Week 4

 Reflection: This week was a national holiday and, therefore, we did not have any lab sessions.

Domain Knowledge: This week we had 2 lectures that were an introduction to ECGs. I will be talking about what i learned this week.

Firstly, ECG is a graphic representation of the changes in magnitude and direction of electrical activity or electrical current generated through the atria and ventricles of the heart.
The wave of depolarization is conducted beyond the epicardium to the surface of the body. This allows surface electrodes to detect the flow of current and electrical impulses of the heart.
Moreover, ECG contains a galvanometer which detects the changes in the surface potential and amplifies the signal, recording the changes on calibrated moving paper. 
Each lead of the ECG has two poles: a positive and a negative one. A current moving towards the positive lead causes a positive deflection, and a current towards the negative lead causes a negative deflection. These deflections occur in relation to an axis which is an imaginary line between the two poles.
Einthoven's triangle describes the axes of the bipolar limb leads as forming an equilateral triangle.
The heart's vectors can be clearly shown in the following picture:

Picture 1: Drawing of heart's vectors, charges at poles, and the number of each lead

In an ECG, the P wave represents atrial depolarization, QRS complex ventricular depolarization, and T wave ventricular repolarization.

There are some terms specific to interpreting ECGs and they are:

• Wave: Represents an electrical event. Always has a positive and a negative deflection
• Positive deflection: A wave above the x-axis on the ECG, representing movement of current to the positive axis
• Negative deflection: A wave below the x-axis on the ECG, representing movement of current to the negative axis
• Amplitude: The height of a wave, from the x-axis to its peak
• Segment: An electrically quiet area in the ECG cycle. There are 3: PR,ST, TP. Measured in mm.
• Intervals: Include waves and complexes, unlike segments. There are 3 intervals: RR, PR, and QT.

Picture 2: ECG showing the segments and intervals

ECG paper has grid made up of small 1mm- sided squares, which are grouped into big 5mm-sided squares. The horizontal axis represents time in sec, and vertically the amplitude in mm or voltage in mV. The paper travels at a speed of 25mm per second. That is 5 large boxes per second.

Picture 3: Values of boxes on ECG paper

Afterwards we went into detailed analysis of ECG waves and identified some rhythms.

The following table describes the characteristics of normal P waves and QRS complexes.

P wave	QRS complex
Normal: Upright, 0.08-0.10 sec duration, smooth and rounded. Less than 2mm high	Normal: Upright, width of 0.06-0.12 sec. Considered Wide if > 0.12 sec

                            Table 1: Characteristics of normal waves

Sinus Rhythm:

• Regular rhythm, 
• P-wave upright with normal rounded morphology, 
• P-R interval is 0.12-0.20 sec, 
• QRS width is 0.06-0.12 sec
• There is a QRS complex after every P wave
• There is a P wave before every QRS complex
• A T wave follows every QRS complex

The characteristics above are the same for all sinus rhythms, however, there is one characteristic that differentiates the different sinus rhythms. This characteristic is the HR or QRS rate.
In normal sinus rhythms the HR= 60-100 bpm. In sinus tachycardia the rate is >100 bpm, and in sinus bradycardia the QRS rate is <60 bpm.
However, there is one more abnormal sinus rhythm that has the same rate as the normal one, 60-100 bpm. This is the sinus arrythmia. The only characteristic of it that is different from the normal sinus rhythm is that sinus arrythmia is irregular. Where the RR interval varies by >0.04 sec. It is considered regularly irregular. This can be caused by changes in Vagal tone during inspiration and expiration, where HR increases during inspiration and decreases on expiration. This is due to changes in intrathoracic pressure which affects the venous return.
Another rhythm we learned was the junctional rhythm. This happens when the SA node ceases to function and the AV node takes over as the pacemaker of the heart. This rhythms characteristics include:

• Regular rhythm 
• P-wave absent. However, there could be a P' wave present before or after the QRS complex
• P'R interval is <0.12 sec
• QRS width is 0.06-0.12 sec
• A T wave follows every QRS complex
• Normally rate is 40-60 bpm (AV node rate)

