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Chapter 4 Basic ECG Concepts & the Normal ECG
Introduction • Cardiac electrical activity originates in the SA node. – Depolarization & repolarization of myocardial cells. – Action potential.
• Electrocardiogram (ECG) – Traces waveforms of cardiac electrical potentials, or heart rhythm. – EKG (German) = ECG (English).
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ECG Paper • Isoelectric, baseline, no electrical activity: – Positive or negative deflection from baseline. – Biphasic deflection.
• Waveform amplitude measured vertically (mm). • Waveform width measured horizontally (seconds).
Figure 4‐1 ECG paper.
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ECG Paper & Speed • Grid lines are standardized for accuracy. – Small light boxes = 1 mm² (1 mm vertical & horizontal) – Large dark boxes = 25 mm²
• Paper speed of the ECG is recorded. – 25 mm/s: Actual measurements – 50 mm/s: Measurements divided by half
Calibration of the ECG • Beginning or end of each 12‐lead reading • 10 mm high (1 mV) & 0.2 second long, or one large box – Can be doubled or cut in half to better interpret ECG – 2 mV, 20 mm, 0.4 second (half the measurement) – 0.5 mV, 5 mm, 0.1 second (double the measurement)
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Figure 4‐2 (a) Standard calibration 1 mV = 10 mm. (b) Half calibration 0.5 mV = 5 mm.
Wave Amplitudes & Automated bld Pressure Cuffs • Clinical Tip: Automatic machines typically use R waves to determine HR. – When QRS complex is small, double the calibration signal. – Paper speed may be 50 mm/s. – If R wave is still insufficient, technician should take BP manually.
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Standard 12‐Lead Printouts • 12 seconds long, including all 12 leads, printed at 25 mm/s using standard calibration signal. – 3 lines of 4 leads horizontally across. – Separated by lead dividers (not used in ECG interpretation). – Rhythm strip below 12‐lead printout, usually lead II, used to determine rate & rhythm.
Figure 4‐3 Standard 12‐lead ECG printout.
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Calculation of HR: Dark Line Method • Dark line method (regular rate, determined w/ calipers) – Find R wave on dark line. Subsequent R waves on dark lines represent rates of 300, 150, 100, 75, 60, 50, 44, & 38 bpm. – R waves on every dark line = 300 bpm. – Every other dark line = 150, every 3rd dark line = 100, etc.
Figure 4‐4 (A) The dark line method of heart rate determination. (B) The second consecutive R wave falls on the fourth dark line; therefore, HR = 75 b/min Reproduced from Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.
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Quick Check Method • Clinical Tip: During exercise in cardiac rehab, target HR range = 96–102 bpm. • Find R wave on dark line. – Second R wave at least 2 little boxes before the 3rd dark line. HR is faster than 102 bpm. Symptoms may appear. Adjustment of workload may be needed. – Second R wave at least 2 little boxes after the 3rd dark line. HR is slower than 96 bpm. Adjustment of workload may be needed.
Calculation of HR: 1500 Method • 1500 method (regular rate) most accurate HR from ECG. – Count the number of small boxes b/n 2 consecutive R waves. – Divide 1,500 (60 sec divided by 0.04 s/small box = 1,500) by the number of small boxes. – Example: 17 small boxes b/n consecutive R waves; thus, 1,500/17 = 88 bpm.
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Figure 4‐5 1500 Method of HR Calculation There are 17 small boxes b/n 2 consecutive R waves; therefore, 1,500/17 = 88 beats per minute. Reproduced from Arrhythmia Recognition: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.
Calculation of HR: 6‐Second Method • The 6‐second method for irregular HR. • 6 seconds = 30 large boxes or the distance b/n three 3‐ second marks at the top or bottom of the rhythm strip. – Count cardiac cycles in 6‐second period. – Multiply the number of cycles by 10 to estimate HR.
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Figure 4‐6 6‐Second Method of Irregular HR Calculation
Development of the 12‐Leads • Einthoven’s triangle – 10 electrodes produce 12 leads: 6 limb leads & 6 chest leads. – 6 limb leads • 3 unipolar & 3 bipolar. • Placement of 4 electrodes: (1) right arm (RA), (2) left arm (LA), (3) right leg (RL), (4) & left leg (LL) on extremities (diagnostic) or torso (functional). • The right leg electrode serves as a ground; no electrical movement.
