PHYSIOLOGY - Cardiac Electrical Activity and Electrocardiograms (ECG) Flashcards
What is an electrocardiogram (ECG)?
An ECG is a record of the electrical activity of the heart during the different phases of the cardiac cycle, graphed in terms of voltage against time
Describe briefly the electrical conduction system of the heart
Within the right atrium, pacemaker cells within the sinoatrial (SA) node spontaneously depolarise, transmitting a depolarisation wave to the left and right atrial cardiomyocytes to allow for coordinated atrial contraction. This depolarisation wave also reaches the atrioventricular (AV) node, allowing the depolarisation wave to travel down the interventricular septum via the Bundle of His and Purkinje fibres to allow for depolarisation of the ventricular endocardium followed by the ventricular epicardium
What are the three phases of an action potential within pacemaker cells?
- Pacemaker potential phase
- Depolarisation phase
- Repolarisation phase
Describe the pacemaker potential phase of an action potential within a pacemaker cell
In response to hyperpolarisation, the voltage-gated sodium channels will open allowing an influx of sodium into the cell which will slowly depolarise the cell until threshold potential (-50mV) is reached
Describe the depolarisation phase of an action potential within a pacemaker cell
When the pacemaker cell reaches threshold potential (-50mV), L-type voltage-gated calcium channels open to allow for an influx of calcium into the cell which will rapidly depolarise the cell
Describe the repolarisation phase of an action potential within a pacemaker cell
Slow voltage gated potassium channels open allowing for potassium to flow out of the cell. Furthermore, the voltage-gated calcium channels will close and the cell will repolarise
What are the four phases of an action potential within cardiomyocytes?
- Depolarisation phase
- Initial repolarisation phase
- Plateau phase
- Repolarisation phase
Describe the depolarisation phase of an action potential within a cardiomyocyte
The opening of fast voltage-gated sodium channels is triggered by action potentials precipitated from the pacemaker cells. The influx of sodium into the cell will depolarise the cell
Describe the initial repolarisation phase of an action potential within a cardiomyocyte
As the slow voltage-gated potassium channels open and potassium flows out of the cell, the fast voltage-gated sodium channels will close. This will begin the repolarisation of the cell
Describe the plateau phase of an action potential within a cardiomyocyte
Voltage-gated calcium channels will open allowing an influx of calcium into the cell which will counterbalance the potassium efflux to stabilise the membrane potential, creating a plateau
Describe the repolarisation phase of an action potential within a cardiomyocyte
The voltage-gated calcium channels will close however the voltage-gated potassium channels will remain open which will begin to repolarise the cell. An ion pump will efflux the calcium within the cell to complete repolarisation
What is the dipole concept?
The dipole concept refers to an electrical dipole in which there is a negative and positive charge separated by distance. If the positive charge is closer to a positive electrode than the negative charge, this will cause an upward deflection of the voltage trace whereas if the negative charge is closer to a positive electrode, this will cause a downward deflection of the voltage trace
In regards to the dipole concept and cardiomyocytes, would an ECG machine measure the charge on the inside or outside of the cell?
An ECG machine would only measure the charge on the outside of a cardiomyocyte
Why will an ECG machine measure 0 when a cardiomyocyte is at rest?
When a cardiomyocyte is at rest, the outside of the cell will be completely positive and thus there will be no dipole formation as there is only a positive charge present
Which stage of the cardiac cycle is represented by the P wave of an ECG?
Atrial depolarisation
Which stage of the cardiac cycle is represented by the QRS complex of an ECG?
Ventricular depolarisation
Which specific stage of ventricular depolarisation is represented by the Q wave of an ECG?
Early ventricular depolarisation
Which specific stage of ventricular depolarisation is represented by the R wave of an ECG?
Ventricular depolarisation
Which specific stage of ventricular depolarisation is represented by the S wave of an ECG?
Late ventricular depolarisation
Which stage of the cardiac cycle is represented by the T wave of an ECG?
