Cardiovascular Physiology

Question 1

What are the components of the cardiac conduction system?

Answer 1

1. SA node
2. 3 Internodal pathways ( anterior, middle -Wenkebach and posterior - Thorel)
3. AV node
4. Bundle of His - left bundle branches off at the top, bundle continues as right bundle brance
5. Purkinje fibres

Question 2

Where are the SA node and AV node located?

Answer 2

SA node - Junction of the SVC and the right atrium

AV node - Right posterior portion of the interatrial septum

Question 3

Where is conduction spread in cardiac tissue the fastest?

Answer 3

The Purkinje system

Question 4

How long does it take for atrial depolarisation?

Answer 4

0.1 s

Question 5

What is the speed of conduction in the AV node?

Answer 5

0.05 m/s (slow)

Delay of 0.1s occurs before excitation spreads to the ventricles.

Note: When there is lack of contribution of Na+ current in the depolarisation (ie in pacemaker potential) a marked loss of conduction is oberved

Question 6

What is the pattern of ventricular depolarisation?

Answer 6

Takes 0.08-0.1s

Left septum > right septum > down to apex > returns along ventricular walls to AV groove > endocardial to epicardial surface

The last part of the heart to be depolarised are the posterobasal portion of the L ventricle, the pulmonary conus and uppermost portion of the septum

Question 7

Which events in the heart correspond to the waves/intervals seen on the ECG?

Answer 7

P wave - atrial depolarisation

PR interval - Atrioventricular conduction

QRS duration - ventricular depolarisation

QT interval - ventricular action potential

T wave - ventriuclar repolarisation

Question 8

What are the bipolar leads in an ECG?

Answer 8

WLeads I, II and III

Record the differences in potential between two limbs

• In lead I, an upward deflection is inscribed when the left arm becomes positive relative to the right
• In lead II the electrodes are on the right arm and left leg
• In lead III the elctrodes are on the left arm and left leg

The three bipolar leads roughly form an equilateral triangle that is called Einthoven’s triangle

Question 9

How do the bipolar leads link to the axial reference system of the heart?

Answer 9

• The positive electrode for lead I is at O^{<span>0</span>}
• The positive electrode for lead II is at 60⁰
• The positive electrode for lead III is at 120⁰

NORMAL cardiac axis is between -30 to +110

Question 10

What is a His Bundle electrogram?

Answer 10

Measures the electrical events in the AV node, bundle of His and Purkinje system

• A deflection when AV node is activated
• H spike during transmission through the His bundle
• V defelction during ventricular depolarisation

Can measure 3 intervals:

1. the PA interval 27ms- represents condcution time from SA to AV node
2. the AH interval 92ms - from A wave to start of H spike, represents AV nodal conduction time
3. the HV interval 43ms - start of H spike to the start of QRS deflection represents conduction time in the Bundle of His and Bundle branches

Question 11

Describe the phenomenon of sinus arrhythmia

Answer 11

During inspiration impulses in the vagi from the stretch receptors in the lungs inhibit the cardio-inhibitory area in the medulla oblongata

The tonic vagal discharge decreases and the heart rate rises

Question 12

What causes 3rd degree heart block?

Answer 12

Disease in the AV node (AV nodal block) or in the conducting system below the AV node (infranodal block)

Question 13

What is the difference between AV nodal and infranodal block

Answer 13

In AV nodal block the remaining nodal tissue becomes the pacemaker - approx 45 BPM

In infranodal block (disease in the bundle of His) the ventricular pacemarker is located more peripherally and the ventricular rate is lower - approx 35 BPM

Question 14

What is an AVNRT?

Answer 14

AV nodal reentrant tachycardia

A regular SVT (the most common cause of paroxysmal SVT) that results from the formation of a re-entry circuit confined to the AV node and peri-nodal atrial tissue

In order for reentry to occur there must be dual pathways with different conduction velocities (ie fast and slow) and refractory periods.

The most common AVNRT is “slow-fast” AVNRT

• A PAC is conducted via the slow pathway, is conducted retrograde up the fast pathway and circus movement develops
• An echo beat develops when the reentrant activity depolarises the atrium

Question 15

What is accelerated AV conduction syndrome?

Answer 15

Wolff-Parkinson White

​An additional aberrant muscular or nodal tissue connection (Bundle of Kent) exists.

