiratory failure (mechanical, toxins)

Inadequate hemoglobin• Hemorrhage or anemia

Inadequate fluid in the vascular system• Hemorrhage or fluid loss (burns, vomiting, diarrhea, sepsis)

Inadequate blood movement• Cardiac pump failure

Impaired oxygen uptakeBiochemical poisoning (hydrogen cyanide)

Shock

Impaired oxygen uptakeCyanide:

Inhibits metal-containing enzymes (i.e., cytochrome oxidase)Halts cellular respiration

Shock

Causes of Shock:Inadequate nutrient delivery

Inadequate nutrient intake• Malnutrition, GI absorption disorder

Inadequate nutrient delivery• Malnutrition, hypoproteinemia

Inadequate fluid in the vascular system• Hemorrhage, fluid loss (burns, vomiting, diarrhea)

Inadequate blood movement• Cardiac pump failure

Impaired nutrient (glucose) uptakeLack of insulin (Diabetes Mellitus)

Shock (Types)

HemorrhagicRespiratoryNeurogenicPsychogenicCardiogenicSepticAnaphylacticMetabolic

Shock (Classifications)

Physiological classifications better describe underlying problem:

Cardiogenic ShockHypovolemic ShockDistributive Shock

Spinal ShockSeptic ShockAnaphylactic

Shock

The pathway to shock follows a common metabolic pattern.

In other words, shock is caused by either impaired oxygen use or impaired glucose use. Decreased tissue perfusion causes both.

This is anaerobic metabolism. It produces far less ATP per glucose molecule and generates lactate (lactic acid) as a byproduct.

Cardiogenic Shock

The heart cannot pump enough blood to meet the metabolic demands of the body.

Cardiogenic Shock

Loss of contractility:AMILoss of critical mass of left ventricleRV pump failureLV aneurysmEnd-stage cardiomyopathyMyocardial contusionAcute myocarditisToxic global LV dysfunctionDysrhythmias/heart blocks

Mechanical impairment of blood flow:

Valvular diseaseAortic dissectionVentricular septal wall ruptureMassive pulmonary embolusPericardial tamponade

As cardiac output declines, the body and heart try to compensate. If compensation is inadequate, or cardiac

outputs declines to a critical level, shock results.

Hypovolemic Shock

Fluid (blood or plasma) is lost from the intravascular space.

Hypovolemic Shock

Trauma:Solid organ injuryPulmonary parenchymal injuryMyocardial laceration/ruptureVascular injuryRetroperitoneal hemorrhageFracturesLacerationsEpistaxisBurns

GI Tract:Esophageal varicesUlcer diseaseGastritis/esophagitisMallory-Weiss tearMalignanciesVascular lesionsInflammatory bowel diseaseIschemic bowel diseaseInfectious GI diseasePancreatitis

Hypovolemic Shock

GI Tract:Infectious diarrheaVomiting

Vascular:AneurysmsDissectionsAV malformations

Reproductive Tract:Vaginal bleeding

MalignanciesMiscarriageMetrorrhagiaRetained products of conceptionPlacenta previa

Ectopic PregnancyRuptured ovarian cyst

Lack of fluid or blood, if severe enough, results in shock.

Neurogenic Shock

Interruption in the CNS connections with the periphery (spinal cord injury).Form of distributive shock.

Neurogenic Shock

Spinal cord injurySpinal anesthetic

The Sympathetic Nervous system

Neurogenic Shock

BP = CO X PVR(Blood Pressure = cardiac output X systemic Vascular Resistance)

CO = HR X SV(Cardiac Output =Heart Rate X Stroke Volume)

Stroke volume is the amount of blood the heart pumps per beat

BP = (HR X SV) X PVRSo…

Blood Pressure=Heart rate X Stroke Volume X Peripheral Vascular Resistance

In Neurogenic shock, PVR drops and so does blood pressure

Neurogenic Shock

Anaphylactic Shock

Shock resulting from widespread hypersensitivity.Form of distributive shock.

