Lecture 7: Heart Rhythmicity and Normal ECG

Question 1

Lead I

Answer 1

• negative terminal connected to the right arm
• positive terminal is connected to the left arm
• looks at heart from left to right
• direction of the lead = 0 degrees
• when right arm is negative w/respect to left arm, ECG records positive

Question 2

Lead II

Answer 2

• negative terminal connected to the right arm
• positive terminal connected to the left leg
• looks at heart from upper right to lower left
• direction of the lead = 60 degrees
• when right arm is negative w/respect to the left leg, ECG records positive

Question 3

Lead III

Answer 3

• negative terminal connected to the left arm
• positive terminal connected to the left leg
• looks at heart from upper left to lower left
• direction of the lead = 120 degrees
• when left arm is negative w/respect to the left leg, ECG records positive

Question 4

Einthoven’s law

Answer 4

Def: if the electrical potentials of any two of the three bipolar limb leads are known at any given moment
Einthoven’s triangle: two arms and left leg form apices of a triangle

Question 5

Vector trends

Answer 5

• when heart vector is in ~perpendicular direction to the axis of the lead, the recorded voltage is very low
• when heart vector has ~same axis as lead, then entire voltage of vector will be recorded
• summated vector of the generated potential at a particular instant is called the “instantaneous mean vector”
• reference vector is horizontal, extends towards a person’s left side; this reference direction = 0 degrees
• avg direction of vector during spread of depolarization wave through the ventricles results in the apex of heart remaining positive w/respect to the base of heart

Question 6

T-wave (vectorial analysis)

Answer 6

• greatest portion of ventricular mass to repolarize first is the entire outer surface of the ventricles, esp near apex
• endocardial areas normally repolarize last
• ventricular vector during repolarization is toward apex
• normal T-wave in all three leads is positive (as is most of the normal QRS complex)

Question 7

P-wave (vectorial analysis)

Answer 7

• atrial depolarization begins in the sinus node
• vector direction is generally in the direction of the axes of the three leads
• ECG is normally positive

Question 8

Atrial T-wave (vectorial analysis)

Answer 8

• spread of depolarization through atrial muscle is slower than in ventricles
• musculature around sinus node becomes depolarized a long time before musculature in the distal parts of atria
• sinus node is first area to repolarize
• @ beginning of repolarization, sinus node is positive w/respect to rest of atria
• atrial t-wave is typically obscured by QRS complex

Question 9

Conditions causing axis deviation

Answer 9

• change in position of heart
• hypertrophy of ventricle
• bundle branch block
• fluid in pericardium
• pulmonary emphysema

Question 10

Abnormal voltages of QRS complex

Answer 10

• high voltage when sum of QRS voltages is > 4 mv; most common cause is hypertrophy
• decreased voltage caused by cardiac myopathies, conditions surrounding heart

Question 11

Bizarre patterns of QRS complex

Answer 11

• normal QRS is 0.06 - 0.08 sec
• hypertrophy/dilation of L/R ventricle may prolong QRS to 0.09 - 0.12 sec
  causes: scar tissue in heart, small blocks in conduction impulses in Purkinje system

Question 12

J-point

Answer 12

reference pt for analyzing current of injury

*occurs @ very end of QRS wave; pt @ which all parts of ventricle (inc. injured areas) have depolarized