HEART

Background reading: Sherwood pages 303-325, Chapter 9.

Objectives:

1. Differentiate between neurogenic and myogenic muscle.

2. Understand the conduction system of the heart and how an ECG wave corresponds to this system.

3. Define and measure pulse wave velocity and recognize its significance.

4. Determine the effect of exercise and Valsalva's maneurver on heart activity.

At eh level of the single muscle cell (muscle fiber), heart and skeletal muscle are very similar. The mechanisms of force generation are the same (i.e. Ca2+ release from the Sarcoplasmic Reticulum, binding to troponin, subsequent exposure of the cross-bridge site allowing for actin-myosin binding and thus contraction) in the two musce types. In both cases, the contraction is the result of a depolarization of the muscle cell. The differences lie in the way by which each is depolarized, by a neurogenic or myogenic mechanism. Skeletal muscle is neurogenic and requires innervation and stimulation by a nerve to depolarize and consequently contract.

Cardiac muscle is myogenic, indicating that it is capable of generating an action potential and depolarization (and consequent contraction) from within the muscle itself. There are no nerves which signal the heart to contract. This does not mean that the heart is not influenced by the autonomic nervous system. Sympathetic and Parasympathetic activity influence heart rate and myocardial contractility, but they do not generate the electrical activity that causes the heart to actually beat. Instead, the sinatrial (SA) node spontaneously depolarizes in a rhytmic fashion, thus setting the pace of heart contraction.

The intrinsic conduction system is a group of specialized cardiac cells that pass an electrical signal throughout the heart. This ensures that heart muscle tissue depolarizes and contracts in a sequential manner (from atria to ventricles) resulting in a coordinated heart beat. The intrinsic conduction systme is composed of the SA (sinoatrial) node, the AV (atrioventrical) node, the bundle of His, right and left bundle branches, and the Purkinje fibers. Referring to the figure below, you can see that these components spread the depolarization waves from the top (atria) of the heart down through the ventricles. In a healthy heart, this highly coordinated series of events happens correctly and the heart ejects blood efficiently to ensure proper blood flow to the entire body.

The autonomic nervous system, comprising the sympathetic and parasympathetic nervous systems, modulates the rhythm and strength of cardiac contraction. These signals from the spinal cord and brain are carried via nerves (the sympathetic and parasympathetic nerves, respectively), and release neurotransmitters onto the heart which speed or slow down the heart, respectively. The sympathetic nervous system also increases the strength of the contraction.

Figure 1: Diagram of the conduction system of the heart.

When the cardiac muscle fibers contract, they exert a force on the wall of the chambers (atria and ventricles) and this decreases the space within the chamber. The force of the compression on blood (a non-compressible fluid) puts blood under pressure. This is how blood pressure is developed, by the contraction of the heart muscle and resulting compression of the blood within the chambers of the heart. There are four chambers of the heart: two small atrial chambers and two large ventricular chambers. The atria receive blood from the veins, pump it into the ventricles, and the ventricles pump blood from the heart out to the major arteries. Under normal circumstances, the heart contraction cycle begins in the atria then spreads to the ventricles and the two sides of the heart (right and left) beat simultaneously.

 

The most common way to study heart activity is through the use of surface electrical recording of the electrical field. The huge number of fibers conracting simultaneously generates and electrical field that is easy to measure from the surface pf tje bpdu isomg am electrocardiogram (ECG). This is caused by the depolarization waves initiated from the SA node and spreading throughout the heart. To understand what the ECG is telling us, we need to briefly reivew the events in a normal heart cycle (from one contraction of the heart until the next contraction).

 

Figure 2: Description of the characteristics of the ECG. Note the characteristics of the P, QRS and T waves and the definition of the various intervals. Note also the timing of the mechanical events of the cardiac cycle. Figure credit: Troyan, M. http://www.bmb.psu.edu/courses/bisci004a/cardio/41card2.htm, date accessed 10/12/03.

Figure 3: Timing of the components of the ECG.