The beat conducts normally down the AV node but spreads to the ventricular end of the aberrant bundle and the impulse is transmitted retrograde to the atrium. Circus movement is established

Short PR interval, prolonged QRS with slurred upstroke, normal interval between start of P and end of QRS (PJ interval)

Question 16

What conditions predispose to automaticity?

Answer 16

• IHD
• Scarring eg from previous repair
• Structural heart disease
• Channelopathies
• Electrolye imbalance
• Sympathomimetic agents
• Infiltrative cardiac disease

Question 17

What are the 3 major abnormalities of membrane polarisation associated with acute MI

Answer 17

ACUTE

1. Rapid repolarisation due to accelerated K+ channel opening (normal region negative relative to infarct furing latter part of repolarisation)
2. Decreased resting membrane potential due to loss of intracellular K+ (Causes current flow into the infarct during diastole)
3. Delayed depolarisation (infarcted region positive relative to healthy tissue during early repolarisation)

LATER

1. Dead muscle becomes electrically silent
2. Infarcted area is relatively negative compared to normal myocardium and does not contribute to normal positivity of ECG complex
   
   1. Q waves appear
   2. Failure or R wave progression
   3. Bundle branch blocks

Question 18

Explain the features seen on an ECG in hyper/hypokalaemia

Answer 18

Moderate hyperkalaemia: K+ >7.0

• Tall peaked T waves due to altered repolarisation

Severe hyperkalaemia: K+ > 8.5

• Paralysis of the atria
• Broad slurred QRS complex
• Tall peaked T waves remain

The resting membrane potential decreases as the extracellular K+ level increases. The fibres eventually become unexcitable and the heart stops in diastole

Hypokalaemia:

• Slight lengthening of the PR interval
• Prominent U waves
• ST segment depression
• Late T wave inversion in precardial leads

Question 19

What are the major differences between the AP in a pacemaker cell and AP in a ventricular myocyte

Answer 19

• Resting membrane potential -90mv in cardiac myocyte, -60mV in pacemaker cell
• Cardiac myocyte has fast depolarisation via Na+ channels whereas pacemaker cell has slow Ca2+ channel dependant depolarisation
• Cardiac myocyte has no prepotential
• Pacemaker potential does not have a plateau phase

Question 20

Describe the action potential of a cardiac pacemaker cell

Answer 20

• Prepotential due to “funny current” I_h through HCN channels(permeable to both Na+ and K+)
• When prepotential reaches the activation threshold Ca2+ channels open
  
  • I_Ca through transient T channels completes the prepotential
  • I_Ca through long-lasting L channels produces the impulse
• There is no sharp, rapid depolarising spike before the plateau
• At the peak of each impulse I_k begins and brings about repolarisation
• Ik then declines and a channel permable to both Na+ and K+ is activated (I_h)

Question 21

Describe the action potential of a ventricular muscle cell

Answer 21

Resting membrnae potential -90mV

Phase 0 Rapid depolarisation due to opening of voltage Na channels

Phase 1 Rapid repolarisation due to closure of voltage gated Na+ channels

Phase 2 Plateau phase due to opening of voltage gated Ca2+ channels

Phase 3 Repolarisation after closure of Ca2+ channels due to K+ efflux

Phase 4 Return to resting membrane potential

Question 22

What is the effect of sympathtic (noradrenergic) and vagal (cholinergic) stimulation on the membrane potential?

Answer 22

Ach binds to M2 receptors (Gi protein couple receptor)> decreased cAMP > slowed Ca2+ channel opening + activation of special K+ channels > Membrane hyperpolarisation > slope of the pre-potential is decreased > decreased firing rate

Adrenaline and noradernaline bind to Beta 1 receptors >increased cAMP > facilitates opening opening of Ca L channels > increased I_Ca > increased firing rate

Question 23

What are the 5 phases of the cardiac cycle

Answer 23

1. Atrial systole
2. Isovolumetric ventricular contraction
3. Ventricular ejection
4. Isovolumetric ventricular relaxation
5. Ventricular filling

Question 24

What are the common mechanisms of cardiac conduction abnormalities?

