CARDIOLOGY Chapter 12 - Guyton

Question 1

Mean QRS vector

Answer 1

+59 degrees is the avg. direction of vector during depolarization, means the apex of heart is positive with respect to base of heart

Question 2

Axis for each standard bipolar lead and each unipolar limb lead.

Answer 2

Lead I axis is 0 degrees, b/c electrodes lie exactly in the horizontal direction; Lead II axis is about +60 degrees b/c electrodes lie on upper right corner of torso and lower left corner of leg; Lead III axis is +120 degrees; aVR axis is +210 degrees; aVF axis is +90 degrees; aVL axis is -30 degrees

Question 3

Normal T wave

Answer 3

all three bipolar limb leads is positive, which is also the polarity of most of the normal QRS complex

Question 4

Hypertrophy

Answer 4

Left ventricle (Left axis shift) caused by hypertension, aortic stenosis, aortic regurgitation, causes a slightly prolonged QRS and high voltage; Right ventricle (right axis shift) caused by pulmonary, hypertension, pulmonary valve stenosis, interventricular septal defect

Question 5

Bundle branch block

Answer 5

left block causes left axis shift b/c right ventricle depolarizes much faster than left ventricle, cause prolonged QRS complex

Question 6

Increased Voltages in Standard Bipolar Limb Leads

Answer 6

Sum of voltages of Leads I-III > 4mV it is considered high voltage EKG; caused by increased ventricular muscle mass (hypertension, marathon runner)

Question 7

Decreased voltages in standard bipolar limb leads.

Answer 7

caused by cardiac muscle abnormalities, old infarctions cause decreased muscle mass, low voltage EKS, and prolonged QRS caused by conditions surrounding heart fluid in pericardium, pleural effusion, emphysema; caused by anterior-posterior rotation of apex of heart

Question 8

Heart position

Answer 8

left shift - caused by expiration, lying down and excess abdominal fat; right shift - exact opposite (skinny, inspiration, standing up)

Question 9

Prolonged QRS Complex

Answer 9

Caused by prolonged conduction of cardiac impulse through the ventricles; Normal QRS is btwn 0.06-0.08 secs, prolonged QRS is caused by cardiac hypertrophy, purkinje system block, or conduction block (if QRS exceeds 0.12 sec)

Question 10

Unusual QRS complex

Answer 10

caused by local conduction blocks which may cause multiple QRS peaks

Question 11

Axis of the Current of Injury

Answer 11

At end of S wave ventricles are fully depolarized (J point), difference between J point and T-P segment is Current of Injury, voltages are plot on the coordinates of 3 leads to determine electrical axis, negative end of vector will originate in the injured/ischemic area of the heart

Question 12

Anterior and Posterior Infarctions

Answer 12

Chest leads determine if current of injury is anterior/posterior; if negative, chest lead is area of negative potential, indicates anterior lesion; if positive current of injury, chest lead indicates posterior lesion; negative end of current of injury vector point towards abnormal cardiac muscle

Question 13

Recovery from Coronary Thrombosis

Answer 13

T-P segment shows a current of injury following acute coronary thrombosis; improves over several weeks when at rest; exercise may cause ischemia of this recovered area, resulting in a current of injury

Question 14

T wave abnormalities

Answer 14

Ventricular repolarization usually occurs in the opposite direction as depolarization which causes an upright T wave in the 3 standard leads; prolongation of repolarization may change T wave axis; Left bundle branch block causes late depolarization and thus a late repolarization of the left ventricle; Mild ischemia particularly in the apex of the heart prevents apex from repolarizing first, inverting T wave; Digitalis toxicity prolongs depolarization in certain part of heart, causes a biphasic T wave