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
