Cardiac dysrhythmias

Chapter 14
Cardiac dysrhythmias


Christian C. Knutsen and Donald M. Yealy


Introduction


Emergency medical services physicians often use the same approach in the field and the hospital to provide patient care, even though the goals in each area differ. The care of patients with dysrhythmias before hospital arrival focuses on treating all life-threatening or imminently life-threatening rhythm changes within minutes. In the emergency department (ED) and in the hospital, the same need exists but more time is available to identify other non-lethal rhythms and deliver definitive long-term treatment.


This chapter discusses a pragmatic method of providing medical oversight for non-arrest dysrhythmias. The most important field observations and actions will be highlighted to help simplify the approach when giving direct medical oversight, creating written protocols, or providing direct patient care. We offer a “low-tech” approach to the problems, emphasizing simple tools including a brief history, physical examination, and standard 3- or 12-lead field ECG. Similarly, we focus on interventions that are effective and easily provided in the out-of-hospital setting. In general, the approach offered is consistent with the 2010 American Heart Association (AHA) Advanced Cardiac Life Support (ACLS) guidelines, although we highlight areas where simplified or alternative approaches exist.


Evaluation


Three basic sources of information are available during the assessment of field dysrhythmias: patient history, physical examination, and the ECG. Rarely will any one of these suffice in guiding treatment. Rather, all three considered together guide care [1,2]. Four steps can be used to manage patients with dysrhythmias in the field. Treatment decisions often can be made before completing all steps, allowing an economy of effort.


Step 1: identify symptoms and how they relate to the rhythm


Two groups of patients present with dysrhythmias: asymptomatic patients with incidental rhythm changes and patients with symptomatic rhythm changes. Incidental dysrhythmias may relate to the symptoms, but are the result and not the cause of another problem, and they do not worsen the immediate outcome. Patients with incidental dysrhythmias or who are asymptomatic rarely require field rhythm-directed treatment. Those with incidental dysrhythmias typically require treatment of any underlying acute condition (e.g. analgesia for pain or fluids for hypovolemia).



A 67-year-old male patient with a history of “extra heart beats” transported for an isolated ankle injury displays a sinus tachycardia (from pain) and occasional premature ventricular complexes, but no other symptoms or abnormalities on physical examination. He requires splinting and analgesia, not antidysrhythmics. This should not be confused with dysrhythmias with symptoms, such as tachycardia or bradycardia associated with chest pain, weakness, breathing difficulties, or syncope.


Step 2: identify stable and unstable patients


Because asymptomatic or incidental dysrhythmias usually require no direct treatment, the prehospital focus shifts to those dysrhythmias associated with symptoms. These patients are classified based on the severity of symptoms as either stable or unstable. Although many patients have symptoms attributable to the change from a “normal” rhythm, most tolerate these well and are stable. However, unstable patients are likely to suffer harm or deteriorate. Providers and EMS physicians must identify these unstable patients and rapidly intervene.


Unstable patients have signs and symptoms of inadequate end-organ perfusion due to the rhythm disturbance [2]. A few brief historical questions and physical examination steps must be rapidly completed to identify these patients early in their evaluation.



  • Hypotension – often arbitrarily defined as a systolic blood pressure below 90 mmHg, though any departure of more than 15% from a known baseline may be functional hypotension.
  • Cardiac dysfunction – seen as chest pain, shortness of breath, or rales (signifying inadequate myocardial perfusion or function).
  • Altered consciousness – from mild agitation or somnolence to obtundation or coma (signifying central nervous system [CNS] hypoperfusion).

Delayed capillary refill and lowered skin temperature can indicate poor perfusion; the subjective nature of these observations and multiple other potential causes limit their use in the field.


Assessing instability is usually a continuum, not an “all-or-nothing” phenomenon. Either a single severe sign or symptom or multiple mild findings is diagnostic of an unstable rhythm. A single mildly abnormal finding suggests “borderline” stability. The blood pressure is the simplest method of assessing circulatory adequacy, but it alone may be insufficient in accurately classifying patients. A patient with a systolic blood pressure of 60 mmHg is always unstable. Another patient with a blood pressure of 90 mmHg systolic, rales, and a depressed sensorium is also unstable. If awake and with no rales, chest pain, or other symptoms, the patient with a systolic blood pressure of 90 mmHg occupies a borderline position due to the singular mild finding. Similarly, agitation suggests mild CNS hypoperfusion and borderline stability, whereas coma is associated with more profound derangement and instability.


In the absence of clear evidence of instability, each patient can receive a more complete evaluation, although the total prehospital time interval should not be prolonged. Unstable patients need rapid therapy, usually with electrical interventions such as external countershock or pacing. Symptomatic but stable or borderline unstable patients can be initially treated with pharmacological agents, with electrical devices nearby in case of deterioration. The more extreme the sign or symptom of instability (e.g. coma versus mild anxiety), the more intensive the initial treatment should be.


Step 3: classify the electrocardiogram findings


After assessing stability, the field providers need to categorize the ECG. Using a traditional approach of separating dysrhythmias into dozens of categories is tempting. In the field evaluation, a simpler scheme should be used based on the assessment of stability and three ECG features: QRS complex rate, regularity, and duration.


Electrocardiogram interpretation is performed in two ways: by medical oversight physicians receiving transmitted tracings, or independently by the field personnel. Transmitted tracings are occasionally hampered by technical problems which can obscure salient features. Field providers can learn the basics of ECG interpretation to identify common and lethal rhythms. However, some errors are common. For example, misclassification of QRS duration and rate occurs in up to 20–30% 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 field and transmitted interpretation. In all steps, ECG interpretation must be done from a printed strip and not “guesstimated” from the monitor screen.


Rate


Initially, the rate should be classified 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 the estimation of rate, sinus P-waves should be sought 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 non-sinus fast rhythms (no discernible P-waves and QRS rate >120/ minute) deserve immediate synchronized countershock with 100 J. Often, lower energy levels can convert specific rhythms, such as supraventricular tachycardia (SVT) or atrial flutter, but little benefit is gained by attempting to make fine distinctions in these unstable patients. Although changes in heart rate that fall into the normal range can cause symptoms, these are usually of little importance in the field management.


Biphasic waveform defibrillators are increasingly common among EMS services. In general, lower energy biphasic waveform shocks are equally or more effective than monophasic shocks [4]. However, no outcome benefit to biphasic waveforms has yet been demonstrated [5]. In addition, the ideal energy for first-shock biphasic waveform defibrillation is uncertain [4]. The defibrillator manufacturer’s recommended energy levels for cardioversion and defibrillation should be used.


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 [6,7]. 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.


Internal, implanted 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. Trying to evaluate the pacemaker in the field is impossible and should await hospital evaluation (see Volume 1, Chapter 15).


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.


Previously, isoproterenol was a second-line therapy for atropine-resistant bradycardias. With the availability of external pacemakers in the field and the poor clinical effectiveness of isoproterenol, this treatment is not currently recommended. In adults, a pressor medication (e.g. dopamine) infusion and a fluid bolus should be administered if transcutaneous pacing has normalized the heart rate but hypotension persists.


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 run 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 providers 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, which 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 a 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 (WCTs) 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 WCT or a narrow-complex tachydysrhythmia (NCT) 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 in unstable patients with a tachydysrhythmia.

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Jun 14, 2016 | Posted by in EMERGENCY MEDICINE | Comments Off on Cardiac dysrhythmias
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