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ECG AND BLOOD PRESSURE

 
Click here for a data sheet of this lab in MS Word.doc
 

I. INTRODUCTION

The purpose of today's laboratory is to familiarize the student with the techniques and concepts associated with the recording of the electrocardiogram (ECG) and arterial blood pressure.

A. The ECG

The fundamental property of cardiac muscle is rhythmicity, that is, the ability of the myocardium to contract and relax spontaneously. This inherent property is developed to the greatest extent in a region known as the sinoatrial node (SA node) which is situated in the posterior wall of the right atrium near the entrance of the superior vena cava. The beat or impulse is generated at the SA node (a.k.a. "pacemaker") and spreads over the atrial muscle producing a contraction of the atria. The impulse reaches a second site of specialized tissue called the atrioventircular node (AV node) also on the right side of the heart between the atrium and the ventricle. Here the impulse is delayed to allow complete atrial contraction. From the AV node the impulse moves rapidly along the right and left bundle branches and, via the Purkinje system, reaches the entire ventricular myocardium within 0.06 sec. The relatively rapid spread of the impulse through both ventricles allows for simultaneous ventricular contraction.

The impulse generated at the SA node is a highly localized electrical current that in turn causes a depolarization of the adjacent muscle cells. The myocardium is said to exist in a functional syncitium (i.e., functionally there are no boundaries to the wave of depolarization) the depolarization of the remaining atrial myocardium is thus affected. An advantage of this mechanism is that the strength of the current generated at the SA node is not diminished when it reaches the more distant muscle cells. Depolarization and repolarization of the myocardium is, of course, associated with the changes in permeability of Na+ and K+ and their respective fluxes through the muscle cell membranes.

The alternate depolarization and repolarization of the heart produces electrical currents that can be detected on the surface of the body. Therefore, it is possible to monitor these electrical waves by amplifying and recording them. Such a recording is referred to as the electrocardiogram (ECG).

B.  The Standard ECG (see Fig. 11.10, page 276 in text)

Link to normal EKG pattern.
Link to examples of a 12 lead ECG showing Myocardial infarction with ST segment deviation.

The Standard ECG
 

A volume conductor is a medium that permits the conduction of electricity in three dimensions. The body, by virtue of its fluid and ion (or electrolyte) content, is essentially a volume conductor. The current generated in any part of the body can reach and be detected in any other part. The electrical impulses originating in the heart are thus picked up at standard locations (defined originally by Einthoven) and produce characteristic wave patterns.  The typical series of waves were arbitrarily designated by Einthoven as the P wave, the QRS complex, and the T wave (see Figure 1 above). These electrical events are the following interpretations:

P wave: depolarization of atrial musculature (0.11 sec).

P-R interval: beginning of P wave to onset of Q; this represents the time to depolarize atrial muscle and the delay in transmission of the AV node (0.2 sec).

QRS complex: depolarization of the ventricles (0.1 sec).

ST segment: between the end of the QRS complex and the beginning of the T-wave; this represents the depolarized state or the duration of the excited states of the ventricular muscle.

T wave: the repolarization of the ventricles.

TP interval: end of T wave to the beginning of succeeding P wave; time between two beats.

As the result of the electrical activity generated and the ensuing contraction of the cardiac muscle, blood which has filled the atria during diastole is ejected from the ventricles.
 

B. Blood Pressure


There are four basic pressures which are classified under the general term "blood pressure." These four pressures and their definitions are as follows:

1. Systolic pressure: This is the highest pressure observed in the artery and is a product of the heart's systole (contraction). It is representative of the total heart energy.

2. Diastolic pressure: This is the lowest pressure observed in the artery and it results from the drop in arterial tension during diastole (rest) of the heart. It represents the sum of factors acting contrary to the cardiac force, e.g. peripheral resistance.

3. Pulse pressure: This is the difference between the systolic and diastolic pressures. It represents the efficiency of the heart indicating the extent to which it overcomes the peripheral resistance.