If there are P' waves they are usually inverted, since the electricity coming from the AV node will be moving in the negative direction, retrograde.
One type of juctional rhythms is the accelerated juctional rythm. The only difference between it and the junctional rhythm the rate. Accelerated junctional rhythms have a rate of 60-100 bpm. Furthermore, junctional tachycardias also differ from juctional rhythms by their rate, where the rate of that tachycardia is >100 bpm. Finally, junctional bradycardia is similar to the junctional rhythm except its QRS rate is <40 bpm.
Moreover, in this lesson we learned how to systematically analyze an ECG rhythm following this set of rules:

1. Scan for any abnormalities in the rhythm
2. Is the rhythm regular or irregular?
3. If there are any P waves
4. Do QRS complexes follow each P wave?
5. Atrial and Ventricular Rates
6. Atrial and Ventricular rhythms
7. P, Q, and T waves configuration/ morphology
8. PR interval
9. QRS complex duration (too wide >0.12sec)
10. Interpretation/ diagnosis

Using these steps i analyzed the sinus and junctional rhythms in the ECG interpretation workbook. The following is my analysis of the first ECG under sinus rhythms:

Picture 4: My analysis of ECG 1 in the sinus rhythms

Enquiry and Research:
I read through the assigned readings in the Huszar's book and the following are notes of any new information i found different from the lecture. I read chapters 2,3,5 and 7.
In chapter 2, I found out that along the edge of ECG paper, either at the top or bottom, regularly spaced short, vertical lines are printed. These lines are an indication of time and are usually spaced 15 large squares apart. That is 3 secs apart. Moreover, on the ECG machine you can increase the amplitude. This is called "gain". This does not affect the depiction of the ECG waveform when printed, but is very helpful when analyzing a rhythm off the monitor (Wesley, 2011).
Also, atrial repolarization usually occurs during ventricular depolarization and it therefore hidden in the QRS complex. Moreover, the shape and length of segments and intervals provides information related to the speed of electrical conduction through the heart. When analyzing an ECG's waveform, the waves and complexes are evaluated based on their shape and timing, intervals on their length and segments based on their relationship with the baseline (Wesley, 2011).
For the ECG leads, whenever a different lead is selected, the ECG machine itself changes the polarity of the electrode. Lead II, most commonly used, is obtained by attaching the negative electrode to the upper right anterior chest wall below the right clavicle and the positive electrode to the lower left anterior chest wall over the apex of the heart (usually at the left intercostal space, midclavicular line). Furthermore, a lead with only one electrode, usually positive, is unipolar. The view of this electrode is usually in relation to a reference point calculated by the ECG machine located in the center of the heart's electrical field. These are usually used in 12-lead ECGs (Wesley, 2011).
Sometimes loose electrodes are in poor contact with the skin. This forms artifacts on the ECG. Loose electrodes is the most common cause of artifacts (Wesley, 2011).

Picture 5: Highlighted notes from chapter 2.

Picture 6: Highlighted notes from chapter 2.

From chapter 3, the following information was acquired. Depolarization of the atria begins near the SA node and progresses across the atria from right to left and downward. In an ECG, the first part of normal sinus P wave represents the depolarization of the right atrium and the second part the depolarization of the left atrium. This makes sense since the current going to the left atrium will be moving towards the negative lead , therefore, the negative deflection of the second part of the P wave. Furthermore, for the QRS complex the depolarization begins at the left side of the interventricular septum near the AV junction and progresses across the interventricular septum from left to right. Then, starting at the endocardial surface of the ventricles, depolarization progresses through the ventricles' walls to the epicardial surface.The first part of the QRS complex represents the depolarization of the interventricular septum and the rest of the QRS complex represents the depolarization of the left and right ventricles. However, since the left ventricle is larger than the right one, the QRS complex mostly represents the depolarization of the left ventricle. However, the depolarization in both is the same since they are coupled, as i learned last week.

