Case 11 - CABG

Question 1

describe myocardial oxygen extraction and coronary anatomy

Answer 1

myocardial o2 extraction

• myocardium extracts 65% of oxygen (highest of any organ)
• maximum extraction, therefore only increase in coronary blood flow can meet increase O2 demand
  
  • rest of tissues has 25% extraction

Coronary Anatomy

RCA

• supplies RA, RV
• usually also supplies AV and SA node
• right dominant circulations gives rise to PDA (85%)
• PDA
  
  • interventricular spetum and inferior wall of LV

LCA

• LAD - septum of LV and anterior wall
• LCX - lateral wall of LV
• left dominant circulation (15% of time) - supply PDA

Question 2

what are the major determinants for myocardial oxygen supply?

Answer 2

Myocardial oxygen supply

1) CPP

• CPP = aortic diastolic pressure - LVEDP
• during systole, LV pressure approaches aortic pressure (allow forward ejection of blood, no coronary perfusion)
  
  • LV perfused only during disastole
  • RV perfused during diastole and systole

2) HR

• LV perfused during diastole
• low HR leads to increased diastolic time

3) Arterial oxygen content
* CaO2 = Hgb x 1.34 x SaO2 + (PaO2 x 0.003)

Question 3

what are the major determinants of myocardial oxygen demand?

Answer 3

1) basal o2 requirements
2) Increase Afterload

• Increase afterload leads to increased LV wall tension 2/2 increase in intraventricular pressure (match afterload for forward ejection of blood)
• inc o2 consumption
• laplace law: T = PR/2H

3) Increase Preload

• Increase preload leads to increased LV wall tension 2/2 increase ventricular radius
• inc o2 consumption

4) contractility
* increaes contractility = uses more oxygen

Question 4

what medications are patients with CAD typically on?

Answer 4

goal: reduce demand o2 demand + inc O2 supply

Meds: BB, anti-platelet, statin

1) BB

• Reduce O2 consumption
  
  • decrease HR
  • decrease contractility
• increase o2 supply
  
  • decrease HR = inc diastolic time for coronary perfusion

2) anti-platelet

• platelet inhibitor, prevent thrombosis

3) statins

• antiinflammatory and antithrombic events

Question 5

patient comes for elective surgery and has CAD. What meds do you advise him to continue during periop period?

Answer 5

1) BB

• if taking BB, then continue BB
• if not taking BB but high risk for CAD or major cardiac events -> start BB
• do not start BB in low risk individuals
  
  • found to be assoc with increase incidence of stroke (2/2 hypotension)

2) anti-PLT agents

• pt with stent undergoing elective surgey -> delay surgery..
  
  • PTCA 4 weeks
  • BMS 6 weeks
  • DES 12 months
• recent stent + emergent surgery
  
  • high risk of surgical bleeding -> stop plavix, continue ASA
  • low risk of surgical bleeding - > continue both meds

3) Statins

• continue during periop period
• withdrawl of statin = inc in-hosp mortality after CABG
  *

Question 6

how does intraaortic ballon pump work?

Answer 6

mechanal device that assists a failing heart

• assist with decreasing myocardial oxygen demand and increasing myocardial oxygen supply
• Deflate during systole
  
  • decrese afterload (dec o2 demand)
  • promote forward flow (inc o2 supply)
• inflate during diastole
  
  • promote coronary perfusion (inc o2 supply)

Question 7

patients with CAD typically have other co-morbidities like HTN, CVA, DM, and CKD. What our pre-op concerns if the patient has all of these diseases?

Answer 7

1) HTN

• relative hypovolemia (pressure maintained by inc SVR)
• induction induced hypotension 2/2 loss of symp tone

2) DM

• atutonomic and peripheral neuropathy
• positioning
• full stomach

3) CVA

• compromised flow to brain
• cerebral autoregulation shifted to right
• higher blood pressure than normal to maintain adequate CPP

4) CKD
* affect elimination of certain meds

Question 8

what are your anesthetic goals for a patient with CAD undergoing CABG? Any anesthetics you would particuarly use?

Answer 8

Goals:

• maintain normal blood pressure
• increase myocardial O2 supply
• decrease myocardial O2 demand

Anesthetic

• no superior anesthetic agent
• choose anesthetic (induction and maintance) based on LV function and coronary pathology
• Volatile anesthetic
  
  • ischemic preconditioing
    
    • protect myocardium against ischemia by their ability to elicit protective cellular responses
    • result = decreaes MI infarct size after periods of ischemia

Question 9

what monitors will you use to detect myocardial ischemia?

Answer 9

1) EKG

• lead II and V5 detect 90% of ischemic episodes
  
  • II - inferior wall
  • V5 - anterior wall
• arrythmia detection
• early ischemia = t wave inversion followed by ST seg depression

2) TEE

• recommended in all patients undergoing CABG
• RWMA
• transgrastric short axis midpapillary view
  
  • view territory RCA, LAD, LCX
    *

Question 10

what are the disadvantages of doing a CABG compared to off-pump CABG?

Answer 10

Disadvantages of CABG - relies on CPB

• cross clamping aorta
  
  • risk of aortic dissection
• CPB
  
  • systemic inflammatory response 2/2 foreign substance in contact with blood
• cerebral dysfunction from emboli
  
  • CBP machine or plaques of aortic wall
• Cannulation complications

Question 11

what may be a disadvantage of off-pump cabg?

Answer 11

sugeon induced hemodyanmic changes

• mechanical stabilizer device or suction device is placed on apex of heart
• twisting or lifting of heart to get to inferior surface can cause compression of RV and distortion of mitral valve annulus:
  
  • severe hypotension
  • mitral regurg

Tx hypotension

• communicate with surgeon as hypotension typically surgeeon manipulation induced
• T-berg position (inc venous return)
• volume expansion
• inotrope and pressors