Cardiopulmonary Bypass

Question 1

What are the three basic components of a CPB machine?

Answer 1

1- tubing that drain venous and return arterial blood
2- a membrane oxygenator, where gas exchange occurs
3- a mechanical pump that provides systemic perfusion.

Question 2

What type of pumps exist in CPB? What is the downside with both?

Answer 2

1- roller pumps
2- centrifugal pumps

Neither type of pump is able to deliver pulsatile blood flow, which can be a cause of renal dysfunction and production of ischemic metabolic products.

Question 3

What is prime?

Answer 3

• the fluid contained within the CPB tubing.
• can contain LR, albumin, mannitol, calcium, heparin, etc. May cause dilutional anemia, which is useful to the degree that it offsets changes in blood viscosity due to hypothermia.

Question 4

Why is anticoagulation with heparin important in CPB?

Answer 4

Contact between a patient’s blood and components of the CPB circuit initiates activation of the coagulation cascade. To prevent thrombosis of the CPB circuit and the patient’s death, systemic anticoagulation is required prior to insertion of cannulae and initiation of CPB.

Question 5

What is the peak onset of heparin? What is the half-life?

Answer 5

Peak onset: < 5 minutes
Half-life: approximately 90 minutes in normothermic patients, progressive increase in half-klife in hypothermic patients.

Question 6

What is the mechanism of action of heparin?

Answer 6

Heparin potentiates the effects of antithrombin III

–> heparin binding alters antithrombin III’s structural configuration and increases its thrombin inhibitory potency by greater than 1000 fold.

By inhibiting thrombin, AT prevents formation of the fibrin clot via intrinsic and extrinsic pathways.

Question 7

What do you do in patients receiving heparin preoperatively and those with congenital AT deficiencies?

Answer 7

Give higher than expected doses of heparin in order to achieve adequate anticoagulation.

During CPB, heparin levels are measured frequently, but only the ACT is a measure of anticoagulant activity. ACT measurement is important in these patients with heparin resistance and antithrombin III deficiency.

Question 8

Why is PTT not used to measure heparin action in cardiac surgery?

Answer 8

Modern PTT assays are so sensitive that heparin levels far lower than those used for safe initiation of CPB causes sample blood to become unclottable.

Question 9

How is adequacy of anticoagulation measured in CPB? What is ACT?

Answer 9

ACTs > 480 seconds are considered acceptable for initiation of CPB.

Activated clotting time (ACT) is measured about 3 to 4 minutes after heparin administration
and every 30 minutes on CPB.

Question 10

What is protamine? How does it work?

Answer 10

• Protamine neutralizes heparin, works as a cation that binds anionic heparin.
• Neutralization ratio is 1 mg protamine to 100 units heparin.

Question 11

What is heparin induced thrombocytopenia (HIT-1)?

Answer 11

A mild and transient decrease in platelet count following the administration of heparin a few days following surgery.

Question 12

What is HIT-2?

Answer 12

Less common and more serious form of HIT, which is an immune-mediated disorder characterized by the formation of antibodies against the heparin-platelet factor 4 complex. Predisposes to platelet clumping and microvascular thrombosis.

Question 13

Why is aminocaproic acid administered?

Answer 13

Aminocaproic acid is an antifibrinolytic agent. It binds to plasminogen and blocks its ability to bind at fibrinogen. This prevents the lysis of fibrin clots. Administration of antifibrinolytics decreases bleeding after CPB.

Question 14

What is the most common method of myocardial protection?

Answer 14

intermittent hyperkalemic cold cardioplegia and moderate systemic hypothermia

Question 15

Why may you see postbypass bleeding?

Answer 15

Persistent oozing following heparin reversal is not uncommon.

The usual causes include inadequate surgical hemostasis or reduced platelet count or function, and neither of these is identified by prolonged activated coagulation time.

DDx: insufficient doses of protamine, dilution of coagulation factors, thrombocytopenia, and platelet dysfunction.

Question 16

Describe the CPB circuit:

Answer 16

Pt’s venous blood –> venus cannula –> venous reservoir –> oxygenator and heat exchanger –> main pump (can be roller or centrifugal) –> arterial filter to remove debris, clumps, and fat –> aortic cannula

Question 17

Why is mannitol added to prime?

Answer 17

Mannitol is an osmotic diuretic that is commonly added to the CPB prime with the belief that it may decrease renal injury.

The theorized renoprotective effects of mannitol are due to its ability to flush nephrotoxic substances from the tubules, scavenge free radicals, and improve medullary blood flow.

Question 18

What are adverse effects associated with hypothermia?

Answer 18

Platelet dysfunction
Reduction in serum ionized calcium concentration caused by enhanced citrate activity
Impaired coagulation
Arrhythmias
Increased risk of infection
Decreased oxygen unloading
Potentiation of neuromuscular blockade
Impaired cardiac contractility.

Question 19

Why is hypothermia used during CPB?

Answer 19

Systemic oxygen demand decreases 9% for every degree of temperature drop –> hypothermia allows for lower CPB pump flows while providing adequate oxygen supply to vital organs.

The main concern of CPB is the prevention of myocardial injury and CNS injury, along with renal and hepatic protection.

