Q31

Draw a diagram of the action potential in the sino-atrial node / any pacemaker tissue.

  • The lower membrane potential*
  • Automaticity
  • Pre-potential*
  • Potassium efflux*
  • Opening of T channels and then later opening of L channels as AP come into place. (* mention calcium channels)
  • Re-polarisation due to potassium efflux

cardio 13

What are the effects of sympathetic and vagal stimulation?

  • Sympathetic stimulation: slope increases
  • Vagal: hyperpolarisation

Q32

What is the stroke volume in a normal adult at rest?

  • 70-90mL

Pass Criteria:

  • Bold to pass

Please draw and label the pressure volume loop of the left ventricle.

  • A
    • Start of systole: mitral (and tricuspid) valves close
    • Isovolumetric contraction until left ventricular pressure > aortic pressure (80mmHg).
    • Aortic (and pulmonary) valves open
  • B
    • Ventricular ejection (rapid at first) peak pressure 120mmHg
    • End systole: momentum of ejected blood overcome by aortic pressure
  • C
    • Aortic valve closes
    • End systolic volume – 50mL
  • C-D
    • Isovolumetric relaxation
    • Left ventricular pressure drops below atrial pressure – mitral valve opens – ventricle begins to fill (rapidly at first)
    • End diastolic volume – 130mL

Pass Criteria:

  • Correct graph needed to pass.
  • Need to demonstrate reasonable understanding of the loop.

Q33

Describe the normal sequence of electrical excitation of the cardiac conduction system and cardiac muscle.

  1. SA Node
  2. Atria
  3. AV Node
  4. Bundle of His
  5. Major bundles (Right and left)
  6. Purkinje fibres
  7. Ventricular muscle

Pass Criteria:

  • Bold to pass

What are the common mechanisms that cause abnormalities of cardiac conduction?

  • Abnormal pacemakers
  • Re-entry circuits
  • Conduction deficits
  • Prolonged repolarisation
  • Accessory pathways
  • Electrolyte disturbance

Pass Criteria:

  • 4 to pass

Please draw and explain the action potential of a cardiac pacemaker cell.

PROMPT – Which electrolytes are responsible for each phase of the action potential?

  • Pre-potential is initially due to a decrease in K+ efflux, then completed by Ca2+ influx through CaT channels
  • The action potential is due to influx of Ca2+ via CaL channels
  • Repolarisation is due to K+ efflux

Pass Criteria:

  • Correct shape of graph
  • Know ion fluxes
    • Pre-potential decrease K+ efflux/Ca2+ influx
    • Action potential influx Ca2+
    • Repolarisation K+ efflux

Q34

What are baroreceptors and where are they located?

  • Stretch receptors in the walls (adventitia) of the heart & blood vessels
  • Important in control of blood pressure (especially short term)
  • Arterial – carotid sinus/aortic arch
  • Low pressure – Atria at entrance of IVC and SVC, pulmonary veins and pulmonary circulation

Pass Criteria:

  • Bold + 2 locations

What stimulates these receptors?

  • Distension of the structures above
  • More sensitive to pulsatile then constant pressure
  • Maximal firing at 150mmHg (at carotid sinus)

Pass Criteria:

  • Bold to pass

What are their effects?

  • Inhibit tonic sympathetic drive and increase vagal drive -> vasodilation, venodilation, hypotension, bradycardia (tachycardia in low pressure baroreceptors), decreased cardiac output.
  • Allows rapid adjustments in BP in response to abrupt changes in posture, blood volume, cardiac output or peripheral resistance

Pass Criteria:

  • 3/5 end effects

Q35

Describe the mechanisms of venous return to the heart.