Ventricular repolarisation
Describe the underlying physiology of the P wave of an ECG
Within the right atrium, pacemaker cells within the sinoatrial (SA) node spontaneously depolarise, leading to the pacemaker cells being negatively charged, however, the depolarisation wave will move slowly through the atrial cardiomyocytes so they will still be at rest and positively charged - resulting in the formation of a dipole with the positive charge closer to the positive electrode on the left forelimb, resulting in an upward deflection of the voltage trace, forming the P wave. When all of the atrial cardiomyocytes are depolarised, all of the cells will be negatively charged and thus there will be no dipole so the voltage trace will return to the baseline
Describe the underlying physiology of the Q wave of an ECG
The depolarisation wave will travel down the interventricular septum via the Bundle of His and Purkinje fibres, with the wave of depolarisation spreading from the left to right side of the interventricular septum leading to the left side being depolarised and thus negatively charged before the right side which will remain temporarily at rest and positively charged - resulting in the formation of a dipole with the negative charge closer to the positive electrode on the left forelimb, resulting in a downward deflection of the voltage trace, forming the Q wave
Describe the underlying physiology of the R wave of an ECG
The wave of depolarisation will spread to the endocardium of the ventricles resulting in a negative charge of the cardiomyocytes. However the epicardium will not yet be depolarised so the cardiomyocytes will remain temporarily at rest and positively charged - resulting in the formation of a dipole with the positive charge closer to the positive electrode on the left forelimb, resulting in an upward deflection of the voltage trace, forming the R wave
Describe the underlying physiology of the S wave of an ECG
The wave of depolarisation reaches the epicardium of the ventricles resulting in a negative charge of the cardiomyocytes - resulting in the formation of a dipole with the negative charge closer to the positive electrode, resulting in a downward deflection of the voltage trace, forming the S wave. When all of the ventricular cardiomyocytes are depolarised, all of the cells will be negatively charged and thus there will be no dipole so the voltage trace will return to the baseline
Describe the underlying physiology of the T wave of an ECG
The epicardium of the ventricles will be the first to repolarise resulting in the cardiomyocytes being positively charged. However the endocardium will not yet be repolarised so the cardiomyocytes will remain temporarily depolarised and negatively charged - resulting in the formation of a dipole with the positive charge closer to the positive electrode on the left forelimb, resulting in an upward deflection of the voltage trace, forming the T wave. When all of the ventricular cardiomyocytes are repolarised, all of the cells will be positively charged and thus there will be no dipole so the voltage trace will return to the baseline
Why is the dipole directed towards the left side of the heart rather than the right side?
The dipole is directed towards the left side of the heart as the apex of the heart is directed towards the left and the ventricular muscle is thicker in the left ventricle compared to the right ventricle
What is the R-R interval in an ECG?
The R-R interval is the time elapsed between the peaks of two successive R waves on an ECG
What is indicated by the R-R interval in an ECG?
Heart rate
How do you calculate heart rate from R-R interval?
Count the squares between the R-R peaks. Each square is equivalent to 0.04 seconds, so multiply the number of squares counted by 0.04. Divide 60 by the value you have calculated:
e.g. if there are 20 squares between the R-R peaks:
20x0.04 = 0.8
60/0.8 = 75bpm
What is the P-R interval in an ECG?
The P-R interval is the time elapsed between the beginning of the P wave and the beginning of the Q wave
What is indicated by the P-R interval in an ECG?
Time between the beginning of atrial depolarisation and the beginning of ventricular depolarisation. Changes in the P-R interval can indicate atrial dysfunction
What is the Q-T interval in an ECG?
The Q-R interval is the time elapsed between the beginning of the Q wave to the end of the T wave
What is indicated by the Q-T interval in an ECG?
Time between the beginning of ventricular depolarisation to the end of ventricular repolarisation. Changes in the Q-T interval can indicate ventricular dysfunction
Which colour of ECG cable should be placed on the right forelimb?
Red ECG cable
Which colour of ECG cable should be placed on the left forelimb?
Yellow ECG cable
Which colour of ECG cable should be placed on the left hindlimb?
Green ECG cable
Which colour of ECG cable should be placed on the right hindlimb?
Black ECG cable
What are the five clinical uses for electrocardiograms (ECGs)?
Calculate heart rate
Detect arrhythmias
Detect heart chamber enlargement
Detect myocardial ischaemia
Provide information on electrolyte imbalance
