Intro to Cardiology Module 1 Guide Part 7

The U wave is usually absent from the ECG. Its electrophysiologic origin is uncertain. The U wave normally mimics the T wave in polarity. Interpreting the ECG and Arrhythmias Interpreting an ECG requires a systematic approach. Below is a systematic method used to interpret an ECG: 1. Rhythm - Regular or Irregular - Looking at the R-R interval, is the rhythm regular or irregular? ○ Atrial activity: presence, regularity, duration, morphology, rate ○ Ventricular activity: presence, regularity, duration, morphology, rate 2. Heart rate ○ Determine the heart rate, including Ventricular heart rate (R-R) and Atrial heart rate (P-P) ○ Use one of the five methods (described below) to determine the heart rate ○ Normal resting heart rate is between 60 to 100 bpm. 3. Measure & calculate intervals ○ P wave - are P waves present? Do they occur regularly? Do all the P waves look similar? ○ PR interval - are the PR intervals consistent? Or do they change throughout the ECG? ○ QRS interval ○ QT interval 4. Determine QRS axis (this will be reviewed in more detail in CARD 2201) 5. ST segment or T wave abnormalities - is the ST segment isoelectric? Depressed? Elevated? Is the T wave rounded and upright? Or inverted? 6. Final Interpretation or Rhythm Analysis i.e. Sinus Bradycardia or Atrial Fibrillation *Don't forget to check the recording variables: Calibration 1.0 mV & Paper Speed 25 mm/sec ECG interpretation will be reviewed in greater detail in CARD 2201 Heart Rate Calculation 6-Second Strip Method This is best for irregular rhythms and is the least accurate of all the methods. The method is to count the number of QRS complexes on a 6second rhythm strip and multiply by 10. The Large Square Method The Large Square method is best for regular rhythms. Count the number of large squares between 2 consecutive QRS complexes and dividing into 300, since there are 300 large squares in one minute. It is easiest to start on a dark line. If the rhythm is irregular, you take the longest R-R interval and the shortest R-R interval and give a range. The R-R value is referring to two consecutive QRS complexes. The Small Square Method The concept is the same as the large square method, except small squares are used, instead of large squares. The method is to count the amount of small squares between two consecutive QRS complexes, and divide into 1500. The Memory Method The memory method is handy if you don’t have a ruler or a calculator. The method involves counting the number of large squares between two QRS complexes and using the rates that you have memorized. Sinus Rhythms The normal cardiac rhythm originates in the sinus node and is therefore called, "sinus rhythm." It is generally regular but has some continual variation due to the input of the sympathetic and parasympathetic aspects of the autonomic nervous system. Sinus rhythm is usually between 60 and 100 bpm. However, you must consider situation factors to determine if the rate is in fact normal (e.g., your HR should not be 100 bpm while sleeping, unless you are having a nightmare). Sinus Bradycardia (less than 60bpm) Sinus bradycardia can be defined as a sinus rhythm of less than 60 beats per minute (bpm). Treatment with atropine can be indicated if the patient is symptomatic. Sinus Arrhythmia Commonly, young people experience an exaggeration of the normal variation of heart rate with respiration (i.e., the slowing down and speeding up that occurs with normal breathing) known as sinus arrhythmia. It is benign and usually of no clinical significance. However, marked sinus arrhythmia is a sign of increased vagal tone, which can be an indicator of cause for vasovagal syncope. Sinus Tachycardia(above 100 bpm) Sinus tachycardia can be defined as a sinus rhythm in excess of 100 bpm. Sinus tachycardia is typically the result of a physiological demand, but may be caused by pain, anxiety, hypovolemia, left ventricular dysfunction, fever, pulmonary embolism, hyperthyroidism, or drug withdrawal. These are just some examples. Treatment is for the cause (e.g., pain, anxiety, hypovolemia, left ventricular dysfunction, fever, drug toxicity, anemia) of the sinus tachycardia, not the rhythm. Atrial Rhythms: Premature atrial contraction (PAC) originates in an ectopic focus in the atria outside of the SA node. PAC is often the result of catecholamine, nicotine, and caffeine influences. In some cases, it is associated with drug toxicity and rarely with pericarditis. The P wave may be premature and hidden in the T wave of the preceding beat. The contour of the P wave is usually different from that of a sinus beat; it depends on the origin of the ectopic focus. Three or more premature atrial contractions (PACs) establish the criteria for supraventricular