  1. Thoracic pump: inspiration resulting in negative pressure in the thorax and positive pressure in the abdomen. Blood flow towards the heart because of venous valves
  2. Effect of heart beat: during systole, AV valves are pulled downward -> increase the capacity of the atria
  3. Muscle pump: contraction of muscles around the veins in the limbs during activity
  4. Differential resistance: resistance of the large veins near the heart is less than peripheral veins

Pass Criteria:

  • Thoracic pump + 1 other

A 40y.o. man presents with extensive burns to the lower half of his body. A CVC is inserted. What factors might affect the central venous pressure of this patient?

  • Decrease CVP:
    • Fluid loss
    • Blood loss
  • Increase CVP:
    • Excessive fluid replacement
    • Other pre-existing conditions e.g. CCF
    • Positive pressure ventilation
    • Increased thoracic pressures

Pass Criteria:

  • 1 example from each bold category

What is the value of mean central venous pressure in normal individuals?

  • 4.6-5.8 mmHg or 6-8cm H2O

Pass Criteria:

  • Reasonable value

Q36

What are baroreceptors and where are they located?

  • Stretch receptors
  • Carotid, aortic, cardiopulmonary. In the adventitia of vessels. The carotid sinus and aortic arch receptors monitor the arterial circulation. Receptors are in the wall of the right and left atria, at the entrance of SVC and IVC and in the pulmonary veins as well as in the pulmonary circulation (collectively the cardiopulmonary receptors).

Pass Criteria:

  • Bold to pass
  • Carotid and aortic plus one other to pass

What is their mechanism of action?

  • Very sensitive to changes in pulse pressure.
  • Exert an inhibitory input via the tractus solitarius in the medulla.
  • Stimulated by distension of the structures in which they are located, therefore discharge at an increased rate when the pressure in these structures rises.
  • Increased baroreceptor discharge inhibits the tonic discharge of sympathetic nerves and excites the vagal innervation of the heart. Result is vasodilatation, venodilation and a fall in BP, bradycardia and decreased cardiac output.

Pass Criteria:

  • Need mention of inhibitory nature of pathway and nerves affected (vagus, sympathetics)

A 60 year old man with a history of atrial fibrillation on warfarin presents to ED following a motor bike accident. His blood pressure on arrival is 80/40. What is the action of baroreceptors in this setting of acute blood loss?

  • Decreased blood volume and decreased venous return results in reduced stimulation of arterial baroreceptors and increased sympathetic output.
  • The result is reflex tachycardia and vasoconstriction.

Pass Criteria:

  • Bold to pass

Q37

What is cardiac output?

  • Output of the heart per unit time
  • Heart rate x Stroke volume

Pass Criteria:

  • Bold to pass

What factors determine cardiac output?

  • Stroke volume is related to the preload (degree of stretch prior to contraction) and afterload (resistance to flow) of the heart and the intrinsic contractility of the myocardial cells
  • Heart rate – sympathetic vs parasympathetic stimulation

What methods can be used to measure cardiac output?

  • Direct Fick method or indicator (or thermal) dilution
  • Can also measure by Doppler ultrasound techniques
  • Fick principle
    • Amount of substance taken up by organ per unit time = (A-V concentration difference) x blood flow. In the heart can use O2.
    • LV output = O2 consumption mL/min/[AO2]-[VO2] (both in mL/L)
  • Indicator dilution
    • Substance injected IV and serial sampling in arterial blood performed, log plotted and extrapolated to find circulation time (indicator must not be lost from circulation)

Pass Criteria:

  • 2 to pass

Q38

What is cardiac output?

  • Output of the heart per unit time
  • Heart rate x Stroke volume

Pass Criteria:

  • Bold to pass

What factors determine cardiac output?

  • Stroke volume is related to the preload (degree of stretch prior to contraction) and afterload (resistance to flow) of the heart and the intrinsic contractility of the myocardial cells
  • Heart rate – sympathetic vs parasympathetic stimulation

What methods can be used to measure cardiac output?