4. Mean pressure: This is the average pressure to which the arteries are subjected. Due to the irregular shape of the blood pressure pulsitile curve, it is not the simple arithmetic mean of the systolic and diastolic pressures, but is more approximated by the following:   Mean pressure = diastolic pressure + 1/3 (pulse pressure)

Blood pressure can be measured directly with a strain gauge in conjunction with an indwelling catheter. This technique is not applicable, however, under most circumstances, so indirect methods for measuring blood pressure have been devised. The indirect method used in the present experiment involves placing an inflatable rubber bag enclosed in a cloth ban around the lower portion of the arm, directly over the brachial artery. The inflatable bag is connected to both a mercury manometer an inflating bulb. This entire unit is referred to as a sphygmomanometer (see page 269 of text).

click here for link to BP measurement

With the cuff applied snuggly over the arm, pressure is introduced into the rubber bag via the inflation bulb to a level sufficient to completely collapse the artery, the exact pressure being indicated by the manometer. When this compression is reduced gradually by releasing the air compressed in the cuff, the artery tends to return to its normal size. As the pressure is being released from the cuff, the artery passes through several phases which may be recognized by listening to the sounds produced by the vibration of the arterial wall as it responds to the varying degrees of in-rushing blood. These sounds can be detected by placing a stethoscope directly over the brachial artery just distal to the blood pressure cuff.

On further reduction of the cuff pressure, this loud, clear sound changes to a dull thumping tone (Korotkoff sounds). The minimum pressure reading at this transition represents the diastolic pressure. The dull thumping sound eventually disappears shortly after the attainment of the diastolic pressure. Since the start of the dull thumping represents the fourth sound change and the start of silence represents the fifth sound change, these are referred to as the 4th and 5th phase diastolic pressures, respectively. While most investigators feel the 4th phase diastolic pressure represents most accurately the true diastolic pressure, some prefer to use the 5th phase because it is easier to detect in most subjects.

click here for BP classification

In releasing the pressure initially, pulsations of the mercury column will be observed before the attainment of the first should (systolic pressure). This is simply due to the pulsating of the blood against the top edge of the cuff and is to be disregarded.
 

II. PROCEDURES

A. Blood Pressure:

  • Please work in groups to perfect your technique for measuring blood pressure. Switch partners as often as possible. Get a lot of practice on different people.
  • For each member of your group, record blood pressures (systolic and diastolic) and gender. Elect one person to record this information on the board (or computer).
  • Record BP for the different conditions as explained by your TA.  Record these values in the data table (table #2) below.
    1. Obtain resting (supine) HR and BP (3 min)
    2. Obtain HR and BP dynamic large muscle mass exercise (3 min)
    3. Supine (3 min)
    4. HR and BP during dynamic small muslce mass exercie (3 min)
    5. supine (3 min)
    6. HR and BP during isometric exercise (2 min)

    B. ECG:

  • Each group should record a standard ECG for one member only.

    • III. RESULTS

  • Each student should present the below data in his/her own lab report.
  • Record the gender of your class member. Calculate MAP for at for your subject dueing different conditions.  (see table 1  below).
  • Record all HR and BP data for your subject in table #2
    • TABLE 1.
    Systolic (mmHg) Diastolic (mmHg) Gender Pulse Pressure Mean Arterial Pressure (MAP) Condition
               
               
               
               
               
               
               
               
               
               
               
    Table 2.
     
    Condition Heart Rate (b/min) Blood Pressure (mmHg) Systolic/Diastolic
    supine    
    dynamic large muscle exercise    
    supine    
    dynamic small muscle exercise    
    supine    
    isometirc    

    IV. DISCUSSION (to be turned in at the end of class)

      1.  What is the blood pressure that is defined as hypertension?
       
       
       
       
       
       
       
       
       
       

      2.  Draw or explain an ECG recording of a patient with ST segment depression. What does ST segment depression represent?
       
       
       
       
       
       
       
       
       
       
       
       
       
       

      3.  What are the effects of change in body position on systolc BP and why do the changes occur?
       
       
       
       
       
       
       
       
       

      4.  What are the effects of fine motor vs. gross motor exercise tasks on systolic BP and why do these changes occur?
       
       
       
       
       
       
       
       
       
       

      5.  Which condition elicited the greatest change in HR?  How can this be explained?