EKG chapter 4 part b

Using the 12-lead tracing

-Look at each lead (from left to right), starting at the top and moving to the bottom in the first column -Then in the second column, next in the third column, and then lastly, in the fourth column

Every QRS complex preceded by one P wave (1:1 ratio)

-P wave and QRS complex must be positive in lead II -P wave and QRS complex must be negative in aVr

PR Interval

-Represents depolarization of heart from SA node through atria, AV node and His-Purkinje system -Distance from beginning of P wave to beginning of Q wave • First portion is P wave while second portion is PR segment -duration is 0.12-0.20 seconds -includes P wave and flat line that precede each QRS complex

T Wave

-Represents ventricular repolarization and the end of ventricular systole -After T wave, heart begins to relax and enters diastole. Ventricular diastole is from the end of the T wave to the start of the next QRS complex. • Ventricles are isoelectrically inactive during this time -T wave is the positive deflection after the S wave (in lead II), but it can be negative if QRS is negative in that lead. - T wave in same direction as QRS complex. -Normal T waves are asymmetrical with a slow upstroke and fast downstroke -T wave characteristics are important indicators of pathologies - inverted T wave is clinically significant if seen in more than one lead- coronary ischemia or LVH

The P Wave

-represents atrial depolarization (electrical activity from the SA node to the AV node) -first positive deflection in all leads except aVr -P wave is negative in aVr • Normal height is < mm (2.5 small boxes) and length is 0.08- 0.12 seconds (two to three small boxes) at a paper speed of 25 mm/sec. • P wave signifies the start of atrial systole , which produces the atrial kick (preload), which increases EDV and initiates Frank-Starling, increasing SV • Mitral valve open and aortic valve closed, allowing left ventricle to fill Should be + in lead II and - in aVR. If + in aVR and - in II, the rhythm originated outside of the SA node, and instead at the AV node, this isreferred to as a "junctional rhythm" -A P wave of varying morphology (biphasic, tall and peaked, or wide and humped) is abnormal and represents various chamber enlargements -Normal P wave height is < 2.5 mm (2.5 small boxes) vertically. Normal P wave length is < 0.10 s, or 2.5 small boxes horizontally at a paper speed of 25 mm/s -PP interval measures the and represents the distance/time between atrial depolarizations or consecutive P waves. PP interval should be constant: consecutive PP intervals should be within 0.04 s of each other, or within 1 small box

Nine-Step Process

can be used to identify the characteristics of each ECG tracing

The PR Interval (PRI)

• Electrical activity from SA node to AV node to Purkinje fibers. • Measured from beginning of P wave to beginning of QRS complex • Time it takes from the beginning of atrial depolarization to beginning of ventricular depolarization (time it takes impulse to move from SA node to AV node through to Purkinje fibers) • Normal PRI measures 0.12-0.20 seconds • < 0.12 s may represent a premature beat or beats originating from somewhere above the AV junction • > 0.20 s may represent a conduction disturbance of the impulse trying to pass through the AV junction (AV blocks)

The Normal Sinus Rhythm (NSR)

• Flow of depolarization is and to left • Normal axis deviation (NAD) is present • HR must be and between and • All wave forms must have normal • A flat ST segment followed by an upright and slightly asymmetrical T wave • A normal QT interval • U wave may or may not be present

ST Segment Depression

• Indicates electrical signal is taking longer to move through the myocardium. - Potential CAD or old MI - During exercise testing, depression is evidence of possible myocardial ischemia - Ventricular hypertrophies and bundle branch blocks

U Wave

• Last small, rounded, upward deflection in lead II - Represents last stages of ventricular repolarization and could signify repolarization of Purkinje fibers or papillary muscles • Typically not seen in a normal ECG. • Can be fused with the previous T wave or voltage so low that is goes unnoticed • Prominent or inverted U waves may be significant -when present follows the T wave is oriented in the same direction -usually less than one third the amplitude of he T wave and slightly asymmetrical

Using the rhythm strip

• Look at the waveforms, intervals, and durations of each from left to right • Compare the findings against NSR

ST Segment and T Wave

• On isoelectric line or baseline, no electrical activity. - Ventricular volume is decreasing at the time, just after aortic valve opens • ST segment is flat line that follows QRS complex • T wave is slightly asymmetrical waveform that follows ST segment • J point = where QRS complex ends and ST segment begins • ST segment deviation (> 1 mm) indicates disease and/or infarction.

