Physiology of Shock Flashcards
What is shock?
Condition of inadequate perfusion to sustain normal organ function
What are the five classes of shock?
Hypovolaemic, cardiogenic, obstructive, distributive, cytotoxic
What is cytotoxic shock?
Uncoupling of tissue oxygen delivery and mitochondrial oxygen uptake = CO poisoning, CN- poisoning
What is hypovolaemic shock?
Insufficient circulating volume = loss of circulating volume causes reduced preload and CO
What are some causes of hypovolaemic shock?
Bleeding, third space losses, severe dehydration (rare) = clinical features depend on degree of hypovolaemia
What are some compensatory mechanisms for hypovolaemia?
Baroreceptor reflexes
Sympathetic mediated neurohormonal response Capillary absorption of interstitial fluid
HPA axis response
Where are stretch sensitive receptors located?
Carotid sinus (CN IX) and aortic arch (CN X)
How does the baroreceptor reflex respond to decreased stretch?
Decreased afferent input to medullary CV centres = inhibition of parasympathetic (CN X) and enhanced sympathetic output
In what ways can the sympathetic nervous system increase cardiac output?
Via chronotropy and inotropy
How does the sympathetic neurohormonal response correct hypovolaemia?
Release of circulating vasoconstrictors to redirect fluid from peripheral and secondary organs
What effect does the redirection of fluids from peripheral organs have?
Causes lactic acidosis which drives chemoreceptors to enhance response
Circulating vasodilators also increased
What are some features of the capillary absorption of interstitial fluid to correct hypovolaemia?
Reduced capillary hydrostatic pressure
Inward net filtration
What occurs in the HPA axis response to hypovolaemia?
Intra-renal baroreceptors mediate renin release from JGA = resulting ang II enhances vasoconstriction and ADH secretion to enhance renal reabsorption of water and Na
What are the three ways the heart can increase cardiac output?
Increase heart rate
Increase stroke volume
Both of the above
What is the Frank Starling relationship?
Greater volume loading of ventricle during diastole results in greater ventricular ejection in systole
What effect does inotropy have on the Frank Starling curve?
Shifts curve up
What effect does a failing heart have on the Frank Starling curve?
Shifts curve down = has less contractility so EDV increases to maintain SV resulting in pulmonary congestion
What are some ways to ensure good fluid practice?
Treat them as a drug
Consider individual patient
Consider both fluid and electrolyte requirements
Consider difference between resuscitation and maintenance
What is cardiogenic shock?
Inability of the heart as a pump to meet circulatory demands = causes reduction in systolic function and cardiac output
What does cardiogenic shock commonly occur as a complication of?
Acute MI
What are some other causes of cardiogenic shock?
Acute valve dysfunction, myocarditis, cardiomyopathy, myocardial contusion
What are the clinical signs of cardiogenic shock?
Poor forward flow = hypotension, shock, fatigue, syncope
Backpressure = pulmonary oedema, elevated JVP, hepatic congestion
What is positive inotropy?
Increase in force of cardiac contraction for any given preload
How can positive inotropy be achieved?
Physiologically by sympathetic nervous system
Using beta and dopaminergic stimulation = dobutamine, adrenaline, dopamine, dopexamine
What is the purpose of the intra-aortic balloon pump?
Provides counterpulsation
What does an intra-aortic balloon pump do?
Inflates during ventricular diastole (augmented diastole)
Deflates during ventricular systole (reduced afterload)
What is obstructive shock?
Physical obstruction to filling of heart reducing preload and cardiac output = may be obstruction to heart or great vessels
What part of the cardiac cycle does obstructive shock affect?
Mainly affects cardiac filling (rather than cardiac ejection)
What are some causes of obstructive shock?
Pulmonary embolism, tamponade, tension pneumothorax
How is obstructive shock treated?
By treating the underlying cause:
PE = anticoagulation +/- thrombolysis
Tamponade = pericardial drainage
Tension pneumothorax = decompression + drainage
What are some x-ray signs of a massive PE?
Dilated hypokinetic right ventricle
Bowing of interventricular septum
Hyperkinetic right ventricular apex (McConnell’s sign)
What are some features of tamponade?
Fluid accumulation in the pericardial sac
Each chamber is compressed so impaired cardiac filling and contraction
What are some features of a tension pneumothorax?
Air trapped in pleural cavity under positive pressure to create one way valve
Lung collapses due to increased pressure
Why does a tension pneumothorax impair cardiac filling and function?
Due to the displacement of mediastinal structures
What are the other names for distributive shock?
Vasodilatory or warm shock
What occurs in distributive shock?
Significant reduction in SVR beyond the compensatory limits of increased cardiac output
What is the cardiac output like in distributive shock?
Initially high but insufficient to maintain forward perfusion
What are the three types of distributive shock?
Septic, anaphylactic and neurogenic
What is septic shock?
Bacterial endotoxin mediated capillary dysfunction
What indicates hypoperfusion in septic shock?
Rising lactate levels = usually before hypotension occurs
Why is quick administration of antibiotics important in septic shock?
Every hour delay of appropriate antibiotics increases mortality by 75%
Why are vasopressors used in septic shock?
Early use of vasopressors improves perfusion and minimises excessive fluid volumes
What causes anaphylactic shock?
Uncontrolled activation and degranulation of mast cells = release of histamine causes uncontrolled vasodilation
How does adrenaline act in anaphylactic shock?
As both a vasoconstrictor and mast cell stabiliser
How is anaphylactic shock diagnosed?
By measuring serum mast cell tryptase levels
What does neurogenic shock usually follow?
Spinal cord or central trauma
What are some features of neurogenic shock?
Hypotension follows loss of descending sympathetic tone
Inappropriate bradycardia generally occurs due to unopposed vagal tone
How is neurogenic shock treated?
Dopamine and vasopressors
What is the traditional view of how CPR works?
Physically pushes blood out of the heart to maintain vital organ perfusion
What is the more modern view of how CPR works?
Cyclical changes in intrathoracic pressure alternately push blood out of and suck blood into chest
What are the shockable rhythms?
Ventricular fibrillation and pulseless ventricular tachycardia
What are the non-shockable rhythms?
Asystole and pulseless electrical activity
How long does it take for significant cerebral damage to occur after cardiac arrest?
4-5 minutes
What are the reversible causes of cardiac arrest?
Hypovolaemia, hypoxia, hyperkalaemia (metabolic), hypothermia, tamponade, thrombosis, tension pneumothorax, toxins