Answer 24

• Abnormal pacemakers - ectopic beats, sinus arrest, atrial of ventricular fibrillation
• Re-entry circuits - tachyarrhythmias
• Conduction defecits- heart blocks, bundle branch blocks
• Prolonged repolarisation - long QTc
• Accessory pathways eg WPW
• Electrolyte disturbance

Question 25

Describe the cardiac cycle

Answer 25

* Atrial systole (phase 1) * Contraction of the atria propels some blood into the ventricles * There is some regurgitation of blood into the veins * Ventricular systole * The AV valves close (S1) * **Isovolumetric ventricular contraction (phase 2):** Ventricle contracts and intraventricular pressure rises sharply * When pressure in ventricles exceed the pressure in the aorta and PA, the aortic and pulmonary valves open * the AV valves bulge into the atria causing a small but sharp rise in atrial pressure ​​​ * **Ventricular ejection (phase 3)** occurs when the aortic and pulmonary valves open * Intraventriuclar pressure rises to a maxiumum and then declines * The amount of blood ejected is 70-90mL (50mL of blood remains in each ventricle at the end of systole and the **EF is about 65%)** * Early diastole * Protodiastole - Ventricular pressure drop rapidly * Momentum of ejected blood is overcome and the aortic and pulmonary valves close (S2) * Pressure drops rapidly but the volume stays the same **(isovolumetric ventricular relaxation (phase 4))** * Isovolumetric ventricular relaxation ends when ventricular pressure falls below atrial pressure and the AV vlaves open permitting the ventricules to fill. * Late diastole * When ventricular pressure falls below atrial pressure, the **AV valves open** * **Ventricular filling (phase 5)** occurs * 70% of ventricular filling occurs passively during diastole * End diastolic volume is 130 mL

Question 26

Describe how the wave forms of an ECG relate to the cardiac cycle

Answer 26

Atrial systole starts **after the P wave** Ventricular systole starts **near the end of the R wave** and ends **just after the T wave**

Question 27

How does the cardiac cycle alter with heart rate?

Answer 27

The duration of systole decreases from 0.27s at a heart rate of 65 to 0.16 at a rate of 200. When the heart rate is increased, diastole is shortened to much greater degree than diastole.

Question 28

Describe a normal pressure-volume loop of the left ventricle

Answer 28

* During diastole the ventricle fills and pressure increases from d to a * Pressure then rises sharply during isovolumetric ventricular contraction from a to b * Volume then falls during ventricular ejection from b to c * At c the aortic valve closes and pressure falls during isovolumetric relaxation from c to d

Question 29

Describe the timing of opening and closing of the valves

Answer 29

* **Right atrial systole** precedes left atrial systole * **Contraction of the left ventricle** starts before the right * **Right ventricular ejection** begins before that of the left * During expiration the P and A valves close at the same time * During inspiration the A valve closes slightly before the P valve

Question 30

What is the dicrotic notch?

Answer 30

A small oscillation in the falling phase of the pulse wave caused by vibrations set up when the aortic valve snaps shut

Question 31

What do the waves of jugular venous pressure waveform represent?

Answer 31

a wave - Atrial systole c wave - bulging of the triCuspid valve into the atria during isovolumetric ventricular contraction v wave - mirrors rise in atrial pressure before the tricuspid valve opens during diastole

Question 32

What are the 4 heart sounds?

Answer 32

1st - Closure of the AV valves 2nd - Closure of the P and A valves 3rd - 1/3 through diastole, rapid ventricular filling 4th - Immediately before 1st, when atrial pressure high or stiff ventricle due to ventricular filling

Question 33

What are the methods of measuring cardiac output ?

Answer 33

1. **The Ficks method** * The amount of substance taken up by an organ is equal to the arterial level minus the venous level times the blood flow * Measure the amount of O2 consumed by the body in a given period and dividing this value by the A-V difference across the lungs * CO = O2 consumption / (AO2 - VO2) 2. **The indicator dilution method** * Known amount of substance is injected into a vein and its concentration in serial samples of blood is determined * CO = amount of indicator injected divided byits average concentration in arterial blood after a single circulation through the heart * CO = injected dose (mg) / average concentration (mg/L) * Thermodilution of cold saline is commonly used indicator 3. **Doppler combined with echo**

Question 34

What is cardiac index?

Answer 34

Cardiac output per square metre of body surface Averages 3.2 L/min/m²