  • Direct Fick method or indicator (or thermal) dilution
  • Can also measure by Doppler ultrasound techniques
  • Fick principle
    • Amount of substance taken up by organ per unit time = (A-V concentration difference) x blood flow. In the heart can use O2.
    • LV output = O2 consumption mL/min/[AO2]-[VO2] (both in mL/L)
  • Indicator dilution
    • Substance injected IV and serial sampling in arterial blood performed, log plotted and extrapolated to find circulation time (indicator must not be lost from circulation)

Pass Criteria:

  • 2 to pass

Following administration of anti-venom for a snakebite, a 60 year old man is noted to be hypotensive. What causes of decreased cardiac output could be causing this man's hypotension?

  1. Variation in heart rate due to induction of arrhythmias or heart block (too fast or too slow)
  2. Reduced preload (venodilatation with reduced venous return due to anaphylxais)
  3. Increased afterload (not too likely in this case)
  4. Reduced contractility (i.e. ischaemia, venoms, drugs)

Q39

Draw and describe an ECG tracing of a single normal heart beat.

PROMPT – What produces the waves and segments?

  • P wave – atrial depolarisation
  • PR – AV conduction
  • QRS – ventricular depolarisation
  • ST – plateau of ventricular depolarisation
  • QT – Ventricular action potential
  • T wave – ventricular repolarisation

Pass Criteria:

  • Bold 5/6

A 65 year old man presents with an inferior myocardial infarction. What features would appear different in this patient's ECG?

  • ST segment elevation in inferior leads
  • ST segment depression in the reciprocal leads

Pass Criteria:

  • Both

At the myocardial cell membrane level, what causes these changes?

  • Abnormally rapid depolarisation in early phase (accelerated opening of K+ channels)
  • Decreased resting membrane potential (due to loss of intracellular K+)
  • Slowed depolarisation of affected cells (cf normal cells)

Pass Criteria:

  • 1 of 3 to pass

Q40

Please draw and label the pressure volume curve of the left ventricle.

  • Graph with appropriate axis, curves and approximate pressuresPVLoop2

Pass Criteria:

  • Correct graph

Describe the pressure and volume changes in the left ventricle at the onset of systole.

PROMPT – What is meant by isovolumetric contraction?

  • (a to b on graph) Start of systole, mitral valve closes.
  • Isovolumetric contraction until left ventricular pressure is greater than aortic pressure (80mmHg) then aortic valve opens.
  • End-systolic volume 50mL.

Pass Criteria:

  • Bold to pass with reasonable understanding of the loop

Describe the pressure and volume changes in the left ventricle at the onset of diastole.

PROMPT – What is meant by isovolumetric relaxation?

  • (c to d on graph) Momentum of ejected blood is overcome by arterial pressure, then the aortic valve closes.
  • Isovolumetric relaxation as the ventricular pressure drops rapidly until below atrial pressure. Then AV valve opens to start ventricular filling.
  • End-diastolic volume 130mL, stroke volume 70-90mL

Pass Criteria:

  • Bold to pass with reasonable understanding of the loop

Q41

Draw & label the membrane potential of normal pacemaker tissue.

 

APPacemakerCell

Pass Criteria:

  • Must identify fast upslope being due to Ca influx and repolarization due to K efflux plus presence of pre-potential

By what mechanisms can tachyarrhythmias be generated?

  • Increased automaticity (AT,VT)
  • Acessory pathways (WPW)
  • Re-entry loops (VT)
  • Early after depolarisations (torsades de pointes)
  • Delayed after deoplarisations (as in digoxin toxicity)

Pass Criteria:

  • Bold + 1 other

What conditions may predispose to increased automaticity?

  • IHD
  • Previous repair of congenital heart disease (scar tissue)
  • Structural heart disease
  • Channelopathies (congenital or acquired)
  • Electrolyte imbalances (K, MG, Ca)
  • Sympathomimetic agents
  • Infiltrative cardiac diseases

Pass Criteria:

  • Mention at least one condition

 


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