Additionally, for the T wave the normal repolarization of the ventricles begins at the epicardial surface of the ventricles and progresses inwardly through the ventricles' walls to the endocardial surface (Wesley, 2011). 

The PR interval represents the time of progression of the electrical impulse from the SA node through the entire conduction system of the heart to the ventricular myocardium. Following atrial depolarization, the electrical impulse passes to the AV node where it slows momentarily before being transmitted to the ventricles. This period of slowing is represented by the PR segment. On the other hand, the QT interval represents the total time for the ventricles to depolarize and repolarize. The RR interval represents the one cardiac cycle during which the atria and ventricles contract and relax once. It is the time between two successive ventricular depolarizations. Also, the TP segment represents the time from the end of ventricular repolarization to the onset of the following atrial depolarization, during which electrical activity of the heart is absent. The ST segment represents the early part of ventricular repolarization. This is a period of electrical silence of the heart during which the mechanical contraction of the ventricles is reaching completion. An abnormal ST segment signifies abnormal ventricular repolarization, a common consequence of myocardial ischemia and injury. It is due to the damaged myocardium repolarizing earlier than the normal myocardium. An abnormal ST segment could be elevated or depressed. These are the result of multiple different causes (Wesley, 2011).

Causes of ST elevation:

• AMI
• Myocardial ischemia
• Prinzmetal angina
• Ventricular aneurysm
• Acute pericarditis
• Hyperkalemia
• Hypothermia
• And others

Causes of ST depression:

• Subendocardial MI (nSTEMI)
• Angina pectoris
• Reciprocal ECG changes in AMI
• Digitalis effect
• Hypokalemia

 Moving to chapter 5, i read about sinus dysrythmias. A normal sinus rhythm with a palpable pulse simultaneously with the QRS indicates that the heart is ejecting blood with each cardiac cycle. However, the presence of a normal sinus rhythm does not guarantee the presence of a pulse nor does it give any indication of the quality of the pulse if present. If a pulse is not palpable with the presence of a normal sinus rhythm this is called pulseless electrical activity (PEA). An abnormality of the sinus rhythm is sinus arrythmia. It is when the rate increases during inspiration and decreases during expiration. It is regularly irregular. Furthermore, sinus bradycardia is caused by any of the following:

• Excessive inhibitory vagal tone on the SA node
• Decrease in sympathetic tone on the SA node
• Administration of calcium channel blockers
• Disease in the SA node
• Acute inferior wall and right ventricular MIs
• Hypothyrodism
• Hypothermia
• Hypoxia
• During sleep and in trained athletes (Wesley, 2011)

In the presence of AMI, mild sinus bradycardia may be beneficial in some patients. This is because the workload of the heart is decreased, reducing oxygen demand of the myocardium, minimizing the extension of the infarction.
Some causes of sinus tachycardia include:

• Ingestion of stimulants
• Increase in catecholamines and sympathetic tone resulting from excitement or anxiety
• Congestive heart failure
• Pulmonary embolism
• AMI
• Fever
• Anemia
• Hypovolemia
• Hypoxia (Wesley, 2011)

Sinus tachycardia in acute coronary syndromes may increase myocardial ischemia and the frequency and severity of chest pain and can cause extension of the infarct.

 Furthermore, i read chapter 7 and these are my notes after,
A P' wave that occurs before the QRS complex is most likely originating in the proximal, upper part of the AV junction. A P' wave that occurs during or after the QRS has most likely originated in the middle or distal part of the AV junction (Wesley, 2011).

Picture 7: Highlighted points from chapter 7.

All the photos in this enquiry and research section of this week were adopted from (Wesley, 2011).
Doing the reading helped me understand and widen my knowledge about the topics covered in the lecture.

References 

Wesley, K. (2011). Huszar's basic dysrythmias and acute coronary syndromes: Interpretation and management (4th ed.). St Louis, MO: Elsevier.