Hemorrhage, Fainting, Exercise Flashcards
What is hypovolaemia ?
A reduction in blood volume?
What are the two types of shock?
Compensatory/non progressive shock
Progressive shock
What blood pressure value is associated with compensatory shock?
Bp remains greater than 45mmHg
What bp value is associated with progressive shock?
Bp falls below 45mmHg
What is the difference in the outcome for a patient with progressive shock vs non progressive shock?
Progressive shock - patient will get worse without therapy
Non progressive - the body’s own compensatory mechanisms will cause recovery
What are the main compensatory mechanisms for shock?
- Baroreceptor reflex
- Chemoreceptor reflex
- Central nervous system response to cerebral ischaemia
- Reabsorption of tissue fluids
- Hormonal vasoconstrictors
- ADH release from pituitary
- Angiotensin II and aldosterone release from adrenal cortex
What happens in the initial response to hemorrhage, a resonse caused by the baroreceptors ?
- General arteriolar constriction = TPR ↑
- Venous constriction = venous return ↑
- HR increases from ≈60bpm to ≈180bpm
Why are other compensatory mechanisms needed, apart from the baroreceptor reflex?
Once MAP drops below 60mmHg, baroreceptor intensity doesn’t increase - plateau
Below ___ mmHg of MAP, baroreceptor reflexes plateau
60mmHg
What stimulates chemoreceptors?
PO2 ↓
What does stimulation of chemoreceptors cause?
- Peripheral vasoconstriction
- Increased venous return as an increased rate and depth of perspiration reduces intrathoracic pressure
What is the respiratory pump?
When venous return is increased due to a reduction in intrathoracic pressure caused by increases respiratory rate
Why is cerebral ischamia a compensatory reflex?
When PO2 ↓ and PCO2 ↑ in the brain
Causes extreme activation of SNS
More vasoconstriction and contractility of heart
What causes the plateau at 50mmHg?
The compensatory mechanism caused by cerebral ischaemia
How does the reabsorption of tissue fluids happen during shock?
- Reduced MAP, increased TPR and increased venous return - capillary hydrostatic pressure ↓
More tissue fluid reabsorbed
why does the oncotic pressure of plasma decrease during hemorrhage?
More tissue fluid = more dilution of blood = lower protein conc
How much tissue fluid can be reabsorbed per hour during hemorrhage ?
1 litre per hour
What increases adrenaline release?
When MAP drops to 40mmHg, adrenal medulla increases adrenaline output by 50 fold
What causes the pituitary to release ADH?
Sensory input from baroreceptors and other vascular stretch receptors
How is salt and water conserved in the kidney?
- Reduction in MAP means less filtration occurs
- Increased production of angiotensin II causes release of aldosterone
- ADH causes water reabsorption
What is Angiotensin II ?
A vasoconstrictor released by the kidneys
What is aldosterone?
A sodium retaining hormone, whos release is stimulated by angiotensin II
How long does it take for sympathetic reflexes to become active after hemmorhage?
30-60 sec
How long does it take for angiotensin and vasopressin mechanisms to respond completely after hemmorhage?
10-60 mins
How long does it take for reabsorption of tissue fluid to become effective after hemmorhage?
1 to 48 hrs
What causes progressive shock?
Decompensatory mechanisms
What determines the outcome of a shock patient?
The relative strength of the compensatory and decompensatory mechanisms
List the decompensatory mechanisms
- Cardiac failure
- Increased tissue hypoxia
- Acidosis
- CNS depression
- Sludged blood
- Endotoxin release