ch.29

Calculating Regularity

An important step in evaluating an ECG is determining regularity. Regularity can be determined by counting the boxes between the waveforms being measured, such as P wave to P wave (P to P) or QRS complex to QRS complex (R to R). A regular rhythm will have the same number of boxes or equal space between waveforms or complexes . This is also called marching out the rhythm. Marching out the waveforms can help demonstrate if waves are falling early or late or are even hiding within another waveform. If the spaces are not equal, then it is considered irregular

AF with rapid ventricular response

If the HR is greater than 100, the AF is considered uncontrolled and is called

Waveforms

The P wave is the first waveform that is normally seen (Fig. 29.4). The P wave represents the SA node sending out an electrical impulse and represents atrial depolarization. This wave should be upright (positive deflection) with a rounded top in lead II. The P wave itself should not be longer than 0.10 sec in length and no higher than 2.5 mm. The QRS wave (or QRS complex) is the next waveform that is normally seen . This complex represents ventricular depolarization. The ventricles depolarize from the endocardial layer (inside of the heart) to the epicardial layer (outermost layer of the heart). The QRS complex can take on many different forms . Normal QRS complexes are usually pointy and skinny in width. As a rule of thumb, a wide QRS (greater than 0.10 sec or two and a half small boxes wide) means that there is something occurring in the ventricles causing the QRS to widen. For example, it may indicate that the impulse originated in the ventricles or that there is a block in the ventricles delaying the impulse from traveling through the ventricles to allow for a normal depolarization. The T wave occurs after the QRS and represents ventricular repolarization (Fig. 29.6). This wave should be upright and rounded in lead II. Even though the T wave is not used for rhythm interpretation, it can be helpful in monitoring extreme electrolyte imbalances such as hypokalemia or hyperkalemia, disturbances with the myocardial oxygen supply, or other cardiac disorders such as pericarditis or ventricular aneurysm. There is no visible waveform that represents atrial repolarization. That is typically "buried" in the QRS complex. The U wave represents Purkinje fiber repolarization and is rarely seen (Fig. 29.7). However, this waveform can be seen in patients with certain medication toxicities (e.g., digoxin toxicity) and electrolyte imbalances (e.g., hypokalemia). When a U wave is seen, it should be a small rounded wave in lead II. Be careful not to confuse it with the P wave.

INTERVALS

The PR interval is the measure of time that it takes an electrical impulse to depolarize the atria and travel to the ventricles. To measure this interval, start from the beginning of the P wave (where it starts to leave the baseline) and count the number of small boxes to the beginning of the QRS complex (not the R wave; Fig. 29.8). The normal PR interval is from 0.12 sec (three small boxes) to 0.20 sec (five small boxes) in length. The QRS interval is the measure of time to depolarize the ventricles. It is measured from where the QRS waveform leaves the baseline to where the QRS returns to the baseline . The normal interval is 0.06 to 0.10 sec in length. As discussed previously, if the QRS is prolonged (more than 0.10 sec; equal to or more than three small boxes), it may be a sign of a disturbance within the ventricle itself (i.e., that the impulse originated in the ventricles or that there is a block in the ventricles delaying impulse travel time through the ventricles).\\ The QT interval is the measure of time that it takes the ventricle to depolarize and then repolarize. To measure a QT interval, start where the QRS leaves baseline and measure to where the T wave returns to baseline (Fig. 29.10). This interval is heart-rate dependent and should never be more than half the distance from one QRS to the next. A normal QT is usually less than or equal to 0.52 sec in length

Lead II

The best lead to identify or interpret the heart's rhythm is _________. _________ mimics the heart's natural electrical direction in a healthy, normally positioned heart.

pulseless electrical activity (PEA)

There are occasions when the electrical conduction system is functional, but the heart muscle is not responding with a contraction. This is called ______ ______. If not treated quickly, the electrical conduction system will eventually stop working because of the lack of oxygen to the heart's pacemaker cells. CAUSES include hypoxia (lack of oxygen), hypovolemia (low blood/fluid volume in the circulatory system), hyper- or hypokalemia (high or low blood potassium levels), acidosis (excessive acid in the body), hypoglycemia (low blood glucose level), hypothermia (low body temperature), medication toxicities (overdose), pericardial tamponade (excessive blood/fluid around the heart), pneumothorax (collapsed lung), myocardial infarction, and pulmonary embolism (blood clot in the lung),

