7 - Basic ECG Review

Info from 'Clinical Assessment in Respiratory Care' 9th edition

indications for ECGs

• cardiac disorders (MI, ventricular hypotrophy, dysrhythmias)

• pre-op as screening tool

pacemaker sites

1. sinoatrial (SA) node (60-100 bpm)

2. atrioventricular (AV) junction: AV node + bundle of His (40-60 bpm)

3. bundle branches (30-40 bpm)

4. Purkinje fibers (30-40 bpm)

P wave

[1]

little bump on ECG that represents atrial depolarization; sudden loss of negative charge in polarized cells (K⁺ out, Na⁺ in)

PR interval

[1+2]

section on ECG from beginning of P wave to beginning of QRS complex; represents delay of AV node to allow filling of ventricles

QRS complex

[3]

jagged line on ECG that represents ventricular depolarization

ST segment

[4]

section on ECG from end of QRS complex to beginning of T wave; represents beginning of ventricular repolarization

T wave

[5]

big bump on ECG that represents ventricular repolarization (Na⁺ out, K⁺ in)

length of time for 5 consecutive big boxes on ECG strip

1 second

length of time for one big box on ECG strip

0.20 seconds

length of time for one small box on ECG strip

0.04 seconds

6-second method

method of counting rate on ECG strip if HR is irregular

• count number of QRS complexes in 6-second strip

• multiply number by 10

large boxes method

method of counting rate on ECG strip if HR is regular

• count number of large boxes (0.2 sec each) between QRS complexes

• divide number into 300 (300 ÷ x) to find ventricular rate

• repeat above for atrial rate using P waves

small boxes method

method for counting rate on ECG strip if HR is regular

• count number of small boxes (0.04 sec each) between QRS complexes

• divide number into 1500 (1500 ÷ x) to find ventricular rate

• repeat above for atrial rate using P waves

ECG leads

• 6 limb leads

  • 4 electrodes

  • I, II, III, aV_R, aV_L, aV_F

  • view heart on frontal plane to find axis

• 6 chest leads

  • 6 electrodes

  • V₁-V₆

  • view heart on horizontal plane

  • overlie right ventricle (V₁-V₂), ventricular septum (V₃-V₄), left ventricle (V₅-V₆)

3- and 5-lead placement

• white → RA

  • right arm / right upper chest below right clavicle

• black → LA

  • left arm / left upper chest below left clavicle

• red → LL

  • left leg / left lower chest above and left of umbilicus

• green → RL

  • right leg / right lower chest above and right of umbilicus

• brown → any precordial lead

  • 4th intercostal space, right sternal border

---

• white, black, red = I, II, III

• all = I, II, III, aV_R, aV_L, aV_F, V

normal direction of QRS axis

left and down (→↓) and -35° to +90°

axis determination

• I positive, II positive → normal

• I positive, II negative → left deviation

• I negative, II positive → right deviation

• I negative, II negative → extreme right deviation

causes of right axis deviation

• left ventricular infarction

• right ventricular hypertrophy

• chronic obstructive lung disease

• acute pulmonary embolism

• infants ≤ 1 year old (normal)

• biventricular hypertrophy

• left posterior fascicular

causes of left axis deviation

• right ventricular infarction

• left ventricular hypertrophy

• abdominal obesity

• ascites / large abdomen

• third-trimester pregnancy

• left anterior fascicular block

ECG interpretation

1. evaluate patient’s overall condition

2. identify HR

3. is rate regular or irregular?

4. evaluate P waves

5. measure PR interval (normal: 0.12-0.20 sec)

6. evaluate QRS complex (normal: <0.10 sec)

7. inspect ST segment

normal sinus rhythm (NSR)

• rate: 60-100 bpm

• rhythm: regular

• P waves: normal

• PR interval: 0.12-0.20 sec

• QRS complexes: <0.10 sec

• ST segments: normal

sinus bradycardia (SB)

• rate: <60 bpm

• rhythm: regular

• P waves: normal

• PR interval: 0.12-0.20 sec

• QRS complexes: <0.10 sec

• ST segments: normal

sinus tachycardia (ST)

• rate: 100-150 bpm

• rhythm: regular

• P waves: normal

• PR interval: 0.12-0.20 sec

• QRS complexes: <0.10 sec

• ST segments: normal

supraventricular tachycardia (SVT)

