Abstract

Specialized neural-like conductive tissues and their approximate firing rates.

Depolarization and repolarization of cell membranes. A) The resting cell membrane is charged positively on the outside and negatively on the inside. B) Following a stimulus (S), positive ions enter the cell reversing this polarity. C) This process continues until the entire cell is depolarized. D) Ions are returned to their normal location and the cell repolarizes to its normal resting potential.

Summary of events of a cardiac cycle. Of the 8 physiologic events listed for a cardiac cycle, only 3 are actually observed on an ECG tracing.

A) Einthoven's triangle and B) standard limb leads I, II, and III.

Standard ECG paper.

The normal ECG tracing.

Sinus bradycardia. Each cycle commences with a P wave and the PR interval is normal. Therefore, rhythms are sinus-paced and differ only in rate: normal sinus rhythm, sinus bradycardia, or sinus tachycardia. In this case, it is sinus bradycardia, because the rate is <60.

Junctional rhythm. There are no P waves and a PR interval cannot be ascertained. Therefore, the sinoatrial node is not pacing this rhythm. But the QRS complexes are narrow, so the pacemaker is above the ventricles. The logical conclusion is that the atrioventricular node or neighboring tissue is pacing the heart. This is called junctional rhythm. Because this node has a slower firing rate than the sinoatrial node (See Figure 1), rates of 50 and 90 are the cutoffs for bradycardic and tachycardic rates, ie, junctional bradycardia or tachycardia.

Normal sinus rhythm with first-degree atrioventricular block. Each cycle commences with a P wave, but the PR interval is prolonged. Therefore, rhythm is sinus-paced but the impulse is being delayed at the atrioventricular node. Rates can be normal, bradycardic, or tachycardic.

Supraventricular tachycardia. There are no P waves and a PR interval cannot be ascertained. Only 1 wave is discernible between QRS complexes and one cannot determine whether a P wave is absent or occurring simultaneously with the T wave. The rhythm is rapid, but one cannot conclude whether it is sinus-paced or paced by some other tissue. It could be sinus tachycardia or junctional tachycardia, but we can't be sure. This dilemma surfaces when rates become greater than 150. Therefore, because the QRS complexes are narrow, we know only that the rhythm is being paced from above the ventricle. Is it sinus or junctional paced? We “cop out” and call it “supraventricular.”

Atrial flutter. Multiple waves appear between each QRS complex and we cannot ascertain whether they are P or T waves. This pattern emerges when an ectopic pacemaker emerges in the atrial muscle and fires more rapidly than the sinuatrial node. This generates multiple depolarizations in the atrial muscle, reflected as so-called flutter waves. Each has a slant to its anterior portion; we can describe this as a saw-toothed pattern. Normally, the atrioventricular node allows only one of them to pass into the ventricle each cycle, which results in a regular ventricular response.

Premature atrial and junctional complexes. Most cycles commence with a P wave, and most PR intervals are normal. Therefore, the rhythm is sinus-paced, but occasionally an extra impulse is fired from an ectopic pacemaker that travels down into the ventricle and creates an extra QRS complex. Notice that normally there is a pause, or a period of time following a T wave until the next P wave commences. In the case of premature complexes, this pause is interrupted. At this point in your training, it is not important to interpret the source of this premature complex; is it atrial or junctional? We know it is coming from above the ventricle, and it is always acceptable to call it a premature atrial complex. The difference between the two has little clinical relevance.

Atrial fibrillation. The waves between each QRS complex are random and indistinct; in essence, they're a mess! Furthermore, the R-R intervals are consistently irregular. This pattern emerges when several ectopic pacemakers emerge in the atrial muscle and all fire more rapidly than the sinuatrial node. This generates multiple depolarizations in the atrial muscle, far more numerous than those with atrial flutter. The atrioventricular node is so overwhelmed with impulses that it cannot allow any to pass through on a regular basis. Therefore, we see this striking irregular ventricular response.

Normal sinus rhythm with second-degree (Mobitz) atrioventricular block. Each cycle commences with a P wave, but occasionally the P wave is not followed by a QRS and another P wave appears. This is called a “dropped beat” and is the fundamental defect in a second-degree or Mobitz block. First look at tracing A. (Don't be disturbed by the fact that the QRS complexes go down instead of up. Waves are waves! Their direction depends on the particular lead used to record the tracing.) Notice that each successive PR interval lengthens until finally 1 P wave stands alone and a beat is dropped. Also notice that after the beat is dropped, the PR intervals commence again to progressively lengthen until another beat is dropped. This strange pattern of PR intervals was first described by a cardiologist named Wenckebach. Therefore, this type of second-degree block is called a Mobitz 1 or Wenckebach block. In tracing B, notice that all PR intervals are identical. They may be normal in length or delayed, but they are all the same; even after a beat is dropped, they resume their duration. This is called a Mobitz 2 block. In this particular example, the ratio of P waves to QRS complexes is 2 : 1. Therefore, the R-R intervals are regular. With any other ratio, eg, 3 : 1 or 4 : 1, the R-R interval would appear irregular.

Ventricular tachycardia. There are no P waves and a PR interval cannot be ascertained. No waves are discernible between QRS complexes, but the R-R intervals are regular and the QRS complexes are wide. The rhythm is rapid and is being paced by tissue in the ventricle. This rhythm differs from supraventricular tachycardia (Figure 10) only in the fact that the QRS complexes are wide rather than narrow.

Idioventricular rhythm. There are no P waves and a PR interval cannot be ascertained. No waves are discernible between QRS complexes, but the R-R intervals are regular and the QRS complexes are wide. The rhythm is slow and is being paced by tissue in the ventricle. This rhythm differs from ventricular tachycardia (Figure 15) only in the fact that the rate is slow; it could just as well be called ventricular bradycardia.

Third-degree (complete) block. There are P waves but the PR intervals appear inconsistent; no pattern is repeated. If impulses were being conducted into the ventricles, the R-R intervals would be irregular and the QRS complexes would be narrow. Neither is the case, however; the R-R intervals are regular and the complexes are slightly widened. (They get wider and wider according to the location of the ventricular pacemaker. In this case, the pacer is probably in the bundle of His, because the complex is relatively narrow.) On closer analysis, one can detect that intervals between P waves (P-P intervals) are consistent and that R-R intervals are consistent. The only explanation is that the SA node is pacing the atria but impulses are not reaching the ventricles. Therefore, the ventricles have developed their own pacemaker and we have a complete (third-degree) heart block.

Premature ventricular complexes. Most cycles contain narrow QRS complexes and could represent any of the supraventricular rhythms described in groups A or B. But occasionally one sees a wide QRS complex interposed between the cardiac cycles. Therefore, the primary rhythm may be sinus- or supraventricular-paced, but occasionally an extra impulse is fired from an ectopic pacemaker within the ventricle and creates a wide QRS complex. These complexes are called premature ventricular complexes and may accompany any of the supraventricular rhythms described thus far. If the complexes on a tracing all resemble one another in shape, a single irritable focus is the culprit and is described as unifocal. If the premature ventricular complexes have variable shapes, multiple foci are implicated and the rhythm is described as multifocal.

Ventricular fibrillation and asystole. Here we have the worst tracings of all. Tracing A is pure chaos with no consistent waves whatsoever—ventricular fibrillation. In tracing B, following a single beat, we have no further evidence of electrical activity. This is called asystole. In either case, the patient is in cardiac arrest with no pulse.