ECG Paper

When a patient receives an ECG or is placed on a cardiac monitor, the heart's electrical activity will be printed on specialized graph paper for interpretation. The ECG graph paper can measure both amplitude and time (Fig. 29.2). The amplitude is measured on the vertical axis in millimeters (mm), whereas time is measured in seconds (sec) along the horizontal axis. The standard paper speed for ECGs is 25 mm per second. The graph paper is segmented into small and large boxes. Each small box is 1 mm in height (amplitude) and 0.04 sec in length. When looking at the graph paper, you will notice that there are darker lines that create a larger box. The larger boxes are divided into areas five small boxes high and five small boxes long. One large box equals 0.20 sec long (0.04 sec × five small boxes = 0.20 sec).

Steps in ECG Interpretation

When evaluating a cardiac rhythm, ask the following questions to help identify the rhythm seen on the ECG: • Is the rate fast, slow, or normal? • Is the rhythm regular; are complexes an equal distance from one to the next? -Are the distances from P wave to P wave equal throughout the strip? This is referred to as the P-P interval. - Are the distances from QRS wave to QRS wave equal throughout the strip? This is referred to as the R-R interval. • Are there P waves present? • Are there QRS complexes present? • Are there T waves present? • Are the intervals within normal limits (e.g., PR interval(0.12 - 0.20) QRS interval (0.06 - 0.10)? • Is there a P wave before every QRS? • Is there a QRS after every P wave?

normal sinus rhythm (NSR)

When the impulse travels along the normal conduction pathway generated by the SA, it is called ________ ______ ______. It generates waveforms that indicate the firing of the SA node, the P wave, followed by a QRS complex indicating ventricular depolarization, and a T wave, indicating ventricular repolarization, all within the correct timing intervals

Second-degree AV block type II (also known as Mobitz II)

also drops QRS complexes, but unlike second-degree type I, the PR intervals are exactly the same length with each complex (see Fig. 29.29). This dysrhythmia is more alarming and is considered a potentially life-threatening dysrhythmia because it can quickly progress to a third-degree AV block.

Atrial Dysrhythmias

are caused by pacemaker cells firing not from the SA node but from somewhere else within the atria. They include premature atrial contractions (PACs), atrial fibrillation (AF), and atrial flutter (AFL). Supraventricular dysrhythmias also arise from the atria with the exception of junctional tachycardias

Heart blocks

are caused by the delay or blockage of electrical conduction at the AV node. The AV node's blood supply comes from the right coronary artery (RCA). If the RCA becomes partially or completely blocked, the AV node is deprived of oxygen, which causes ischemia. If the AV node tissue becomes ischemic or dies, the electrical impulses originating in the atria will have difficulty traveling through the AV node to the ventricles along the usual pathway. The electrical impulses will need to find an alternative pathway, which could cause conduction delays or even an electrical disassociation of the atria and the ventricles. There are four types of heart blocks: first degree, second-degree type I, second-degree type II, and third degree.

Ventricular rhythms

are those rhythms that originate somewhere within the ventricles. When an impulse starts in the ventricle, there is no P wave, and the QRS is usually wide (≥ 0.12 sec or three small boxes). The normal rate for pacemaker cells within the ventricles is 40 bpm or less. Types of ventricular rhythms include premature ventricular contractions (PVCs), ventricular tachycardia (VT), ventricular fibrillation (VF), and idioventricular rhythms (IVRs).

Premature ventricular contractions (PVCs)

are wide and atypical (or bizarre-looking) QRS complexes that fire earlier than expected from within the ventricles (Fig. 29.22). As with other premature contractions, there is a compensatory pause at the end to allow the heart's conduction system to reset. There are no P waves visible prior to the QRS because the impulse originated in the ventricle. Premature ventricular contractions coming from one ventricular pacemaker cell are called unifocal PVCs (Fig. 29.22A). If they come from multiple ventricular pacemaker cells, they are called multifocal PVCs (Fig. 29.22B). Two PVCs in a row is called a couplet (Fig. 29.22C). Three PVCs in a row is called a triplet or a three-beat run of VT (Fig. 29.22D). A PVC that occurs every other beat is called bigeminy (Fig. 29.22E). A PVC falling every third beat is called trigeminy

