Electrocardiography (Proceedings)


Electrocardiography (Proceedings)

Nov 01, 2010

Electrocardiography is an integral part of the cardiological exam. It is the only way to determine heart rhythm accurately and to determine if there are any conduction abnormalities. This is also the most useful part of an ECG. ECGs can do other things however, these are not nearly as important.

An ECG is a recording of the electrical activity of the heart recorded on the surface of the body (or from the esophagus). Standard lead systems have been developed, these help determine the orientation of the depolarizing forces, an indirect indicator of the position of the heart in the chest cavity. These lead systems are however not needed to determine rhythm, you could in theory record the ECG from the fingertips if you have to.

I. ECG basics

      1. How can an ECG be recorded: A standard lead system can be used an electrodes attached to the limbs. This does require that the patient be in right lateral recumbency. This must be done if amplitude measurements are to be made (only in lead II). Alternatively a direct chest lead system can be used for rhythm diagnosis. Telemetry, holter monitors (24 hour ambulatory ECG) and cardiac event recorders (push a button and it records ECG immediately before and after the button is pushed) are other specialized ECG forms.
      2. What can an ECG do extremely well: Only the ECG can determine rhythm or conduction abnormalities.
      3. What can an ECG do well: The ECG can be helpful in evaluating for heart enlargement. These changes are however not specific nor sensitive. It is more sensitive in cats because there are less confounding breed differences. The ECG is also more helpful with certain congenital heart diseases. It can be helpful for diagnosing pericardial effusion and hyperkalemia.
      4. What can an ECG not do: It cannot give a definitive answer regarding heart size, imaging studies are needed. ECGs also do not reflect the mechanical strength of the heart, an ECG can be normal and no pulse may be present.
      5. What are the indications for an ECG: arrhythmia on auscultation, syncope, heart murmur, pre-, intra- and postop monitoring, dyspnea, cyanosis, drug monitoring (digoxin, beta blocker, tricyclic antidepressants), emergency cases (trauma, GDV, urethral obstruction, hyperkalemia suspect), periocardiocentesis (VPCs indicate the needle is tickling the heart), certain breeds (doberman, boxer), unexplained brady- or tachycardia.

II. Interpreting ECGs

      1. Waveforms and how they are generated: in the heart, as with many things the fastest wins, usually this is the sinus node, however any cell in the heart can develop pacemaker role. The electrical impulse depolarizes the heart muscle leading to contraction.
           a. P-wave: generated by the sinus node as the pacemaker, it is a sign of the depolarization of the right and then left atrium, usually positive in Lead II. Sometimes it is necessary to check the other leads recorded as the P-wave may be more obvious on other leads. A negative Lead I P-wave is an indication of faulty electrode placement.
           b. P-R interval: reflects the time it takes for a sinus beat to begin, depolarize the atria and get conducted through the AV node to begin initiating ventricular depolarization.
           c. QRS complex: The QRS complex represents ventricular depolarization. The Q wave is the first negative deflection before the R-wave. The R-wave is the first positive deflection after the P. The S-wave is the first negative deflection after an R. Remember, not all parts of a QRS have to be present.
      2. Determine heart rate
      3. Determine rhythm: This is something that requires practice.
      4. Measure complexes: measurement is overrated, it is mainly needed for enlargement patterns which are at times not very useful. Two things are measured, amplitudes and durations. Amplitudes can only be measured when a standard, by-the-book Lead II is recorded. You need to know the calibration of the machine, usually 1 cm (10 small boxes) is 1 mV. The other leads are helpful to determine MEA.
           a. Amplitudes: P-wave, QRS complex, ST segment depression or elevation.

Tall P= right atrial enlargement (P-pulmonale, no more than 4 small boxes tall in a dog)

Tall QRS=ventricular enlargement (no more than 25 to 30 boxes tall in a dog, 9 in a cat)

ST segment changes= hypoxia, epicarditis, electrolyte problems, etc.

           b. Durations: P-wave, PR, QRS duration

Increased P=left atrial enlargement (dog no more than 2 small boxes)

Increased QRS=ventricular enlargement or conduction abnormality (normal no more than 2 boxes, with bundle branch block then QRS duration >8msec, 4 boxes at 50 mm/sec)

PR=prolongation is 1st degree heart block (more than 6.5 small boxes in a dog), shortening is consistent with ventricular preexcitation.

Before embarking on antiarrhythmic drug therapy it is important to consider the goals present. My personal feeling is that 2 goals are potentially present

      1. Treat clinical signs resulting from an arrhythmia
      2. Prevent sudden death in a patient from an arrhythmia

Unfortunately, many of the drugs we use cannot accomplish the latter goal and those medications that could are difficult to use in an emergent situation because of the side effects present.

Clinical signs from an arrhythmia can result because of rapid or slow heart rates. With rapid heart rates ventricular filling is inadequate and therefore also cardiac output. This also often eliminates the atrial "kick" from contraction of the atria, this is obviously the case if it is a ventricular arrhythmia. With slow heart rates cardiac output will also suffer since cardiac output is related to stroke volume times heart rate.