Haemorrhage, Fainting, Exercise Flashcards
What is haemorrhage?
Severe blood loss
What does haemorrhage cause?
Hypovolaemia
What happens in hypovolaemia?
Reduction in venous return
Reduction in cardiac output
Reduction in MAP
Reduction in prefusion of regional circulations (circulatory shock)
What kind of shock is present if there is a pressure greater than 45mmHg?
Nonprogressive/compensatory shock
What happens during compensatory shock?
Bodily compensatory mechanism alone cause full recovery
What happens if pressure drops below 45mmHg?
Progressive shock which worsens until death without therapy
What will trigger sympathetic reflexes initiated by arterial baroreceptors and other vascular stretch receptors?
Initial haemorrhage and drop in MAP
What happens as a result of baroreceptor and vascular reflexes?
General systemic arteriolar constriction increases total peripheral resistance
Veins and venous reservoirs constrict; helps to maintain adequate venous return despite diminished blood volume
Heart rate increases to ~180bpm
When does the intensity of baroreceptor reflex plateau?
MAP<60mmHg
What will inadequate blood flow stimulate?
Strongly stimulates peripheral chemoreceptors
What does the chemoreceptor reflex do?
Increases rate and depth of respiration promoting venous return by reducing intrathoracic pressure
When does MAP plateau for a 2nd time?
~50mmHg
What causes the second plateau in MAP?
Activation of CNS ischaemic response resulting from increased PCO2 and decreased PO2
What does the CNS ischaemic response cause?
Extreme stimulation of the SNS which results in pronounced vasoconstriction and increased cardiac contractility
What decreases capillary hydrostatic pressure?
Reduced MAP
Increased TPR
Reduced venous pressure
What does decreased capillary hydrostatic pressure promote?
Reabsorption of large quantities of interstitial fluid
Why does colloid osmotic pressure of plasma decrease?
Dilution of blood by tissue fluid
When does the concentration of adrenaline in plasma increase greatly?
MAP of ~40mmHg
What is a potent vasoconstrictor?
Vasopressin/ADH
What stimulates the release of vasopressin?
Sensory input from baroreceptors and other vascular stretch receptors
What stimulates the release of vasopressin?
Sensory input from baroreceptors and other vascular stretch receptors
What does a reduction in MAP cause renally?
Reduces renal glomerular filtration rate
Increases renal sympathetic nerve activity
ADH promotes renal water reabsorption
What does increased renal sympathetic nerve activity do?
Increased production of angiotensin II stimulating the release of aldosterone
Increases salt and water reabsorption in the nephrons
How long after haemorrhage are sympathetic reflexes maximally activated?
30-60 sec after haemorrhage
What do the reflexes allow in regard to suvival?
Full recovery from blood loss of ~30-40%
How long does it take for angiotensin and vasopressin mechanisms to fully respond?
10-60 minutes
How long does the readjustment of blood volume by absorption of interstitial fluid take?
1hr to 48hrs
What is progressive shock?
Positive feedback mechanisms that further reduce cardiac output - decompensatory mechanisms
What dictates the outcome of a patient in progressive shock?
Relative strength of compensatory/decompensatory mechanisms
Describe the vicious cycle of cardiac failure.
Reduction in ventricular function leads to
Reduction in cardiac output leads to
Reduction in coronary blood flow resulting in
Reduction in ventricular function
Describe the vicious cycle of increased tissue hypoxia.
Decreased tissue blood supply leads to
Increased tissue hypoxia leads to
Increased vasodilatory metabolites leads to
Reduction in total peripheral resistance leads to
Decreased MAP which
Reduces tissue blood supply
How does acidosis work?
Reduced O2 delivery increases cellular production of lactate and impaired kidney function slows excretion of protons
Further metabolic acidosis further depresses cardiac function and reduces vasoconstriction by decreasing sensitivity to noradrenaline
How does CNS depression work?
Reduction in cerebral perfusion depresses activity of cardiovascular control centres
Further reduction in sympathetic outflow
What causes blood agglutination?
Acidosis
What does blood agglutination result in?
Minute blood clots leading to very small plugs in small vessels (sludged blood)
What can acidosis do once blood agglutination has began?
Cause cellular release of thromboxane A which promotes further platelet aggregation
What is endotoxin release?
The phagocytic activity of macrophages which usually detoxify endotoxins in the liver is depressed by shock and thereby increases endotoxins level in blood
This can lead to widespread vasodilation (septic shock) and depress cardiac function
What is syncope?
