Cardiophysiology I – PDF

Learning Outcomes

• Describe the intrinsic conduction system of the heart.
• Describe the various components of a typical EKG.
• Connect the components of a typical EKG to activities of the heart.
• Define important terms connected like tachycardia, bradycardia, fibrillation, arrhythmia.
• Be able to obtain ECG recordings.
• Identify and interpret the different components of a typical EKG.

This lab covers the intrinsic conduction of the heart. Basically, it discusses what is involved in getting the heart to do what it does. The nervous system, like other organs systems and parts of the body, coordinates and communicate with the heart. However, the heart has its own control or intrinsic conduction system. This intrinsic conduction system controls the contraction and relaxation of the heart, which in term allows the heart to receive and pump blood to the body. The main parts of this conduction system are the sinoatrial node or SA node, the atrioventricular node or AV node, the bundle of HIS, bundle branches, and Purkinje fibers. Signals generated from the SA node will be propagated to the AV node and eventually to the Purkinje fibers which by then will coordinate the contraction of the ventricles. Unlike skeletal and smooth muscles, cardiac muscles are capable of initiating action potentials that will enable them to contract without the outside help of the central nervous system. This is referred to as automaticity. Let’s take a closer look at the cardiac intrinsic conduction system.

Figure 1: Conduction system of the heart. Anterior view of the heart showing the components of the conduction system of the heart (Credit: OpenLab Anatomy and Physiology, CC BY 3.0).

The Cardiac Intrinsic Conduction System

The heart muscle cells can contract without input from the CNS. The heart has its own integrated conduction system which initiates and directs contraction of both atria and ventricles. These cells are special cells known as pacemaker cells or conducting cells. They have capabilities that are different from regular cardiac myocytes. Regular myocytes can contract and relax but are limited to in initiating their own contraction. In contrast, pacemaker cells have an innate ability to contract on their own. This is known as automaticity. Initiation of normal activity begins with the SA node.  It is located in the walls of the right atrium. The impulses, once initiated follow a set path. From the SA node impulses will be transmitted to the AV node. This transmission is not immediate. There is a short delay, about 50 ms, from the time it is initiated at the SA node until it passes over to the AV node via the intermodal pathway. This delay is essential to allow the atria to contract.  Both atria must contract and relax at the same time. After the SA node initiates the impulse it passes over to the left atrium via the Bachmann’s bundle. Once the AV node receives the impulse it passes it over to the bundle of His, after a short delay, which will them transmits it over to the left and right bundle branches located in the interventricular septum. These bundle branches will transmit to the Purkinje fibers located in the ventricular walls. The impulses in the Purkinje fibers will initiate and complete the contraction of the ventricles. The right ventricle has a special muscular connection between the interventricular septum and the left ventricular wall called the moderator band, which help in transmitting impulses to the ventricular walls. The impulses from the interventricular septum will also be passed on to papillary muscles to prevent prolapsing of the AV valves during ventricular contraction.

Electrocardiogram and Heart Activities

Contraction of the heart depends on electrical activity initiated by the SA node. Similarly, the nervous system also depends on electrical activities when transmitting impulses. Both cardiac cells and skeletal muscles undergo depolarization followed by repolarization during an action potential. However; cardiac cells, unlike skeletal muscle cells, have a more rapid depolarization and a plateau phase.

Cardiac muscle cells also have a longer refractory period which is essential for efficient contraction and pumping of blood. Like skeletal muscles, depolarization occurs during the influx of Na+ into the cells. Within 3-5 ms Na+ channels close followed by opening of Ca2+ channels. These channels remain open and influx of Ca2+ into the cells lead to the plateau phase during cardiac cell action potential. Eventually these Ca2+ channels will close and K+ channels will open allowing K+ to exit the cells resulting in repolarization. During a cardiac cell’s action potential there is a period when another action potential  will not be able to be generated because NA+ channels will not be able to respond other stimulus, because they are open, closed or are inactivated. This is known as the absolute refractive period. This delay in repolarization is necessary to allow the contraction of the heart to occur and be efficient. Immediately following this absolute refractive period, is a point when, depending on the intensity of the stimulus, another action potential can be generated if it is strong enough. This period is known as the relative refractive period. It is important to note that other components of the intrinsic conduction system are also able to initiate impulses on their own, although at a slower pace than the SA node. For instance, if the SA node is unable to function as a result of coronary artery diseases, the other components of the intrinsic conduction system will be able to depolarize and initiate action potentials, but at a slower pace.

