Step three: classify the electrocardiogram findings
After assessing stability, EMS clinicians should categorize the ECG using a simple scheme. Using a traditional approach of separating dysrhythmias into dozens of categories is tempting but unneeded. In the field evaluation, assess stability and three ECG features: QRS complex rate, regularity, and duration.
ECG interpretation may occur in multiple different ways. They include EMS clinicians interpreting independently, with or without the aid of an integrated computerized interpretation, and medical oversight physicians who interpret transmitted tracings. Some errors are common. For example, EMS personnel misclassify QRS duration and rate in up to 20% to 30% of tachycardias [3]. Protocols and medical oversight decisions must assume that the potential for misclassification exists and attempt to minimize attendant adverse outcomes. The strategies outlined herein apply to both in‐field and transmitted interpretations. In all steps, one key step to ECG interpretation is using a printed strip and not a “guesstimate” from the monitor screen.
Rate
Initially, classify the rate as fast (>120/minute), slow (<60/minute), or normal/near normal (60‐120/minute) based on the frequency of QRS complexes over 6 seconds multiplied by 10. After estimating rate, look for sinus P waves in those patients with normal or fast rates. Sinus P waves always precede the QRS complexes and have a consistent appearance and relationship (i.e., distance) to the QRS complexes.
As a simple rule, all unstable patients with nonsinus fast rhythms (no discernible P waves and QRS rate >120/ min) deserve immediate synchronized countershock with 100 J, increasing quickly if unsuccessful. Lower energy levels may convert specific rhythms, such as supraventricular tachycardia (SVT) or atrial flutter, but little benefit is gained by attempting to make fine distinctions in unstable patients. Although changes in heart rate that fall into the normal range can cause symptoms, these are usually of little importance during field management.
In general, lower energy biphasic waveform shocks are equally or more effective than monophasic shocks [4, 5]. However, no outcome benefit to biphasic waveforms exists [6]. If possible, use the defibrillator manufacturer’s recommended energy levels for cardioversion and defibrillation, recognizing that 100 J is a good first dose if unstable.
Patients with slow dysrhythmias only require classification of their stability. All other details (e.g., P wave characteristics, Type I or II second‐degree block, junctional versus ventricular escape) add little value in prehospital management. Slow stable dysrhythmias need no intervention besides continued monitoring for deterioration. Slow unstable dysrhythmias require external pacing (preferred) or atropine (0.5‐1 mg IV in adults, repeated up to 2‐3 mg total). Transcutaneous pacing is best started as early as possible to maximize the potential for mechanical or clinical capture and restoration of perfusion [7, 8]. Also, do not delay pacing in unstable patients to administer atropine. Conversely, concerns of clinical deterioration after atropine are unwarranted when the correct dose is given to those with symptomatic bradycardia, though there may be no response. In adults, a vasopressor infusion and an IV fluid bolus should be administered if transcutaneous pacing (or atropine) has normalized the heart rate but hypotension persists.
Internal pacemakers should prevent bradycardias, but they may malfunction. When a patient has pacer spikes on the ECG and is still bradycardic, the pacemaker is not working properly, and the patient should be treated in the same fashion previously described with atropine or external pacing. The pacer pads should be kept 10 cm or more away from the internal pacemaker pouch. In‐depth evaluation of pacemaker function should be deferred to the emergency department (see Chapter 11).
Bradycardias resulting from beta‐blocker or calcium channel blocker overdoses may be refractory to atropine. In these cases, glucagon (1‐3 mg IV) may improve the heart rate. Again, drug administration should not delay transcutaneous pacing.
Regularity and duration
In contrast to bradycardia, if the ventricular rate is fast, the regularity and duration of the QRS complexes should be assessed. Regularity is divided into two categories: mostly or completely regular and chaotic (i.e., “irregularly irregular” without any pattern). Chaotic rhythms are usually due to atrial fibrillation, irrespective of the appearance of the baseline or QRS duration. Other less common causes include multifocal atrial tachycardia and frequent extrasystoles (i.e., atrial, ventricular, or junctional).
To simplify the process of measuring duration and assessing regularity, EMS personnel should obtain an ECG strip. From this, they or the medical oversight physician can measure in “small boxes” how wide the QRS duration is and look for irregularity. Each small box represents 0.04 seconds at normal paper speed. Having EMS clinicians seek out “How many small boxes wide is the QRS complex?” will limit mathematic or conversion errors. Similarly, evaluating printed strips also helps detect irregularity that may be difficult to appreciate on a monitor screen if the ventricular rate is greater than 150/minute. In these cases, close tracking on a 6‐second ECG strip may help detect chaos and identify atrial fibrillation.
Those rhythms with a QRS duration of less than three small boxes (0.12 seconds) are narrow‐complex dysrhythmias. Conversely, any rhythm with a QRS duration of greater than three small boxes is a wide‐complex dysrhythmia. Nearly all narrow‐complex rhythms originate from atrial or nodal (i.e., supraventricular) sources. Wide‐complex rhythms can originate from a ventricular or supraventricular source. In the latter situation, some abnormality in ventricular conduction is responsible for the prolonged QRS duration. In the field, attempts to separate the myriad causes of wide‐complex tachydysrhythmias rarely alter therapy and are unnecessary. Treatment should be based on the clinical stability of the patient, basic history, and the simple ECG characteristics previously defined.
Unstable tachydysrhythmias
Aside from sinus tachycardia, all unstable patients with a wide or narrow‐complex tachydysrhythmia deserve countershock(s), irrespective of the exact source, ventricular or supraventricular. The QRS duration will help dictate care after countershock but does not fundamentally drive the initial care for unstable patients with tachydysrhythmias.
The initial energy level used to treat tachycardias is based on the QRS pattern. If the QRS pattern is regular or nearly regular in any unstable patient with a tachydysrhythmia and a palpable pulse, synchronized cardioversion with 100 J should be used, followed by stepwise energy increases to 200 J with a biphasic device or 360 J with a monophasic device, if necessary. Some rhythms may require less energy, but attempts to titrate this lifesaving therapy for unstable patients is of little practical benefit. Synchronized countershock is recommended to avoid postcountershock ventricular fibrillation (VF). However, sensing problems often make reliable identification of the QRS complex needed for synchronization impossible. We recommend an unsynchronized shock promptly if any sensing problem occurs. Any patient without pulses and an irregular tachydysrhythmia should be immediately given a high‐energy unsynchronized countershock.
Patients with internal pacemakers or automatic implantable cardioverter defibrillators (AICDs) are still at risk of cardiac dysrhythmias. Although meant to cardiovert dysrhythmias, AICDs do not always convert these rhythms, and sometimes these devices deliver shocks inappropriately. If a patient has an unstable tachydysrhythmia and the AICD is not firing or is ineffective, externally cardiovert as previously recommended, with pads in the anterior‐posterior configuration and 10 cm away from the internal device pouch. Postconversion care with medical therapy will be unaffected.
If an AICD is repeatedly firing absent a ventricular dysrhythmia, a magnet held over the device may inactivate