Killer Bee

Anaphylactic Shock

Drugs:Penicillin and related antibioticsAspirinTrimethoprim-sulfamethoxazole (Bactrim, Septra)VancomycinNSAIDs

Other:Hymenoptera stingsInsect parts and moldsX-Ray contrast media (ionic)

Foods and Additives:ShellfishSoy beansNutsWheatMilkEggsMonosodium glutamateNitrates and nitritesTartrazine dyes (food colors)

In other words, anaphylaxis leads to fluid loss AND vasodilation, which causes decreased cardiac output and therefore decreased

tissue perfusion = SHOCK

Septic Shock

Component of systemic inflammatory response syndrome (SIRS).Form of distributive shock.

Septic Shock

Patient has nidus of infection.Causative organism releases:

EndotoxinToxic shock syndrome toxin-1Toxin A (Pseudomonas aeruginosa)

Structure ComponentsTeichoic acid antigenEndotoxin

Activates immune system cascade

Sepsis causes a large cascade of problems. Bottom line, it causes

vasodilation and hypovolemia which results

in SHOCK.

Stages of Shock

CompensatedThe body’s compensatory mechanisms are able to maintain some degree of tissue perfusion.

DecompensatedThe body’s compensatory mechanisms fail to maintain tissue perfusion (blood pressure falls).

IrreversibleTissue and cellular damage is so massive that the organism dies even if perfusion is restored.

Clinical Findings

What is the first physiological factor in the development of shock?VO2 < MRO2

Oxygen uptake is less than metabolic needs

So, what are the first symptoms you would expect to find?

↑ respiratory rate↑ heart rate

Clinical Findings

What is often the second physiological response to the development of shock?Peripheral vasoconstrictionWhat symptoms would you expect to see?

pale skincool skinweakened peripheral pulses

Clinical Findings

As shock progresses, what physiological effects are seen?End-organ perfusion fallsWhat symptoms would you expect to see?

altered mental statusdecreased urine output

Clinical Findings

As compensatory mechanisms fully engage, what signs and symptoms would you expect to see?

tachycardiatachypneapupillary dilationdecreased capillary refillpale cool skin

Clinical Findings

When compensatory mechanisms fail, what signs and symptoms would you expect to see?

hypotensionfalling SpO2

bradycardialoss of consciousnessdysrhythmiasdeath

Cardiogenic Shock

Treatment:OxygenMonitorsNitrates (if possible)Morphine or fentanylPressor support (dopamine or dobutamine)If no pulmonary edema, consider small fluid bolusesIABP (Intra Aortic Balloon Pump)Definitive therapy (fibrinolytic therapy, PTCA, CABG, ventricular assist device, cardiac transplant)

Hypovolemic Shock

Treatment:OxygenSupine positionMonitorsIV accessFluid replacementPressor support (rarely needed)Correct underlying cause

Hypovolemic Shock

Fluid replacement:Hypovolemia:

Isotonic crystalloidsColloids

Hemorrhage:Whole bloodPacked RBCs (Red Blood Cells)HBOCs (Hemoglobin Based Oxygen Carriers. I.e. Artificial Blood)Isotonic Crystalloids

Hypovolemic Shock

Caveat:If shock due to trauma, and bleeding cannot be controlled, give only enough small fluid boluses to maintain radial pulse (SBP≈ 80 mm Hg).If bleeding can be controlled, control bleeding and administer enough fluid or blood to restore normal blood pressure.

Neurogenic Shock

Treatment:ABCDEFluid resuscitation with crystalloidPA catheter helpful in preventing overhydration.Look for other causes of hypotensionConsider vasopressor support with dopamine or dobutamineTransfer patient to regional spine center

Anaphylactic Shock

Treatment:Airway (have low threshold for early intubation)Oxygenation and ventilationEpinephrine (IV, IM, Subcutaneously)IV Fluids (crystalloids)Antihistamines

BenadrylZantac

SteroidsBeta agonistsAminophyllinePressor support (dopamine, dobutamine or epinephrine)

Septic Shock

Treatment:Airway and ventilatory managementOxygenationIV fluids (crystalloids)Pressor support (dopamine, norepinephrine)Empiric antibioticsRemoval of source of infectionNaHCO3?Steroids?Anti-endotoxin antibodies

Shock Treatments

Not supported by clinical evidence:MAST/PASGHigh-dose steroids for acute SCITrendelenburg position

Less important than formerly thought:Pressure infusion devicesIO access

Summary

To understand the shock, you must first understand the pathophysiology.Once you understand the pathophysiology, then recognition of the signs and symptoms and treatment becomes intuitive.