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Anterior Chest View
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Mason‐Likar (modified) electrode placement
Precordial (Chest Leads) • Arranged across the chest in a horizontal plane • Record forces moving anteriorly & posteriorly • Each electrode is (+) • Whole body is common ground
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Bipolar Limb Leads (3) • Einthoven’s triangle • Mvt of electricity from negative to positive pole as compared to the heart. • R wave amplitude in leads I & III must be equal amplitude of R wave in lead II. I = LA – RA III = LL – LA II = LL – RA
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Figure 4‐7 Einthoven’s triangle.
Figure 4‐8 Einthoven’s Equation I + III = II The R wave in I (4 mm) + the R wave in III (10 mm) = the R wave in II (14 mm). Reproduced from Klabunde, RE. Cardiovascular Physiology Concepts: Electrocardiogram St&ard Limb Leads (Bipolar). http://www.cvphysiology.com/Arrhythmias/A013a.htm. Accessed on March 16, 2014.
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Unipolar Limb Leads (3) • Augmented vector leads are developed from a combination of leads. – RA, LA, or LL electrode as the positive pole & the combination of 2 others as the negative pole, which “augments” signal strength from the measuring electrode. – aVF , aVL , aVR.
Hexaxial Reference System • Helps to determine heart's electrical axis in the frontal plane. – Identify negative & positive poles of each lead. – Identify positive poles of different leads located in the same region. – Identify angle associated with positive & negative pole of each lead. – Helps to determine positive, negative, & biphasic deflections of QRS complex.
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Figure 4‐9 Hexaxial reference system.
Precordial Leads (6) • Negative pole, imaginary center of heart, to positive pole of electrodes • Chest or ventral (V) leads, horizontal plane • V1–6 (Table 4‐1)
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Figure 4‐10 Precordial leads in the horizontal plane.
The Normal Cardiac Cycle • 1 P wave for every QRS complex (1:1 ratio). • The P wave & QRS complex must be positive in lead II. • The P wave & QRS complex must be negative in lead aVR. • HR must be b/n 60 & 99 bpm. • Normal axis deviation (NAD) is present. • All wave forms must have normal morphology in every cycle.
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Figure 4‐11 Basic ECG of the cardiac cycle.
The P Wave • Electrical activity from the SA node to the AV node; atrial depolarization & atrial kick, ↑ ventricular filling. • First positive deflection in all leads except aVR. • Normal height is 3 mm.
Figure 4‐14 The QRS Complex The QRS complex is 0.10 seconds from the beginning of the QRS complex to the end of the complex.
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QRS Complex in Precordial Leads • V1: • A small positive r wave (septal r) & negative deflection (S wave) occurs = rS.
• V6: • A small negative q wave (septal q) & positive deflection (R wave) = qR or qRs.
ST Segment • On isoelectric line or baseline, no electrical activity. – Ventricular volume is ↓ng at the time & just after the aortic valve opens.
• J point = where the QRS complex ends & the ST segment begins. • ST segment deviation (> 1 mm) indicates disease &/or infarction.
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Figure 4‐17 ST Segment ST segment variations from baseline greater than 1 mm constitute ST elevation or ST depression. Reproduced from Grauer, K. ECG Interpretation Review #34 (Acute MI ‐ vs Nonspecific Changes ‐ ST Elevation/Depression). http://ecg‐ interpretation.blogspot.com/2012/01/ecg‐interpretation‐review‐34‐acute‐mi.html. Accessed on March 21, 2014.
ST Segment Depression • Electrical signal is taking longer to move through the myocardium. – Potential coronary artery disease or old myocardial injury. – During exercise testing, depression is evidence of possible myocardial ischemia. – Ventricular hypertrophies & bundle branch blocks.
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ST Segment Elevation • Insufficient bld supply to myocardium. – Evidence of a recent or impending myocardial infarction. – During exercise testing, elevation is cause for immediate concern, especially if patient is symptomatic. – Could represent myocardial injury or infarction.
T Wave • The positive deflection after the S wave (in lead II). – End of the T wave to the start of the next QRS complex. – Same direction as QRS complex.
• Ventricles are isoelectrically inactive, repolarization, diastole. • Normal T waves are asymmetrical; inverted T wave is clinically significant if seen in more than one lead.
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U Wave • Last small, rounded, upward deflection in lead II. – Last stages of ventricular repolarization.
• Typically not seen in a normal ECG. • Can be fused with the previous T wave.
Figure 4‐18 T wave. Reproduced from Introduction to 12‐Lead ECG: The Art of Interpretation, courtesy of Tomas B. Garcia, MD.
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Q‐T Interval • Start of QRS complex to end of T wave. • Can be corrected for HR. – Bazett’s formula, QTC. – Normal QTc