tachycardia (SVT). Supraventricular tachycardias are often called paroxysmal due to their sudden onset and termination and run in the neighborhood of 140 to 220 bpm. Supraventricular tachycardia (SVT) is a term used to describe tachycardias that originate above the level of the ventricle. Examples of SVTs include: ● ● ● ● ● - Atrial tachycardia Atrioventricular re-entry tachycardia Atrioventricular nodal re-entry tachycardia Atrial Flutter Atrial Fibrillation Atrial flutter is similar to atrial fibrillation, which is discussed below. The major difference is rhythmicity; atrial flutter is a slower, more organized rhythm with atrial rates between 200 and 350 bpm. The distinguishing characteristics of atrial flutter are the recognizable flutter waves (f waves). These waves combined with the preceding QRS complexes appear to have a saw-tooth configuration. There is usually an associated physiological AV block of at least 2:1. More commonly the ratio is 4:1, and sometimes 6:1. When the conduction ratio is 2:1 and the flutter waves are obscured, it is difficult to recognize atrial flutter. Vagal maneuvers (e.g., carotid sinus massage or the Valsalva maneuver) may help to unmask the rhythm by slowing conduction and increasing the degree of AV block. - Atrial fibrillation (AF) is one of the most common adult arrhythmias. It is the result of multiple atrial foci discharging at rates of >350 bpm, producing undulating deflections (termed fine “f” waves). There is chaotic beating of the right and left atrium, which prevents an orderly contribution to the active filling of the ventricles. The number of impulses reaching the ventricles can be in excess of 140 to 180 bpm by the refractory period of the atrioventricular (AV) node; in this case it is considered a rapid ventricular response. The ECG characteristics of atrial fibrillation are: ● no P waves ● usually irregular ventricular response (i.e., variable RR interval) Atrial fibrillation (AF) with an irregular ventricular response (RR interval) is characterized by the: ● absence of discernible P waves ● QRS being normal ● T wave being obscured by fibrillatory (f) waves AF is thought to be a result of myocardial tissue damage/injury and elevated atrial pressure. Other common causes include: ● pathologies that dilate the atria (mitral stenosis is the most common pathology) ● hypertrophic and dilated cardiomyopathy ● hyperthyroidism Heart Blocks Four Atrioventricular conduction defects are discussed below. These are the First degree, Second degree Type I, Second degree Type II, and Third degree AV blocks. 1. First-degree AV Block With first-degree AV block, the refractory period of the AV node is delayed. The ECG characteristic of first-degree AV block is a PR interval exceeding 0.20 seconds. 2. Second-degree AV Block With second-degree AV block, the conduction of sinus or other supraventricular impulses fail to be conducted to the ventricles. There are two types of second-degree AV block: Type I (Wenckebach) and Type II (Mobitz II) and they may be intermittent or continuous. Type I (Wenckebach) is characterized by a progressive prolongation of the PR interval until a P wave is not conducted to the ventricles. This is because the impulse conducted to the ventricles becomes progressively more difficult to propagate. Type II (Mobitz II) is characterized by a conduction pattern in which the PR interval remains constant, but intermittently, a P wave is blocked (i.e., not conducted to the ventricle). Third-degree AV Block Third-degree AV block, or complete heart block, implies that all sinus or supraventricular impulses fail to conduct through the AV node and penetrate the ventricles. To prevent cardiac standstill, a latent subsidiary pacemaker is necessary. Often there is sinus activity and subsidiary pacemaker activity that are totally independent of each other. This rhythm is often described as P waves marching through the QRS, which implies that there is no relationship between the P wave and the QRS complexes, as they beat independently. This results in no consistent PR interval. The resulting QRS width and rate indicate whether the subsidiary pacemaker is junctional (i.e., AV node) or infranodal (i.e., bundle branchPurkinje system in ventricle) in origin. ● If the subsidiary pacemaker is junctional, the rate is 40 to 50 bpm and the QRS is narrow (i.e., less than 0.10 seconds). ● If the subsidiary pacemaker is infranodal (i.e., ventricular), the rate is less than 40 bpm and the QRS is wide (>0.12 sec). Symptoms associated with complete heart block may range from none to Stokes-Adams attacks. Stokes-Adams attacks are the result of diminished cardiac output and blood flow to the brain and cause fatigue, difficult breathing, loss of consciousness, and even convulsions.