Question 20

T or F: Reductions in priming volume have resulted

in decreased inflammatory response and a reduction in transfusion requirements.

Answer 20

True

Question 21

T or F: The acute hemodilution from the patient’s circulating blood volume mixing with the prime volume can cause an acute reduction in mean arterial pressure and the hemoglobin concentration.

Answer 21

True

Question 22

What are characteristics of cardioplegia?

Answer 22

Cardioplegia is a hyperkalemic solution containing various metabolic energy substrates that is perfused through the coronary vasculature. It induces diastolic electromechanical dissociation.

Myocardial oxygen and energy requirements are reduced to those of cellular maintenance.

Cardioplegia is perfused either anterograde via the aortic root coronary ostia or retrograde through the right atrial coronary sinus.

Question 23

Review the physiologic responses to CPB. Address the following:

• stress hormones
• exposure of blood to CPB circuit
• platelets
• hemodilution

Answer 23

1- Stress hormones, including catecholamines, cortisol, angiotensin, and vasopressin, increase
in part because of decreased metabolism of these substances.

2- Exposure of blood to the CPB circuit results in complement activation, initiation of the coagulation cascade, and platelet activation. A systemic inflammatory response is also initiated.

3- Platelet dysfunction associated with CPB may contribute to post-CPB bleeding.

4- Hemodilution associated with onset of CPB decreases the serum concentrations of most
drugs, but decreased hepatic and renal perfusion during CPB will eventually increase the serum concentration of drugs administered by continuous infusion.

Question 24

What are the pH-stat and alpha-stat methods of blood gas measurement? Why do we discuss these?

Answer 24

The approach to the interpretation of blood gases during CPB, especially when deep hypothermia is used, has been associated with controversy. There are differing opinions as to whether blood gases should be corrected for the temperature BECAUSE THE SOLUBILITY OF GASES DECREASES WITH HYPOTHERMIA.

• All blood gases are warmed to 37 prior to measurement. The values that are obtained are known as the temperature-uncorrected values. If the patient’s blood is at a lower temperature, the ABG values can be corrected to the patient’s temperature using a normogram, which accounts for changes in oxygen and CO2 solubility and pH caused by temperature. When measurements are corrected to be appropriate to a patient’s lower body temperature, the corrected pH will be higher and the corrected PaCO2 will be lower.
• In pH-stat measurements, the obtained value is corrected on a nomogram, and the reported values refer to the partial pressure at the hypothermic temperatures. CO2 is then added to the ventilating gas to maintain constant pH over varying temperatures.
• Alpha-stat blood gas measurement refers to reporting blood gases uncorrected for temperature, resulting in a high value when the blood gas is temperature corrected.
• In adults alpha-stat management leads to improved outcomes and is commonly used.
• Neonatal data indicates a trend toward neurologic improvement using pH-stat management, which is
  therefore used in this population

Question 25

Why is cardiac pacing frequently useful after bypass?

Answer 25

Between the ischemic insult of bypass, the residual effect of cardioplegia, and effects from
hypothermia, cardiac conduction may be impaired, and myocardial wall motion is suboptimal.
Sequential cardiac pacing at a rate of 80 to 100 beats/min can significantly improve cardiac
output.

Question 26

T or F: The CPB reservoir should never be allowed to empty during CPB since massive air embolismis a consequence.

Answer 26

True

Question 27

T or F: Factors involved in myocardial preservation include cardioplegia, myocardial hypothermia, and ventricular venting. Consequences of inadequate myocardial preservation include decreased cardiac output, ischemia, dysrhythmias, and failure to wean from CPB.

Answer 27

True

Question 28

T or F: pH stat may increase the risk of cerebral embolization.

Answer 28

true

Question 29

T or F: retrograde flow is possible with the centrifugal pump but not with the roller pump.

Answer 29

True

Question 30

T or F: The roller pump has a predictable speed and is less expensive than the centrifugal pump.

Answer 30

True

Question 31

T or F: Which view is recommended for detecting areas of myocardial ischemia that are perfused by all three main coronary arteries (right, left anterior descending and circumflex arteries)?

Answer 31

Transgastric mid-papillary short axis view is recommended for monitoring of ischemia becasue it shows portions of the myocardium that are perfused by all three main coronary arteries.

Question 32

Two contraindications to an intra-aortic balloon pump include:

Answer 32

1- severe PVD of the aortoiliac and femoral arteries due to increased risk of lower extremity ischemia

2- severe AI, because the AI will be worsened by the balloon pump

Question 33

For an intra-aortic ballon pump to have optimal effect, the inflation and deflation must be timed appropriately to the patient’s cardiac cycle.

Answer 33

True- the IABP is triggered from the R wave of the EKG signal but the balloon inflation begins in the middle of the T-wave, with deflation prior to the end of the QRS complex. Thus the balloon should inflate at the beginning of diastole and deflates prior to the arterial upstroke.

Question 34

T or F: CPB is a risk factor for kidney injury due to hypoperfusion, nonpulsatile flow, and inflammation.

Answer 34

True

Question 35

T or F: the most powerful predictor of atrial fibrillation post-cardiac surgery is age.

Answer 35

true