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A high pressure high resistance system.
1) systemic
2) pulmonary
systemic
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A low pressure lowresistance system.1) systemic
2) pulmonary
pulmonary
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blood pressure =
flow x resistance
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What 3 things determine 'resistance' in the circulatory system?
- 1) vessel length
- 2) vessel diameter
- 3) viscosity of blood
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What determines 'flow' of blood pressure equation (flow x resistance = BP)
cardiac output = HR X sv
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What is cardiac output?
- HR X SV
- Amount of blood (L/min) pumping through the circulatory system
- (4-8 l/min)
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What 3 components affect cardiac output?
- 1) preload
- 2) afterload
- 3) contractility
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What is afterload
resistance to ventricular ejection
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What is preload
amount of blood volume filling ventricles.
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What happens with inadequate preload? (2 things)
- decreased
- 1) force of contraction
- 2) stroke volume
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What is contractility?
ability of myocardial muscle fibres to contract
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5 things that alter preload?
- 1) total circulating volume
- 2) venous tone (vasodil or vaso constr)
- 3) mechanical ventilation/body position
- 4) ventricular compliance
- 5) arrhythmias
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How does blood volume affect preload?
- decreased by hypovolemia: diuresis; blood loss.
- Increased by excess fluids: eg IV solution or heart or renal failure
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How does venous tone affect preload?
- vasodilation decreased preload
- vasoconstriction increased preload
- (eg drugs)
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How does body position affect preload?
raise legs increases venous return (by 500 mls)
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How does mechanical ventilation affect preload?
increases intra-thoracic pressure which decreases venous return
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how does ventricular compliance affect preload
non compliant ventricle will elevate preload as the ventricle can't stretch to accomodate
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How does arrhythmias affect preload?
- 30% of stroke volume lost with no p wave. Arrhythmias decrease preload.
- bradycardia and tachycardia decrease preload.
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How can you assess preload? (5 things)
- a)Heart rate/rhythm
- b) neck veins
- c) CVP
- d) heart sounds (s3/s4 - extra sounds from turbulence of noncompliant ventricles)
- e) lung sounds/crackles
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How does increased afterload affect the patient?
- increased myocardial oxygen demand
- (from normal 50% to 90%)
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What 4 things can increase left heart afterload?
- aortic stenosis (impedes flow)
- increased blood viscosity
- sympathetic stimulation (vasoconstriction)
- noncompliant arteries
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Does systolic or diastolic BP affect the left ventricular afterload?
diastolic
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What automatic compensatory mechanism occurs when oxygen cannot reach an alveolus?
capillary surrounding the alveolus constricts
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How can you measure right ventricular afterload?
pulmonary artery catheter. Also by oxygen sats
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In COPD or emphysema, is right ventricular afterload increased or decreased
increased - causes right sided heart failure or corpulmonale
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What can affect contractility?
- 1) drugs (positive inotropes increase; negative inotropes (eg antiarrhythmics) decrease contractility
- 2)ANS
- 3) oxygen and ph levels
- 4) loss of muscle mass
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How do drugs affect contractility?
- Positive inotropes increase contractility (eg; dignoxin, dopamine dobutamine; epi; norepi; phenylephrine)
- Negative inotropes decrease contractility (eg anti-arrhythmics; lidocaine, procainamide verapamil amiodarone)
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Which anti-arrhythmic causes the lease decrease in contractility?
amiodarone
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When using anti-arrhythmics what parameter must be monitored carefully?
BP - decreasing contractility decreases CO so BP can fall.
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How does sympathetic stimulation affect contractility?
neurotransmitters epi and norepi are released and stimulate beta receptors in heart. this increases heart rate and contractility. (dopamine and dobutamine can mimic this)
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How does parasympathetic stimulation affect contractility
blocks the beta receptors (eg metoprolol and propranolol) decreasing contractility and decreases heart rate
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