Junctional rhythms

begin within the AV node. The AV node is a cluster of pacemaker cells positioned between the atria and the ventricles. The AV node generates impulses at a rate between 40 and 60 bpm (Fig. 29.20A). If the rate increases to 61 to 100 bpm, it is identified as an accelerated junctional rhythm (Fig. 29.20B). If the HR is greater than 100 bpm, it is identified as a junctional tachycardia (Fig. 29.20C). In order for both the atria and the ventricles to depolarize during a junctional rhythm, the electrical impulse from the AV node must split and travel in different directions. The ventricular conduction pathway will accept the electrical impulse as usual. However, in order for the atria to depolarize, the electrical impulse must travel backward (the opposite direction of the normal conduction), producing a retrograde P wave. This will display as an inverted or upside-down P wave, an absent P wave that is buried in the QRS complex, or a retrograde P wave at the end of the QRS wave.

Atrial fibrillation

has no P waves. It is best described as multiple pacemaker cells generating independent electrical impulses and causing chaos within the atria (see Figs. 29.16A, B, and C). It is characterized as irregularly irregular. The QRS complexes are usually narrow with irregular R-R intervals. Although the naked eye cannot measure an atrial rate in this rhythm, it can range from 300 to 600 bpm. The HR or ventricular response is determined by the AV node's ability to accept and transmit the impulses to the ventricle. When the AV node maintains the HR at less than 100 bpm, the AF is considered controlled (Fig. 29.16A). If the HR is greater than 100, the AF is considered uncontrolled and is called AF with rapid ventricular response COMPLICATIONS Loss of Cardiac Output Because the atrial pacemaker cells are firing and competing against one another at such a rapid rate, the atria can only quiver instead of beating or contracting/squeezing normally. Without a normal atrial contraction, the heart can lose approximately 30% of its CO. This is referred to as loss of atrial kick, the filling force contributed by atrial contraction immediately before ventricular systole to maximize ventricular preload. This decrease in CO can cause significant symptoms for some patients, such as syncope, palpitations, and SOB. CLOTS Because the atria are not squeezing or contracting efficiently, blood pools in the atria, which can predispose the patient to clot formation. Blood clots moving through the circulation put the patient at risk for embolic events such as strokes. Care should be taken when administering any treatments that could convert AF to NSR because of the increased risk of an embolus (blood clot) being forced out into the circulatory system ANTIARRHYMTIC Amiodarone (Cordarone, Pacerone) Dronedarone (Multaq) Dofetilide (Tikosyn) ACTION Slows the cardiac action potential, thus slowing the heart rate VT VF AF SPECIAL CONSIDERATIONS Amiodarone (Cordarone, Pacerone): • IV preparation for continuous infusion must be in a glass bottle. Monitor: • Pulmonary function test • Thyroid function • Liver function

Atrial flutter (AFL)

is a dysrhythmia produced by a pacemaker cell other than the SA node. Because the SA node is not the primary pacemaker in this rhythm, there are no P waves. Flutter waves (F waves), however, are present. F waves resemble a sawtooth pattern between narrow QRS complexes (Fig. 29.17). The atrial rate can range from 250 to 350 bpm. The number of F waves to a QRS complex can vary depending on the AV node's ability to accept and transmit impulses through to the ventricles which could produce a normal HR (60-100 bpm) or have an RVR response (HR >150 bpm). Like AF, AFL can be a chronic or short-term dysrhythmia.

Ventricular fibrillation (VF)

is a lethal dysrhythmia requiring immediate treatment. It is the most frequently seen rhythm in cardiac arrests occurring outside of the hospital. Ventricular fibrillation occurs when the ventricle has multiple chaotic impulses rapidly firing (Fig. 29.24). This chaotic firing prevents the ventricles from pushing blood out of the heart, stopping CO and causing death. There are absolutely no identifiable P waves or QRS waves. The rhythm displayed on the ECG is a shaky or quivering line that can be very coarse ANTIARRHYMTIC Amiodarone (Cordarone, Pacerone) Dronedarone (Multaq) Dofetilide (Tikosyn) ACTION Slows the cardiac action potential, thus slowing the heart rate VT VF AF SPECIAL CONSIDERATIONS Amiodarone (Cordarone, Pacerone): • IV preparation for continuous infusion must be in a glass bottle. Monitor: • Pulmonary function test • Thyroid function • Liver function

