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.