Cardiac Muscle and Electrical Activity Flashcards
What is the heart?
Muscular pump which creates pressure head to push blood through blood vessels
What is the approximate weight of the heart?
250g-350g
What is the cardiac cycle?
The rhythmic contraction and relaxation of the heart that generates heart pumping action
Which parts of the heart act as a unit?
Both atria
Both ventricles
What increases the pressure of the heart?
Contraction of the muscle
What drives blood to move through the body?
Pressure difference droves blood flow (pressure gradient)
Which kind of contraction comes first: atrial or ventricular?
atrial
What are adjacent myocardial cells linked by?
Intercalated discs containing gap junctions
What are gap-junctions?
Fluid filled channels that allow the rapid spread of action potentials from cell to cell
What name is given to cardiac muscle in regard to its simultaneous contraction?
Syncytium
What structure triggers the contraction of myocardial cells?
Autorhythmic or pacemaker cells
What kind of cells are pacemaker cells?
Specialised noncontractile myocardial cells
What special property do autorhythmic cells possess?
Their membranes may spontaneously depolarise and generate action potentials
Where are the autorhythmic cells concentrated?
Sinoatrial node
Where is the sinoatrial node located?
The right atrium near the opening of the superior vena cava
What is the path of depolarisations in myocardial cells?
Atrial myocardial cells
Pause in fibrous layer
Ventricular myocardial cells
Atrial syncytium and ventriucalr syncytium
What is the path of depolarisations in myocardial cells?
Atrial myocardial cells
Pause in fibrous layer
Ventricular myocardial cells
Atrial syncytium and ventricular syncytium
How do spontaneous action potentials generated by autorhythmic cells in the SA spread to atrial contractile myocytes and then the ventricles?
Cardiac conduction system
What is the velocity of impulses spread through atrial fibres?
1m/sec
What conducts the impulse from the right atrium into the left atrium?
Specialised fibres called Bachmann’s bundle
How are impulses spread following atrial conduction?
Atrioventricular node
Where is the AV node located?
Base of the right atrium near the interatrial septum
What is the conduction velocity of the AV node?
0.05m/sec
What does the slow in atrial and ventricular conduction velocity ensure?
A delay between their contraction
What does the delay between atrial and ventricular conduction permit?
Optimal ventricular filling during atrial conduction
What is the conduction path of the atrioventricular node?
Bundle of His, bundle branches, Purkinje fibres
What is the conduction velocity in Purkinje fibres?
1-4m/sec
What generates each heart beat?
Spontaneous depolarisation of the SA node
Why can’t the AV node set its own rate of excitation in spite of possessing pacemaker cells?
The action potentials from the SA node travel faster to the AV node than it generating its own action potentials
When can the AV node become the predominant pacemaker cell in the heart?
If the SA node is destroyed
Where would pacemaker cells generate action potentials in the case that both the AV node and SA node were destroyed?
Purkinje fibres
What are the two types of action potentials in individual cells in cardiac tissue?
Fast response and slow response
Where is fast response generated?
Atrial and ventricular myocytes
Where is slow response generated?
Autorhythmic cells in sinoatrial and atrioventricular nodes
What are the phases of action potentials in cardiac muscle?
Upstroke
Early repolarisation
Plateau
Repolarisation
FInal repolarisation
What are the phases of action potentials in cardiac muscle?
Upstroke
Early repolarisation
Plateau
Repolarisation
Final repolarisation
When can a relative refractory period occur?
During repolarisation - Phase 3
What is the major difference between cardiac and skeletal muscle’s action potentials?
Cardiac action potentials can last up to 300 msec while skeletal action potentials typically last for only 25 msec
What phase of action potentials define cardiac muscle?
Phase 2 of cardiac action potentials - plateau
What does arrival of AP at a contractile myocardial cell do?
Open voltage gated Na+ channels
What channels open at the arrival of APs?
Voltage-gated Na+ channels rapidly
Voltage-gated Ca++ channels slowly
At what voltage does repolarisation begin in cardiac muscle cells?
+20mV
What causes the plateau phase of cardiac action potentials?
Slow inward diffusion of Ca++ balances outward diffusion of K+
What happens following the plateau phase of a cardiac AP?
Ca++ channels close and K+ channels complete repolarisation
What is Calcium Induced Calcium Release?
Inward movement of extracellular Ca++ during depolarisation opens Ca++ channels on the SR
How long are myocardial cells refractory?
During almost their entire contraction
Would tetany suit cardiac function?
No
What are the predominant Ca++ channels in cardiac muscle?
L-type Ca++ channels - long-lasting
What does opening L-Type Ca++ channels increase?
Conductance- large concentration gradient driving the influx of Ca++
Does large conductance strengthen or weaken the contraction?
Strengthen
What essential features does the influx of Ca++ have on myocardium?
Prolongs the AP and triggers the contraction of muscle fibers
What decreases the duration of the action potential in myocardial cells?
