Cardiology in Critical Care
The Electrocardiogram
nursebob
10/15/2005

The EKG Measures the Heart's Electrical Activity

            As it beats, the heart generates small electrical currents. A recording of this electrical activity is called an "EKG" (electrocardiograph). The terms EKG and ECG mean the same thing. EKG comes from the German language while ECG comes from English. A standard EKG is obtained by placing 12 small electrodes on the patient's body in a specific pattern. The EKG machine is then turned on, and the recording is made in just a few moments. The test is entirely painless.

            Each of the heart's beats can be divided into three main parts. The first part is the small P wave which represents the atrial contraction. The second part is the tall QRS spike which represents the ventricular contraction. The third part is the large T wave which represents the relaxation of the ventricles. By analyzing the exact pattern of the EKG, doctors can learn a great deal about how the heart is working.

The Heart Beat: Contracting and Relaxing

            The heart beat is usually divided into two main phases called "diastole" and "systole." During the first phase (diastole), the heart relaxes and fills with blood. During the second phase (systole), the heart contracts and pumps out the blood. The heart typically spends about 2/3 of its time in diastole and 1/3 of its time in systole. Keeping this activity well timed is the job of the heart's conduction system.

            The signal to begin a contraction starts in a small region at the top of the right atrium called the "sinoatrial (SA) node." From this point, the contraction spreads over the two atria like a wave. The atrial contraction pumps the blood down into the ventricles. Next, the signal to contract reaches a small region of the heart called the "atrioventricular (AV) node." As its name implies, the AV node sits right between the atria and the ventricles. The AV node is smart, and it delays the signal just long enough to allow the atria to finish contracting. From here, the signal to contract travels down special fibers that connect to the bottom of the heart. This causes the ventricles to contract from the bottom to the top, and ensures that the blood is pumped up and out, into the lungs and the body. At the end of the cycle, the ventricles relax and the whole process begins again.

 

ECG Paper – What it means!

You must understand that two things are being measured when looking at a ECG, time and voltage.  The ECG is a wave recording of the electro activity of the heart placed on graph paper.  In order to adequately read EKGs you must know what is happening during each wave. All rhythm interpretation must be correlated with other signs and the condition of the patient for successful outcome of any resuscitation attempt. Always remember the admonition “treat the patient, not the monitor.” Is There a pulse?

 

You Must Know What Is Happening When!

When the P occurs, the SA node fires and atrial depolarization produces a sinus initiated rhythm. They P wave should not be longer than 0.20 seconds. A prolonged P waive may indicate a heart block.

P-Wave:  Not greater than 0.20 seconds. If it is longer may indicate first degree heart block. SA Node fires.  Atrial Depolarization  Sinus initiated rhythm.

QRS:  Depolarization of the ventricles.  Ventricular Contraction. Atrial Repoliarization. (Can't always be seen on EKG)

T-Wave:  Ventricle repolarized.  May be affected by changes in serum K levels.

Down and Dirty Rhythm Strip Analysis

1.   Determine Heart Rate. Count small boxes between two R waves. Divide into1500 Gives BPM.  Normally 60-100 bpm in adults.  When the rate is greater it is some type of a Tachycardia.

2.  Eyeball rhythm. It should be regular and even.

3.  Are P waves present? Are they Normal?  P-R interval 0.12 - 0.20 sec and consistant.

 Monitor Lead Placement

Most monitors have standardized colors which allow proper monitor lead placement.

1. The white lead is placed on the right side of the chest. This is usually called the “right arm lead.”

2. The black lead is placed on the left side of the chest. This is called the “left arm lead.”

3. The green lead is placed on the right lower abdomen, it is called the “right leg lead.”

4. The red lead is placed on the right lower abdomen, it is called the “left leg lead.”

5. The brown lead is placed slightly to the right of the midsternum, it is called the “V lead or chest lead.”

 

After a heart attack.

 

Laboratory values after heart attack.

1. White Blood Cells - elevated after MI.

2. BUN, Creatine maybe elevated after MI.

3. Glucose maybe elevated after MI. (Bodies reaction to stress)

4. Coagulation Studies.

            a. PT, PTT
            b. PT normal 11-15 seconds.

            c. Patient should be two times higher than control to be therapeutic.
            d. PTT is used to monitor heparin.


MI Locations

ST elevations in MI

I III AVF - Inferior Right
V1 V2 - Septial
V2 V3 V4 - Anterior LAD
V5 V6 AVL I - Lateral Circumflex
Left anterior descending affects V1 V2 V3 V4
Inferior Right Coronary Artery affects II III AVF
Lateral Left Circumflex Artery affects I AVL V5 V6