Week 6 Flashcards
Elastic arteries EAs
ABP reflects the driving pressure for blood flow
ABP determined in the elastic arteries
Rhythmic ejection of blood from ventricle leads to pulsatile pressure in aorta and large arteries
Functions of EAs:
- dampen the pulsatile pressure to ensure continuous flow into the circulation (‘dampening function’ Windkessel effect)
-ensures blood pressure is maintained during diastole (act as tubes to ensure flow to periphery ‘conduit function’)
Pulse pressure in the aorta
Systolic pressure peak -120mmHg
Diastolic pressure 80mmHg
Pulse pressure= SP-DP
Mean ABP= SP-DP/3 +DP
Windkessel effect
During systole elastic wall of aorta distends (energy converted to elastic energy)
Aortic wall recoils during diastole to propel blood forward (energy released)
Driving pressure sustained during diastole-> continuous blood flow
As blood is passed into aorta only 20-50% goes straight into peripheral circulation
Pressure wave progressively damped in smaller arteries
The mean pressure constantly dropping throughout peripheral circulation due to the resistance offered by different types of vessels
Blood renters heart at minimal pressure- central venous pressure- most of original energy lost due to friction and dissipated as heat
Effects of ageing on pulse pressure
Compliance= change in vol/ change in pressure
With age the elastic components of the aorta and its branches degenerate, collagen becomes more prominent and arteries stiffen
Ejection of same or even reduced SV leads to increased systolic pressure due to reduced elasticity
Reduced elastic recoil leads to lower diastolic pressure
So pulse pressure increases with age
Arteriosclerosis- loss of compliance and stiffening of arteries with age
Haemodynamics
Flow proportional to change in pressure
Flow inversely proportional to resistance
Change pressure= arterial pressure-venous pressure
Resistance to blood flow in single vessel determined by:
-vessel length
-blood viscosity can change due to proteins and RBCs and drop in temp can increase viscosity
-radius, main determinant
R a n xL/ pi x r^4. L= length n=viscosity r=radius
So small changes in radius can have very large effect on R
Poiseuille’s equation
Flow= change P x pi r^4/ n xL
Peripheral arterial occlusive disease PAOD
Atherosclerotic plaques in large and medium sized arteries (particularly lower limb)
Decrease radius increase resistance to flow
Ischaemia distal to stenosis
Particularly problematic and painful during exercise
Tissue hypoxia-> pain (intermittent Claudication)
Imbalance between oxygen supply and demand in increased metabolic demand eg in exercise it wont be met so get tissue hypoxia
Blood flow through vasculature
Poiseuilles equation only precisely applies if there’s laminar flow
Laminar flow: normal pattern of flow, highly efficient
-concentric layers blood, immobile layer at endothelium, flow is fastest in centre
Turbulent flow: occurs where flow velocity is high, inefficient , cannot apply Poiseuille’s law
-eg at large artery branches, pregnancy, exercise, anaemia, valve defects/arterial stenosis
-vibrations heard as sounds- murmurs, korotkoff sounds
Taking blood pressure korotkoff sounds
Cuff> 120mmHg stops flow brachial artery occluded:
-no sounds
Cuff 120-80mmHg pulsatile, turbulent flow:
-korotkoff sounds indicate level of systolic BP
Cuff <80mmHg laminar flow:
-no sounds diastolic pressure, artery filling, open
ABP
Level of arterial blood pressure is determined in elastic arteries and is the driving force for flow
Determined by:
-blood volume in the arterial system CO
-resistance to blood flow TPR
ABP= cardiac output x total peripheral resistance
ABP= CO x TPR
What determines SP
1.Stroke volume (anything that increases SV such as changes in cardiac filling and contractility will increase SP)
2. Aortic/arterial distensibility
3. Ejection velocity
4. DP of previous beat
Therefore increases in the following increase SP:
-EDV (preload)
-contractility
-decreased aortic compliance
-increased venous return
-exercise, circulating Catecholamines
-ageing
So SP gives information about cardiac output
What determines DP
Arteriolar resistance
Therefore the following increase DP:
-vasoconstriction
-arteriosclerosis
-atherosclerosis
HR: a very high heart rate increases DP because cardiac cycle is shorter so not enough time for the diastolic pressure to fall back down to normal level so DP increases
DP gives information about total peripheral resistance
Different components of ECG
An ECG provides a visual representation of the spread of electrical events through the heart. This electrical activity is detected on the surface of the body using recording electrodes. ‘Leads’ consisting of a pair of electrodes (one positive and one negative) are used to record the electrical activity
The shape of an ECG is dependent on which leads are used, and any abnormal conduction of electrical current. The convention is to use an idealised ECG shape to understand differences between the types of ECGs recorded, as well as differences within a person in different circumstances
Waves of ECG
P wave- atrial depolarisation
QRS complex- ventricular depolarisation
T wave- ventricular repolarisation
Approximate durations for heart rate 60bpm
P wave- 0.10s
PR interval- 0.12-0.20s
QRS complex- 0.08s
T wave- 0.16s
QT interval- 0.40s
ECG leads
A pair of electrodes forms what is known as an ECG lead. Theres 12 standard ECG leads
-3 standard limb leads, I, II, III
-3 augmented limb leads, aVR, aVL, aVF
-6 unipolar chest leads V1 to V6