The first deflection, termed the P wave is due to the depolarization of the atria, the large QRS is due to the depolarization of the ventricles and concomitant repolarization of the atria, and the T wave is ventricular repolarization. By counting the number of PQRST "complexes" per unit time, one can determine the rate of heart contraction. One can also measure the time difference between subsequent R waves, termed the R-R interval. The closer together they occur, the faster the heart rate.

By measuring the internal differences in the events during the ECG, one can infer changes in the ability of the electrical signal to move througout the various regions of the hert. Figure 3 also shows the average time in seconds for each wave segment of the ECG.

Using a second transducer, which shows blood flow within the finger, we will also do another analysis that can be used to assess the Pulse Wave Velocity (PWV). Each time the ventricles contract and eject a bolus of blood into the arteries, the presssure on the arterial walls increases. This pressure travels along the arterial walls at a faster rate than the blood, creating a pulse wave. The velocity of the pulse wave is measured by calculating the distance between the heart (approximately in the center of the chest cavity) and the finger where the transducer is attached and dividing that distance by the time difference between ventricular contraction (The end of the QRS complex) and the generation of pulse pressure in the finger.

 

Lab Protocol:

 

Getting an ECG:

Attach a positive (red) electrode to the inside of your right wrist, attach a negative (black) electrode to the inside of your left ankle, and finally, attach the ground electrode (green) to the inside of your right ankle. The figure below refers to a standard three unipolar limb lead ECG set up where we will be measuring "Lead II". Different "Leads" give you a different perspective on the ECG as they are oriented to measure potential differences from various anatomical viewpoints. In a clinical situation, a twelve lead ECG would be used to present even greater perspective and allow a well-rained clinician to detect various defects of locate specific areas of tissue damage in the heart.

 

 Figure 4: An example of the placement of leads in a three-lead ECG.

Use the velcro strap attached to the pulse wave sensor to attach to your index finger. While recording, keep the finger still.

Start the ECG lab by clicking on the white right arrow. You should see two windows: one for the ECG, and one for the pulse wave. Record for a few seconds. Hit the "Stop Data" button and the ECG and Pulse wave form will appear together in the right screen.

What you will see:

Figure 5: Lab View Screen for ECG.

Lab Protocol:

A. Normal ECG: Make all of the measures below (A, B, C) at rest with the person seated and quiet.

1. Heart rate

Measure the time difference from one R wave to the next (note that your readout wil be in seconds). To determine the beats/min, take the reciprocal of the R-R interval and multiply by 60. For example, if the R-R interval is 0.5 seconds:

[1/0.5sec/beat]*60 sec/min- 120 beats/min

Measure the time difference for the various segments in the ECG:

2. Measures within a single cardiac cycle:

 

 

3. Calculation of Pulse wave velocity (PWV):

Pulse Wave Velocity (m/sec) = Distance (meters)/Delay Duration (seconds)

B. Effect of exercise on heart activity: Have subject continuously exercise by running in place or in the hallway/stairwell for 5 minutes. Please actually exert yourself or you will not see significant differences. Make the same measurements that you made for A above.

C. Effect of Valsalva's Maneuver on ECG:

The Valsalva's maneuver (VM) is performed by attempting to forcibly exhale while keeping the mouth and nose closed. It is used as a diagnostic tool to evaluate the condition of the heart and is sometimes done as a treatment to correct abnormal heart rhythms or relieve chest pain.

 Unhook the subject form the ECG and keep the pulse wave transducer attached to the finger. When the subject is relaxes and quietly resting, begin recording a new pulse trace. After about 10 seconds druign V.M., and for the 10 seconds immediatley afterwards. Measure the peak to peak intervals continously throughut this period (Yes, every single one!). When these rates are plotted on your graph (as described on the assignment), the x-axis should be titled "Consecutive Heart Beats", but is not actually "Time". Calculate the heart rate (after entering the formula in excel do not calculate all 30 seconds by hand!) for each heart beat and plot this on the y-axis.

This is a total 30 second trace. Note on your graph where each period of rest, V.M., and recovery occur.

 LINK TO HEART WRITE-UP