The electrical activity of the heart can be recorded using electrocardiogram (ECG), also known as EKG. This has been a very important instrument to diagnose heart conditions. The readings picked up by electrodes placed on a patient, is transmitted to an instrument capable of displaying representations of the different heart electrical activities. These activities translate to contraction and relaxation of heart walls and chambers. On an ECG, there are waves, complexes, intervals and segments. Waves can be positive, above the straight line or isoelectric line, or negative, below the isoelectric line. More than one consecutive wave makes a complex. Distance between two waves is a segment, while one or more waves and a segment is known as intervals. Each feature of the ECG represents different heart activities. The P wave represents atrial depolarization. The QRS complex, which includes three waves (QRS), represents depolarization of the ventricles. Depolarization is not contraction but will lead to contraction. The third wave, usually positive, represents repolarization of the ventricles; an event that will lead to relaxation of the ventricles. Repolarization of the atria takes place, but it is not seen on an ECG because it is masked by the bigger QRS.  Other components include PR interval, from the start of the P wave to the start of the Q wave; the P segment, from the end of the P wave to the start of the Q wave; the ST segment, from the endo the S wave to the start of the T wave; the QT interval, from the start of Q wave to the end of T wave. The table below gives the various interpretations of segments and intervals seen on an ECG. Also given are the normal values for each component. Interpretation of ECG also relies on the duration of each of these components. There is a normal range for each component. Deviation from these ranges can be indicative of conditions affecting the normal function of the heart. To facilitate reading and interpretation of an ECG, the strip or paper is divided into squares. On an ECG stripe each small square represents 0.04 sec. and one large square represents 0.2 secs.

Heart rate can be calculated, if the heart rate is regular, by dividing 1500 (# of small squares in a minute) by the number of small squares between two R waves (or 2 P waves) or dividing 300 by the number of large squares. See examples below.

1500/# of small squares
If there are 22.5 small squares between two RR intervals, then the heart rate will be
1500/22.5 = 66.66 bpm

300/#of large squares
If there are 4.5 large squares, then the heart rate will be
300/4.5 = 66.6

If the rhythm is irregular then one can count the number of RR intervals in 6 sec (30 large squares) and multiply by 10.

Normal heart rate is between 60 – 100 bpm, however average resting heart rate is in the 70s for both men and women. Heart rates below 60bpm are referred to as bradycardia and rate above 100bpm is tachycardia.

Activity 1 – The ECG

Conditions involving irregular heart activities

• Cardiac arrhythmias or Irregular heart rate – deviation from normal heart rate.
  • Atrial fibrillation (A-fib) – absence of P wave. Atria and ventricle activity are not coordinated.
  • Ventricular fibrillation (V-fib) – uncoordinated activities or contraction of the ventricles
  • Premature ventricular contractions (PVCs) – “skipped heartbeat” sensation. There is an extra heartbeat in the ventricles.
  • Bradycardia – Abnormally low heart rate (less than 60 bpm)
  • Tachycardia – Abnormally high heart rate (above 100 bpm)
• Heart blocks – impairment of electrical conduction from the SA node to the purkinje fibers. May be of different types depending on the affected area.
  • First-degree heart block – slow electrical impulses through the conduction system
  • Second-degree heart block – signals are unable to reach the ventricles.
  • Third-degree heart block – complete blockage . electrical impulses do not reach the ventricles.
• Long QT syndrome (LQTS) – The QT interval is longer than normal indicating the ventricles are taking longer to contract and relax.

 

Activity 2 – Interpreting an ECG

Measuring electrical activity of the heart – EKG

A typical EKG machine has 12 leads (angles) calculated using 10 electrodes. These electrodes are placed on the body to measure the hearts electrical activities. In this lab we will only make use of 3 to four electrodes which will be used to determine three leads (I-III). These leads will then be used to determine the heart activity.  The three leads forms what is known as Einthoven triangle, it is presumed that the heart is at the center of this triangle. The electrodes to be used will be; one electrode on the right arm (RA), one on the left arm (LA), one on the left leg (LL) and one on the right leg (RL). If the RL is not available, the three others will do. The leads are as follows;

Lead I – connects right arm to left arm –
Lead II – connects right arm to left leg – ideal for examining the various waves or                                     deflections.
Lead III – connects left arm to left leg

After the electrodes have been set and the readings have been taken, a printout of the readings will be given. Description of the squares and measurements has been described previously. To get good readings follow these particular guidelines.

1. Be sure to be still when the readings are being taken.
2. Remove watches and other metals to prevent interference.
3. Be sure to wait to give the instrument time to take the best reading.

Activity 3 – ECG Measurements