Supraventricular tachycardia (SVT)

is a rapid heart rhythm that originates above the ventricles. It most commonly appears as a regular, narrow QRS complex tachycardia. By definition, SVT is any narrow complex rhythm greater than 100 bpm, but can have HRs from 150 to 250 bpm (Fig. 29.18A). The rate may be slower depending on the patient's condition and the underlying cause of SVT. Another presentation is paroxysmal supraventricular tachycardia (PSVT), which is intermittent, coming on quickly and ending quickly (Fig. 29.18B). Supraventricular tachycardia can be considered an umbrella term to capture one of five narrow complex tachycardia rhythms: • Sinus tachycardia (ST; see Fig. 29.14), previously discussed in the sinus dysrhythmia section • Atrial tachycardia (AT), which is similar to ST except the electrical impulse is not generated from the sinus node. The electrical impulse is generated somewhere in the atria and can have unifocal (uniform appearance—originating from a single source; Fig. 29.19A) or multifocal (nonuniform appearance—originating from multiple sources; Fig. 29.19B) presentation. • Atrial fibrillation with RVR (see Fig. 29.16C) • Atrial flutter with RVR (see Fig. 29.17) • Junctional tachycardia (JT), which is discussed in the junctional section later in the chapter (see Fig. 29.20C)

The treatment for pulseless electrical activity (PEA)

is chest compressions, epinephrine, and treating the possible causes. With this in mind, it should not be assumed that a patient will have a pulse because you see a rhythm on a cardiac/heart monitor; make sure that you palpate for a pulse to confirm that the heart is pumping and circulating blood.

Ventricular tachycardia (VT)

is defined as three or more PVCs (wide and fast impulses originating from the ventricles) in a row. The ventricular rate is usually greater than 150 bpm. It can be monomorphic (originating from one pacemaker cell; Fig. 29.23A) or polymorphic (originating from multiple pacemaker cells; Fig. 29.23B). VT can be a life-threatening dysrhythmia as a result of the significant reduction in CO that can occur. A patient in VT might be able to maintain a pulse and a blood pressure for a limited time, but VT will cause death if prolonged or untreated. ANTIARRHYMTIC Amiodarone (Cordarone, Pacerone) Dronedarone (Multaq) Dofetilide (Tikosyn) ACTION Slows the cardiac action potential, thus slowing the heart rate VT VF AF SPECIAL CONSIDERATIONS Amiodarone (Cordarone, Pacerone): • IV preparation for continuous infusion must be in a glass bottle. Monitor: • Pulmonary function test • Thyroid function • Liver function

First-degree AV block

looks very similar to an NSR except the PR interval is prolonged (> 0.20 sec or five blocks long; Fig. 29.27). This is due to the atrial depolarization being delayed in the AV node. Every atrial impulse gets through the AV node; it just takes longer. It is the same time interval each time.

Second-degree AV block type I (also known as Wenckebach or Mobitz I)

occurs when not all atrial impulses get through the AV node to the ventricles. There are more P waves than QRS complexes, and the PR interval gets progressively longer until a QRS is dropped (also referred to as dropping QRSs; Fig. 29.28). After the dropped QRS, the PR interval resets, and the process starts over. A helpful mnemonic is longer, longer, longer, dropped; then you have a Wenckebach. When identifying a heart block, look at the PR interval pattern after the dropped QRS complex. This is not considered a life-threatening dysrhythmia, and it is typicallyan intermittent rhythm; however, on rare occasions, it can be continuous.

Third-degree AV block or complete heart block (CHB)

occurs when the AV node is completely blocked and prevents any impulses from entering or exiting. There is no communication between the atria and the ventricles. The ECG records more P waves than QRS complexes (Fig. 29.31). The atrial rate is usually between 60 and 100 bpm, whereas the ventricular rate is usually less than or equal to 40 bpm. In CHB, QRS complexes march out regularly and are independent of the P waves. Similarly, the P waves march throughout the rhythm strip at a regular rate. Sometimes the P waves are hidden within a QRS complex or the T wave, making it very important to march out the P waves to see where they fall.