Ventricular Tachyarrhythmia (Ventricular Fibrillation or Pulseless VT)

With ventricular fibrillation (VF) the ventricles do not contract but instead twitch rapidly in a completely ineffective way. No cardiac output occurs, and the patient loses consciousness within seconds. The characteristic ECG pattern, with its unmistakable fast oscillatory waves, is illustrated in Figure 19-1.

VF may appear spontaneously, as noted in Chapter 14, but is often preceded by another ventricular arrhythmia (usually ventricular tachycardia [VT] or premature ventricular beats). Figure 19-2 shows a run of VT degenerating into VF during cardiac arrest.

The treatment of VF was described in Chapter 16. The patient should be immediately defibrillated, given a direct current electric shock (360 joules) to the heart by means of paddles or pads placed on the chest wall (usually in an anterior-posterior position).

VT and VF are the only “shockable” sudden cardiac arrest rhythms. An example of successful defibrillation is presented in Figure 19-6D.

Success in defibrillating any patient depends on a number of factors. The single most important factor in treating VF is haste: the less delay in defibrillation, the greater the chance of succeeding.

Sometimes repeated shocks must be administered before the patient is successfully resuscitated. In other cases, all attempts fail. Finally, external cardiac compression must be continued between attempts at defibrillation.

In addition to defibrillation, additional measures include intravenous drugs to support the circulation (epinephrine or vasopressin) and antiarrhythmic agents such as amiodarone or lidocaine, and magnesium sulfate (in cases of torsades de pointes and when hypomagnesemia is present).

Ventricular Asystole and Brady-Asystolic Rhythms

The normal pacemaker of the heart is the sinus node, which is located in the right atrium. Failure of the sinus node to function (sinus arrest) leads to ventricular standstill (asystole) if no other subsidiary pacemaker (e.g., in the atria, atrioventricular [AV] junction, or ventricles) takes over. In such cases the ECG records a straight-line pattern (see Fig. 19-3), indicating asystole. Whenever you encounter a straight-line pattern, you need to confirm this finding in at least two leads (as seen in most conventional telemetry systems) and check to see that all electrodes are connected to the patient. Electrodes often become disconnected during a cardiac arrest, leading to the mistaken diagnosis of asystole. Very low amplitude VF (so-called “fine VF”) may also mimic a straight-line pattern. Increasing the gain on the monitor may reveal this “hidden” VF pattern.

The treatment of asystole also requires continued external cardiac compression; however, unlike VT or VF, defibrillation is not appropriate, nor effective. Sometimes spontaneous cardiac electrical activity resumes. Drugs such as vasopressin or epinephrine may help support the circulation or stimulate cardiac electrical activity. Patients with refractory ventricular standstill require a temporary pacemaker, inserted into the right ventricle through the internal jugular or femoral veins. Noninvasive, transcutaneous pacing uses special electrodes that are pasted on the chest wall. However, transcutaneous pacing may only be effective with bradycardia, not frank asystole, and it may be quite painful in conscious patients.

Not uncommonly with ventricular standstill, you also see occasional QRS complexes appearing at infrequent intervals against the background of the basic straight-line rhythm. These are escape beats and represent the attempt of intrinsic cardiac pacemakers to restart the heart’s beating (see Chapter 14). Examples of escape rhythms with underlying ventricular standstill are shown in Figure 19-4. In some cases the escape beats are narrow, indicating their origin from either the atria or the AV junction (see Fig. 19-4A). In others they come from a lower focus in the ventricles, producing a slow idioventricular rhythm with wide QRS complexes (see Fig. 19-4B). The term brady-asystolic pattern is used to describe this type of cardiac arrest ECG.

Hyperkalemia, as well as other potentially reversible causes, such as drugs or ischemia, should always be excluded in cases of brady-asystolic rhythms.

Escape beats should not be confused with artifacts produced by external cardiac compression. Artifacts are large, wide deflections that occur with each compression (see Fig. 19-5). Their size varies with the strength of the compression, and their direction varies with the lead in which they appear (i.e., usually negative in leads II, III, and aVF, positive in leads aVR and aVL).

Pulseless Electrical Activity (Electromechanical Dissociation)

In occasional patients with cardiac arrest, the person is unconscious and does not have a palpable pulse or blood pressure despite the presence of recurring QRS complexes and even P waves on the ECG. In other words, the patient has cardiac electrical activity but insufficient mechanical heart contractions to pump blood effectively. This syndrome is called pulseless electrical activity (PEA) or electromechanical dissociation (EMD). Similar to asystole, defibrillation is not appropriate therapy for PEA.

PEA with a physiologic rate can arise in a number of settings. When assessing a patient with PEA, you must consider potentially reversible causes first. Box 19-3 presents an adaptation of the classic “5 Hs and the 5 Ts” that may lead to PEA.

One of the most common settings in which PEA occurs is when the myocardium has sustained severe generalized injury that may not be reversible, such as with myocardial infarction (MI). In such cases, even though the heart’s conduction system may be intact enough to generate a relatively normal rhythm, the amount of functional ventricular muscle is insufficient to respond to this electrical signal with an adequate contraction. Sometimes the myocardial depression is temporary and reversible (“stunned myocardium”), and the patient may respond to resuscitative efforts.

In summary, the main ECG patterns seen with cardiac arrest are a sustained ventricular tachyarrhythmia or VF, ventricular asystole (including brady-asystolic patterns), and PEA. During the course of resuscitating any patient, you may see two or even all three of these ECG patterns at different times during the arrest. Figure 19-6 shows the “ECG history” of a cardiac arrest.