Chapter 1

Question 1

How is electric current in heart generated?

Answer 1

• Cardiac cells are electrically polarized at resting state: (-) inside w respect to outside
  
  • Charge maintained by Na-K-ATPase pump (2K+ in + 3Na+ out)
  • E-required process
• Depolarization = loss of internal (-) charge
  
  • Pacemaker cells spontaneously depol
  • Represents flow of electricity detected by electrodes
• Repolarization = restoring resting polarity
• Action potential (AP) = cycle of depol + repol

Question 2

What are the cells of the heart?

Answer 2

1. Pacemaker cells - depol spontaneously (5-10 mcm)
   
   • Rate det by innate electrical char of cells + ext. neurohormal input
   • Dom pacemaker cell = sinal atrial (SA) node, rate 60-100bpm
     
     • Rate varies w ANS: sympathetic stim (adrenaline) + vagal stim + body CO demands
2. Electrical conducting cells - Purkinje fibers + Bachmann’s bundle (rapid activation R–>L atrium)
3. Myocardiocytes - resp for contracting + relaxing to deliv blood to rest of body (50-100mcm)
   
   • Contain many actin + myosin protein
   • Spreads slowly across myocardium
   • Excitation-contration coupling
     
     1. Depol
     2. Ca++ release w/in cell –> allows actin-myosin interaction
     3. Contraction

Question 3

What are components of EKG?

Answer 3

1. Horizontal axis = time (0.04s x 5 = 0.2s)
2. Vertical axis = voltage (0.1mV x 5 = 0.5mV)
3. P wave = atrial depol (first half = R atrial component, second half = L atrial component)
4. Electrical gate (fibrous skin) at jxn b/w atria + ventricles –> pause in conduction –> allows atria to fully empty before ventr contraction
5. QRS complex = ventricular depol (earliest pt = interventricular septal depol)
   
   • RV + LV depol at same time but mostly see effects of L b/c LV = 3x RV
   • Atrial repol hidden by QRS complex
6. T wave = ventricular repol

Question 4

Describe electrical pathway of ventriclar contration.

Answer 4

1. SA node fires –> atrial depol + contraction
   
   • RA begins + fin before LA
2. Atrial depol reaches fibrous skeleton –> electrical conduction prevented from dir comm b/w atria + ventricules
3. Electrical conduction must be funneled along interventricular septum thru atrioventricular (AV) node –> slows conduction
4. Bundle of His
5. L + R bundle branches
   
   • R branch: carries current down R side of septum to apex of RV
   • L branch: divided into septal, anterior, posterior fascicle
6. Purkinje fibers
7. Entire myocardium

Question 5

Describe the components of QRS wave.

Answer 5

• Q wave = first deflection down
  
  • ONLY if downward deflection is first wave of the complex; otherwise = R wave
• R wave = first deflection up
• S wave = first deflection down following upward deflection
• If entire config consists of only 1 downward deflection = QS wave

Question 6

What is the diff b/w segment + interval?

Answer 6

Segment = straight line conn 2 waves

Interval = encompasses at least 1 wave + conn. straight line

Question 7

Define positive, negative, biphasic deflection.

Answer 7

Positive deflection = depol towards (+) electrode or repol away from (+) electrode

Negative deflection = depol away from (+) electrode or towards (+) electrode

Biphasic deflection = wave moves perpendicular to (+) electrode

Question 8

How is the heart positioned w/in the chest cavity?

Answer 8

• Rotation of heart w/in chest cavity dedirects atrial depol –> EKG variations among diff ppl
• RV = anteriorly + medially w/in body cavity
• LV = posteriorly + laterally within body cavity

Question 9

What are the 6 limb leads?

Answer 9

• Vertical frontal plane
• 2 electrodes on arms + 2 on legs
• Standard leads = leads I, II, III
• Augmented leads = amplified tracings w only 1 (+) lead, all others = negative; leads aVL, aVR, aVR,
• Leads:
  
  • I: (-)RA (+)LA, 0 degrees
  • II: (-)RA (+) legs, +60 degrees
  • III: (-)LA (+) legs, +120 degrees
  • aVL: (+)LA (-30 degrees)
  • aVR: (+)RA, -150 degrees
  • aVF: (+) legs, +90 degrees

Question 10

What are the 6 precordial leads?

Answer 10

• Horizontal plane, chest
• Each electrode made (+) in turn w whole body as common ground
• Leads:
  
  • V1: 4th intercostal space, R of sternum
  • V2: 4th intercostal space, L of sternum
  • V3: b/w V2 + V4
  • V4: 5th intercostal space, midclavicular
  • V5: b/w V4 + V6
  • V6: 5th intercostal space, midaxillary

Question 11

Which leads best view which parts of the heart?

Answer 11

• Inferior leads: II, III, aVF,
• L lateral leads: I, avL, V5, V6
• R-sided (limb) leads: aVR, V1
• Anterior leads: V2, V3, V4

Question 12

What happens during these cardiac intervals + segments?

Answer 12

• PR interval - start of atrial depol –> start of ventricular depol, including delay in conduction at AV node (0.12-0.2s)
• PR segment - end of atrial depol –> start of ventricular depol
• QRS interval - duration of QRS complex (0.06-0.1s)
• ST segment - end of ventricular depol –> start of ventricular repol, usually horizontal or gently upsloping in all leads
• QT interval - begining of ventricular depol –> ventricular repol
  
  • composes 40% normal cardiac cycle
  • More of QT interval devoted to repol than depol
  • Faster HR = faster repol –> shorter QT interval + v.v

Question 13

Describe appearance of P wave among 12 leads.

Answer 13

• Atrial depol begins at SA node w vector of current flow pointing R–>L, pointed slightly inferiorly
• Atria = small –> amplitude doesn’t usually exceed 0.25mV
• Positive deflection: I, II, AVF, AVL, V5, V6
• Negative deflection: aVR
• Variable: V2, V3, V4

Question 14

Descibe the apperance of QRS complex among 12 leads.

Answer 14

• Interventricular septum = first to depol due to rapid depol by L bundle branch (L–>R direction)
  
  • Not always visible (septal Q wave)
  • Norm amplitude not greater than 0.1mV
  • Negative deflection: I, avL, V5, V6, sometimes V3, V4
• Ventricles much more massive, QRS >> P wave
• LV >> RV –> average vector of current flow swings leftward b/w 0-90 degrees
  
  • Lg positive deflection (R wave): I, II, V5, V6
  • Lg negative deflection (S wave): aVR, V1
  • Transition zone (biphasic, L wave = R wave in amplitude) = V3, V4
• R wave progression = progressively increase in R wave from V1-V5 (V6 typically a lil smaller than V5)

Question 15

Descibe T wave appearance.

Answer 15

• T wave highly variable in apperance, very susceptible to cardiac + noncardiac influences (hormonal, neurologic, etc.)
  
  • Repol requires a lot of cellular E –> not passive unlike depol
• Repol travels in opp dir as depol beginning w last area to have been depolarized
  
  • Leads w tall R waves typically have (+) T waves