All intervals must be within normal limits

• PR interval 0.12-0.2 s • QRS complex < 0.1 s

ST Segment

• Period between end of S wave and start of T wave • Should normally be on isoelectric line or baseline, because no electrical activity in heart at this time - Ventricular volume decreasing is at the time, just after aortic valve opens (early ventricular systole) -J point= where QRS complex ends and ST segment begins. • Where ST segment elevation/depression measured • ST segment deviation (> 1 mm) indicates disease and/or infarction.

QRS Complex Nomenclature

• QRS complexes can have many different appearances depending on the specific lead • After the P wave, if the initial deflection is negative, it is a Q wave • Sometimes Q waves may be absent in a normal ECG • Q wave s or the amplitude of the R wave is considered abnormal • After P wave, if the initial deflection is positive, or if there is any positive deflection after a Q wave, it is an R wave. • Any negative deflection after an R wave is an S wave. • Lowercase letter signifies a wave of < 3 mm; uppercase letters identify waves > 3 mm.

QRS Complex in Precordial Leads

• R wave represents conduction of electrical activity through the ventricles from the Purkinje fibers to the endocardium • R wave peak time is the time from the start of the QRS complex to the end of the R wave (beginning of negative deflection of the S wave). This is equivalent to the time it takes for the electrical signal to spread from the endocardium (Purkinje fibers) to the epicardium • Expected to be shorter in V1 and V2 (right chest leads) because the right ventral wall is thinner than the left ventricular wall • S wave represents conduction of electrical activity away from apex of ventricles to base of ventricles via intracardiac conduction of the electrical signal. Movement of the signal flows from endocardium to epicardium

QRS complex

• Represents movement of electrical impulse through ventricles causing their contraction • Consists of: • Q wave • 1st negative deflection following PR segment • R wave • 1st positive deflection following Q wave or PR segment • S wave • negative deflection after R wave -sharply pointed narrow waveform -amplitude is from 5-30 mm -duration is 0.06-0.10 seconds • Complete (left) ventricular depolarization. • Measured from beginning of complex (Q or R) to the end of the complex (R or S). • Sometimes, additional R or S waves may be present. They are referred to as R' and S' • Normal QRS complex measures 0.06-0.10 s (1.5-2.5 small boxes)

ST Segment Elevation

• Sign of insufficient blood supply to myocardium. - Evidence of a recent or impending MI - Seen in aVR in RBBB and is expected to be seen if the RBB produces ST depression in the other augmented leads and V1-V3 - Could represent myocardial injury or infarction - Seen in MI in leads that visualize area of infarct, while ST depression should be seen in reciprocal leads (leads that look at heart from opposite direction) - During exercise testing, any elevation is cause for immediate concern, especially if patient is symptomatic. Could indicate MI, and should be treated as such until ruled out.

Q-T Interval

• Start of QRS complex to end of T wave. Measures time of ventricular depolarization and repolarization • Can be corrected for . - Bazett's formula, QTC. - Normal QTc < 0.44 sec and less than half of R-R interval. - elongated Q-T intervals increase risk for Torsades de Pointes (TdP) and sudden death. - Patients with a history of ventricular tachyarrhythmia.

P Wave produced by

• initiation of impulse in SA node • depolarization of RA and LA • impulse passing through AV junction • First positive deflection in all leads except aVR. Waveform is upright and slightly asymmetrical -amplitude is 0.5-2.5 mm -duration is 0.06- 0.10 seconds