Ca++ channel antagonists such as verapamil and diltiazem
What does verapamil and diltiazem do?
Decrease the duration of the action potential and diminish the contractility of the myocardial cells
What word best describes the generation of slow response action potential in pacemaker cells?
Spontaneous
What differentiates the slow response and fast response action potentials?
Phase 4 is less negative in slow response
Phase 0 is not as large and less rapid
Early repolarisation is not apparent
Plateau phase is less prolonged and not as flat
At what voltage do the slow Na+ and Ca++ channels open?
At -60 mV
What term is used for Na+ and Ca++ channels?
HCN channels
What is threshold for myocardial cells?
-50mV
When does the cell revert to a closed or activatable state?
Phase 3 relatively and fully closed in phase 4
What unique features do pacemaker cells have in regard to refraction?
Have a prolonged relative refractory period
Post-repolarisation refractoiness
What is an electrocardiography?
Recording of the electrical activity in the heart
What happens during depolarisation on the surface of the heart tissue?
Section of outer heart muscle becomes negative
Remaining surfaces of the heart stay positive
Creates potential difference
What is the P wave on the ECG?
Results from atrial depolarisation
What is the QRS wave on an ECG?
Results from ventricular depolarisation
What is the T wave on the ECG?
Results from ventricular repolarisation
Which wave is usually hidden and by what other wave?
Atrial t wave obscured by larger QRS wave
How long does the P-Q interval last?
0.16 sec
What is the P-Q interval representing?
Delay in conduction of impulse into ventricles
How long should the Q-T interval last?
0.35 sec
What does the Q-T interval represent?
Duration of ventricular contraction
What does the R-R interval represent?
Duration between two consecutive R waves/ cardiac cycles/ heart beats
How long is the average R-R interval?
0.83 sec
What is the principle of ECG?
Depolarisation wave enters ventricles via septum and spreads toward apex
For almost entire ventricular depolarisation electrical current flows from base to apex in circuitous routes
Heart electrical currents have vector properties - magnitude and direction
What is the mean electrical axis of the heart?
Orientation of cardiac vector during QRS is +60 degrees from horizontal plane
What conducts electrical activity to the surface in the body?
Tissue fluid
What is the most commonly used way to record ECGs?
Bipolar limb leads
Explain how the most common form of recording ECGs work.
One positive and one negative recording electrode on either side of the heart on limbs
reference electrode is placed on the left leg
Connected via wires to the electrocardiograph forming a complete circuit
What name is given to the positions to the three positions of the leads which provide different pictures of electrical activity in the heart?
Einthoven’s triangle
For Lead I, where is the positive electrode and negative electrode placed?
+ - Left arm
- - Right arm
For Lead II, where is the positive electrode and negative electrode placed?
+ - Left leg
- - Right arm
For Lead III, where is the positive electrode and negative electrode placed?
+ - Left leg
- - Left arm
If the direction of the cardiac vector is towards the positive electrode, what is seen on the ECG?
Upward deflection on ECG
If the direction of the cardiac vector is parallel to the direction of the lead, what is seen on the ECG
Maximum deflection
If the direction of the cardiac vector is parallel to the direction of the lead, what is seen on the ECG?
Maximum deflection
In the typical person, which limb lead has the largest deflection?
Lead II
Are all reasons that the electrical axis changes in the heart pathological?
No
How may axis deviations be detected?
Changes in the direction and magnitude of QRS in leads I, II, III
What are unipolar leads?
Single recording electrode place on the body?
How many leads are associated with unipolar leads?
3 augmented unipolar limb leads
What are the codes for the unipolar leads and where are they placed?
Left arm (aVL)
Right arm (aVR)
Left leg (aVF)
How many electrodes are placed on the patient for unipolar leads?
6 electrodes
What are normal changes in the mean electrical axis?
Short/stocky people have a greater than 60 degrees vector
Long/thin people have a less than 60 degrees vector
What causes abnormal changes in mean electrical axis?
Heart hypertrophy - the axis shifts towards hypertrophied side
What is heart hypertrophy?
Abnormal thickening of one side of the heart
May occur in heart failure
What are ECGs good at detecting?
Cardiac arrhythmias
What are cardiac arrhythmias?
Disturbances in generation of depolarisations or their propagation
What is sinus tachycardia?
Increase in heart rate
What is sinus bradycardia?
Decrease in heart rate
What is atrial fibrillation?
Damage to atrial tissue which prevents a coherent wave of the polarisation and depolarisation spreading across the atrial tissue
How may one diagnose atrial fibrillation?
Absence of P wave on ECG
What is the major issue in atrial fibrillation?
Accumulation of blood in atria resulting promotion of blood clot formation
What is ventricular fibrillation?
Damage to ventricular fibres or Purkinje fibres which prevents coherent wave of polarisation passing through ventricular tissue
Why is ventricular fibrillation so serious?
NO coherent contraction and relaxation of ventricles, quickly compromising blood flow to circulatory system; fatal in3/4 minutes
