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