What is an ECG?The electrocardiogram (ECG) is arepresentation of the electrical events ofthe cardiac cycle.Each event has a distinctivewaveform, the study of which can lead togreater insight into a patient’s cardiacpathophysiology.
ECG LeadsLeads are electrodes which measure thedifference in electrical potential betweeneither:1. Two different points on the body (bipolarleads)2. One point on the body and a virtual referencepoint with zero electrical potential, located inthe center of the heart (unipolar leads)
EKG Leads The standard EKG has 12 leads: 3 Standard Limb Leads 3 Augmented Limb Leads 6 Precordial LeadsThe axis of a particular lead represents theviewpoint from which it looks at the heart.
Elements of the ECG: P wave: Depolarization of both atria; Relationship between P and QRS helps distinguishvarious cardiac arrhythmias Shape and duration of P may indicate atrialenlargement PR interval: from onset of P wave to onset of QRS Normal duration = 0.12-2.0 sec (120-200 ms) (3-4horizontal boxes) Represents atria to ventricular conduction time(through His bundle) Prolonged PR interval may indicate a 1st degree heartbloc
QRS complex: Ventricular depolarization Larger than P wave because of greater muscle mass ofventricles Normal duration = 0.08-0.12 seconds Its duration, amplitude, and morphology are useful indiagnosing cardiac arrhythmias, ventricularhypertrophy, MI, electrolyte derangement, etc. Q wave greater than 1/3 the height of the Rwave, greater than 0.04 sec are abnormal and mayrepresent MI
ST segment:Connects the QRS complex and T waveDuration of 0.08-0.12 sec.T wave:Represents repolarization or recovery of ventriclesInterval from beginning of QRS to apex of T is referred to as theabsolute refractory periodQT IntervalMeasured from beginning of QRS to the end of the T waveNormal QT is usually about 0.40 secQT interval varies based on heart rate
Determining the Heart RateRule of 300 6Second Rule
Rule of 300Take the number of “big boxes” between neighboringQRS complexes, and divide this into 300. The result willbe approximately equal to the rateAlthough fast, this method only works for regularrhythms.
What is the heart rate? www.uptodate.com (300 / 6) = 50 bpm
What is the heart rate? www.uptodate.com (300 / ~ 4) = ~ 75 bpm
Themay be easiest300 It Rule of to memorize the following table: # of big Rate boxes 1 300 2 150 3 100 4 75 5 60 6 50
6 Second RuleAs most EKGs record 10 seconds of rhythm perpage, one can simply count the number of beatspresent on the EKG and multiply by 10 to get thenumber of beats per 60 seconds.This method works well for irregular rhythms.
What is the heart rate? The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
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