Week 3

Reflection
Using the Gibb’s Reflective Model  I will reflect on the third lab session we had for this semester. It took place on the 16th of September.

Description: In the lab session this week I worked through the Practical Simulation Workbook for approaching the cardiac patient. At first I did the pre-reading from my core textbook, then I attempted to memorize the PQRST acronym, since we uses to always use DOLOR+S previously. After that I watched some videos included in the workbook, to refresh my memory on the VSS skills. Following that I assessed my confidence levels performing these skills. I was confident to do all the skills except performing a manual BP measurement and using the acronym PQRST. Furthermore, I read further in my core text book about the different cardiac conditions, such as Acute Coronary Syndrome, and learned about what a cardiac patient presentation looks like.

Feelings: I was thinking of how vulnerable these patients are, and how important it is to identify them ASAP. I felt very confident in most of the skills, however, I felt unsure of how to perform a manual BP. Moreover, I felt confused about the PQRST acronym and kept forgetting what it stood for. However, I felt very interested doing the readings, since they provided me with insight about the cardiac patient.

Evaluation: It was a good academic experience to be able to work through the workbook. This made me realize where I needed to improve in my clinical skills, and provided me with greater knowledge about cardiac patients.

Analysis: It is very important to work through the theory before applying it in the lab, in a scenario. Working through the workbook helped me achieve the theory base I needed to be able to assess and identify a cardiac patient. Additionally, I believe I am not that good at using PQRST because I’m so used to using the acronym DOLOR+S for pain assessment.

Conclusion: I could have practiced my manual BP skill more. Also, I could have applied the PQRST to a scenario to help me memorize it and understand it better.

Action Plan: My plan is to constantly practice measuring a manual BP at home and whenever I get the chance at college. Moreover, I plan to continue training on using PQRST until I get used to using it instead of DOLOR+S.

Domain Knowledge:

This week's lecture discussed the anatomy of the myocardium, how it contracts, and its properties. Moreover, depolarization, repolarization, resting membrane potential and refractory periods are highlighted. Also, some essential concepts were covered in the end. 

For the anatomy of the myocardium, the following are my notes about it:

Picture 1: Anatomy of a sarcomere

The properties of the myocardium include:

• Contractility: the ability of fibers to shorten when stimulated
• Conductivity: fibers can transmit action potentials easily
• Excitability: the capacity to respond to stimuli
• Automaticity: ability of the heart to spontaneously depolarize independent of neurohumoral control
• Refractoriness: the time the cell will not respond to a stimulus
• Expansibility: ability of the heart to stretch as it fills 

These characteristics of the heart are easily understood, since their names are descriptive of what they are.

The depolarization and repolarization of myocardial cells occur as a chain of events that can be illustrated in the following flow chart:

Picture 2: Flow chart of events leading to depolarization and repolarization of myocytes

Depolarization and repolarization occur as 4 phases described below:

Picture 3: Notes about the events in the phases of depolarization and repolarization

Picture 4: Graphical depiction of the electricity at each phase and how it reflects on an ECG

ECG captures the electrical current during depolarization and repolarization of the myocardium. Repolarization currents run in the opposite direction to those from depolarization.

Refractory periods are periods in which the cells will not respond to a stimulus. These occur between phase 0 and the end of phase 3. On the ECG, that is from the beginning of the QRS complex to the end of the T wave. There are two: Absolute and relative.

Absolute Refractory period occurs from phase 0 till halfway of phase 3, this is the peak of the T wave on the ECG. This is where the myocardium cannot pass on another action potential and cannot contract again. This is because the cardiac cells will not have repolarized to their threshold potential (around -70mV). Threshold potential is the point where another action potential can be conducted. The cell does not have to repolarize to the resting membrane potential. Repolarizing to the threshold potential is enough to convey another action potential. 

Furthermore, the relative refractory period is from halfway through phase 3 till the end of it. This is shown on ECG as the peak of the T wave till it's end. In this period, the cardiac cells would have repolarized to the threshold potential and can be stimulated by another action potential. 

A very interesting note is about the shape of the ECG. Any deflections in the positive direction represent the direction of electricity vectors in the heart moving from the base to the apex. Any negative deflections are due to vectors moving from the apex of the heart upwards.

The last essential concepts learned are: 

Picture 5: A summary of the essential concepts learned about the electrophysiology of the heart

Enquiry and Research:

I read further to revise the microscopic anatomy of myocardial cells. I found that the cardiac muscle is striated, short, fat, branched and interconnected. Also, each fiber contains one or at most two centrally located nuclei. Adjacent cells are connected by intercalated discs. These discs contain desmosomes and gap junctions. Desmosomes prevent adjacent cells from separating during contraction. Gap junctions electrically couple cardiac cells so that the myocardium acts as a single coordinated unit. Moreover, 25-35% of cardiac cells' volume is mitochondria. This makes the cardiac cells highly resistant to fatigue. Most of the remaining volume is occupied by the sarcomeres. Each cardiac cell contains multiple sarcomeres (Marieb & Hoehn, 2014).
Moreover, i found a very good graphical representation of the action potentials at different locations in the heart. This is shown below:

Picture 6: Graphical representation of action potential in different areas of heart. Adopted from (Marieb & Hoehn, 2014)

My teacher discussed the use of synchronized cardioversion and how it should be delivered with caution to avoid ventricular fibrillation (VF). I read further on this and found that synchronized cardioversion is done mostly at a specific point of the QRS complex. It is avoided during the T wave especially the middle and second half of the T wave, or at the relative refractory period. If given during the T wave, or the vulnerable period, it can induce VF (AHA, 2010).

References

American Heart Association. (2010). 2010 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care science part 6: Electrical therapies. Circulation, 122, S706-S719. doi: 10.1161/CIRCULATIONAHA.110.970954

Marieb, E.N., & Hoehn, K.N. (2014). The cardiovascular system: The heart. In E.N. Marieb & K.N. Hoehn (Eds.), Human anatomy and physiology (pp. 713-741). Essex, England: Pearson Education.