Sudden loss of consciousness caused by insufficient supply of O2 to neuronal cells in brain
What are most synoptic episodes caused by?
Hypoxia or rapid hypotension (MAP not sufficient to perfuse cerebral circulation)
What is the most common form of syncope and what is it caused by?
Vasovagal syncope
Triggers that stimulate the NTS resulting in activation of the vagal centre which reduces heart rate
Inhibition of spinal sympathetic nerves reducing vasoconstrictor tone
Rapid fall in MAP reducing blood flow to brain
What are some triggers of vasovagal syncope?
Heat exposure
Sight of blood/having blood drawn
Intense fear or emotional shock
Where do the triggers usually originate in?
Cerebral cortex
What is orthostatic hypotension?
Sudden, but transient drop, in MAP caused by rapid movement from supine to standing position - caused by gravity
What happens during orthostatic hypotension?
Decreased venous return leads to a reduction in cardiac output which results in a lower MAP
When might syncope happen with orthostatic hypotension?
Chronic hypotension
Hypovolaemia
Medication - alpha 1 blockers
What usually restores the normal MAP?
Baroreceptor reflex
Where do cardiovascular and respiratory control centres receive input from?
Muscular mechanoreceptors
Baroreceptors
Vascular and muscular chemoreceptors
What does anticipation/initiation of exercise do?
Inhibits vagal impulses to the heart and decreases sympathetic discharge
Results in increases in myocardial contractility and cardiac output, and tachycardia
What does functional sympatholysis do?
Prevents constriction in cerebral and coronary circulations
What is there an increased concentration of in circulation?
Increased catecholamines enhancing effects of SN stimulation
What is active hyperaemia?
Increased blood flow due to increased tissue activity
How does CO increase during exercise?
Tachycardia
Why does Stroke volume only increase a little during exercise?
Tachycardia reduces filling time and decreases EDV
What effect does exercise have on venous return and what causes this?
Increased venous return due to:
SNS induced vasoconstriction of venules
Contracting skeletal/respiratory muscles pumps blood
Increased depth/rate of ventilation decreases intrathoracic pressure, increasing blood flow into thoracic cavity (increased delta P)
What effect does intense exercise using few muscles have on TPR?
Large increase in TPR as only the working muscles are vasodilated
What effect does a whole-body exercise have on TPR?
TPR decreases during exercise as there is vasodilation in large masses of active muscle
Is MAP increased during exercise?
Greatly increased in weightlifting
Slight increase in whole-body exercise as CO still predominates
What does an increase in MAP increase?
Perfusion pressure into skeletal muscle
Blood vessel diameters
Does exercise cause a greater increase in systolic or diastolic pressure?
Systolic as there is an increased SV
What happens when the exercise is finished?
HR and CO are quickly reduced
Accumulation of vasodilatory metabolites keeps TPR low
MAP may fall briefly but is corrected by baroreceptor reflex
What is VO2?
Volume of O2 consumed by the body per minute during exercise
What is the equation for VO2?
VO2 = HR x SV x Arteriovenous O2 difference (PaO2 - PvO2)
What name is given to O2 consumption at maximum exercise intensity?
VO2 Max
What is the resting VO2?
3.5ml/min/Kg
What falls as additional O2 is consumed by contracting muscle?
PO2 levels
What is unloading of O2 facilitated by?
Right shift of oxyHgb curve
Increase in temperature
Increase in [2,3 DPG]
Increase in PCO2
Decrease in pH
What stops an increase in exercise intensity?
Maximum pumping capacity of heart is attained -determines VO2 max in fit individuals
What happens once the VO2 max has been attained and what is this called?
Anaerobic respiration increases
Acidosis
Stimulates high rate of ventilation by chemoreceptors
Anaerobic threshold
What determines exercise tolerance?
Anaerobic threshold
Acidosis
Muscle pain
Subjective feeling of exhaustion
What happens as a result of regular exercise?
Improved capacity to deliver O2 to active muscles
Improved utilisation of O2 by muscles
Results in progressive increase in VO2 max
What increases exercise fitness in the cardiovascular system?
Lower HR
Increased SV (ventricular filling)
Decreased TPR
Improved extraction of O2 from blood (Increased arteriovenous O2 difference)
What causes a lower HR and higher SV in fit individuals?
Higher vagal tone and lower sympathetic tone
What causes a decreased TPR in fit individuals?
Capillary density in muscles increases, improved endothelial function - increasing NO production
What improves the extraction of O2 from blood in fit individuals?
Increased size and number of mitochondria