• rate: >150 bpm

• rhythm: regular

• P waves: not seen

• PR interval: not measurable

• QRS complexes: <0.10 sec

• ST segments: not seen

sinus dysrhythmia (SD)

• rate: <60-100 bpm

• rhythm: irregular

• P waves: normal

• PR interval: 0.12-0.20 sec

• QRS complexes: <0.10 sec

• ST segments: normal

atrial flutter (AF)

• rate: atrial 180-400 bpm, ventricular slower

• rhythm: regular

• P waves: sawtooth, uniform

• PR interval: not measurable

• QRS complexes: <0.10 sec

• ST segments: may not be seen

atrial fibrillation (A-Fib)

• rate: variable

• rhythm: irregularly irregular

• P waves: variable

• PR interval: not measurable

• QRS complexes: <0.10 sec

• ST segments: may not be seen

premature atrial contractions (PAC)

• rate: any

• rhythm: any (usually irregular)

• P waves: early

• PR interval: 0.12-0.20 sec

• QRS complexes: <0.10 sec

• ST segments: normal

premature ventricular contractions (PVC)

• rate: variable

• rhythm: regular until interrupted

• P waves: not associated

• PR interval: not measurable

• QRS complexes: >0.12 sec, premature, abnormal, followed by compensatory pause

• ST segments: not seen after abnormal QRS

• subtypes: unifocal, multifocal, couplets, R-on-T

ventricular tachycardia (VT, V-Tach)

• rate: 140-300 bpm

• rhythm: regular

• P waves: not seen

• PR interval: not measurable

• QRS complexes: abnormal, >0.10 sec

• ST segments: not seen

• subtype: Torsades de Pointe (party streamer shaped)

ventricular fibrillation (V-Fib)

• rate: none

• rhythm: irregular

• P waves: absent

• PR interval: absent

• QRS complexes: absent or low-amp

• ST segments: absent

asystole (flatline)

• rate: none

• rhythm: none

• P waves: none

• PR interval: none

• QRS complexes: none

• ST segments: none

first-degree AV block (first-degree heart block)

• rate: any

• rhythm: any

• P waves: normal

• PR interval: >0.20 sec

• QRS complexes: <0.10 sec

• ST segments: normal

second-degree AV block type I (Mobite I, Wenckebach)

• rate: varies (ventricular < atrial)

• rhythm: irregular

• P waves: normal

• PR interval: varies, lengthens, then none

• QRS complexes: <0.10 sec

• ST segments: normal

second-degree AV block type II (Mobite II)

• rate: any (ventricular < atrial)

• rhythm: atrial regular, ventricular regular/irregular

• P waves: normal

• PR interval: normal or prolonged

• QRS complexes: <0.10 sec or absent

• ST segments: normal

third-degree AV block (complete heart block)

• rate: slow (ventricular < atrial)

• rhythm: regular

• P waves: normal

• PR interval: varies, no relation to QRS

• QRS complexes: usually >0.10 sec

• ST segments: normal

junctional rhythm (JR)

AV junction is pacemaker

• P wave may be absent (1), inverted (2), or after QRS (3)

• QRS normal

evidence of cardiac ischemia, injury, or infarction

• ischemia (1)

  • depressed ST segment (≥1 mm below baseline) or inverted T wave

  • injury potentially reversible

• acute myocardial injury (2)

  • elevated ST segment

  • ST returns to normal with restored perfusion

• MI (3)

  • elevated ST segment and T wave changes in early stages

  • later, pathologic Q waves

    • appear hours to days after event

    • usually remain for rest of patient’s life

pulseless electrical activity (PEA)

electrical activity in the heart with no pulse

causes of dysrhythmias

• hypoxia

• ischemia

• sympathetic stimulation

• drugs

• electrolyte imbalance

• hypertrophy

• rate

• stretch

ECG patterns with chronic lung disease

• severe COPD

  • right axis deviation

  • P waves large due to right atrial enlargement (leads II, III, aV_F)

  • prominent and negative P wave in lead I

• cor pulmonale

  • increased R wave size on leads V₁-V₃

  • reduced size of QRS in leads I-III, V₅-V₆