UBP 4.5 (Short Form): Cardiovascular - CABG & Mitral Valve Replacement

Question 1

Pre-operative Management:

This case will require the use of a cardiopulmonary bypass circuit.

Describe the difference between centrifugal and roller pumps.

(A 67-year-old, 128 kg, 5’8”, male presents for quadruple CABG and mitral valve replacement. He complains of worsening chest pain, is an insulin dependent diabetic, has chronic hypertension, and smokes cigarettes. He reports sleeping in the sitting position and takes prevacid for severe acid reflux.)

Answer 1

With a roller pump, forward flow is produced with the partial compression of tubing by two roller heads.

A roller pump is –

• not sensitive to preload or afterload (i.e. kinking, occlusion, increased SVR, clamp placement),
• can deliver pulsatile flow, and
• reliably produces a certain amount of flow based on pump speed.

The disadvantages of this type of pump include:

1. relatively increased damage to red blood cells;
2. the potential delivery of large quantities of air to the patient if air was entrained into the pump;
3. the risk of over-pressurization leading to subsequent tubing separation or rupture (due to the lack of sensitivity to changes in afterload); and
4. the risk of preload occlusion leading to negative pressure-induced cavitation (the development of microscopic bubbles).

With the centrifugal (kinetic) pump, rotational force is responsible for forward flow.

A centrifugal pump is less damaging to red blood cells; is sensitive to changes in preload and afterload; and will cease to function if a significant amount of air is entrained.

However, while the risks of over-pressurization, cavitation, and delivering large quantities of air to the patient are eliminated, this type of pump is incapable of delivering pulsatile flow and is only able to partially compensate (via increased speed) for decreases in forward flow resulting from increases in distal pressure.

Question 2

Pre-operative Management:

Would alpha-stat or pH-stat management be preferable?

What is the difference between the two strategies?

(A 67-year-old, 128 kg, 5’8”, male presents for quadruple CABG and mitral valve replacement. He complains of worsening chest pain, is an insulin dependent diabetic, has chronic hypertension, and smokes cigarettes. He reports sleeping in the sitting position and takes prevacid for severe acid reflux.)

Answer 2

Although either strategy would probably be acceptable, there is some evidence of slightly improved neuropsychologic outcomes with the use of alpha-stat management in adults.

The two strategies describe different approaches to managing CO2 in the hypothermic patient, where hypothermia leads to an increase in the solubility of CO2 with subsequent reductions in arterial pH and the partial pressure of CO2 (it does not reduce the total CO2 content).

A practitioner using the pH-stat strategy of gas tension management would add CO2 to the oxygenator as necessary to maintain a PaCO2 of 40 mmHg and a pH of 7.40;

a practitioner using the alpha-stat strategy of gas tension management would NOT add CO2 with the onset of hypothermia, maintaining electrochemical neutrality.

Since the primary mechanism of brain injury in adults is thought to be related to embolic events rather than ischemia, the enhanced cerebral blood flow associated with the addition of CO2 to the oxygenator may prove more harmful than helpful.

Question 3

Pre-operative Management:

Would your strategy be different for a pediatric patient? (alpha-stat vs pH-stat)

(A 67-year-old, 128 kg, 5’8”, male presents for quadruple CABG and mitral valve replacement. He complains of worsening chest pain, is an insulin dependent diabetic, has chronic hypertension, and smokes cigarettes. He reports sleeping in the sitting position and takes prevacid for severe acid reflux.)

Answer 3

In contrast to an adult, the primary mechanism of brain injury in the pediatric patient is thought to be related to ischemia.

Therefore, I would employ the pH-stat strategy, recognizing that the enhanced cerebral blood flow associated with this strategy may prove beneficial by facilitating cerebral cooling prior to cardiopulmonary arrest, thus providing superior brain protection.

Note: There is some evidence that cerebral metabolic recovery following cardiopulmonary arrest is improved using the alpha-stat strategy.

Therefore, some practitioners are utilizing a crossover strategy, using pH-stat management during the first 10 minutes of cooling (to facilitate cerebral cooling) and then switching to alpha-stat management for the remainder of the case, to facilitate cerebral metabolic recovery.

However, since the benefits of this combination strategy are unproven, most practitioners utilize the pH-stat management strategy throughout the case.

Question 4

Pre-operative Management:

How high does the activated clotting time (ACT) need to be prior to initiation of cardiopulmonary bypass?

What if you are using aprotinin?

(A 67-year-old, 128 kg, 5’8”, male presents for quadruple CABG and mitral valve replacement. He complains of worsening chest pain, is an insulin dependent diabetic, has chronic hypertension, and smokes cigarettes. He reports sleeping in the sitting position and takes prevacid for severe acid reflux.)

Answer 4

Heparinization is considered adequate when the ACT is greater than 480 seconds.

If the ACT is too low, additional doses of heparin (100 U/kg) should be administered.

Since aprotinin artifactually prolongs the celite ACT through effects on the intrinsic pathway and inhibition of kallikrein, it is preferable to utilize the kaolin ACT, which is NOT affected.

If the kaolin ACT is not available, a celite ACT greater than 700-750 seconds is required for adequate heparinization.

Question 5

Pre-operative Management:

Does it matter if his mitral regurgitation is acute or chronic in nature?

Describe the pathophysiology of acute mitral regurgitation.

(A 67-year-old, 128 kg, 5’8”, male presents for quadruple CABG and mitral valve replacement. He complains of worsening chest pain, is an insulin dependent diabetic, has chronic hypertension, and smokes cigarettes. He reports sleeping in the sitting position and takes prevacid for severe acid reflux.)

Answer 5

It does matter,

because acute mitral regurgitation results in left atrial and ventricular volume overload without the compensatory ventricular dilation that occurs with chronic mitral regurgitation.

This results in markedly increased left atrial and left ventricular end-diastolic pressures, leading to decreased cardiac output, pulmonary congestion, pulmonary edema, and right ventricular failure.

Furthermore, elevated LVEDP combined with compensatory tachycardia places this patient at increased risk of worsening myocardial ischemia.

Question 6

Pre-operative Management:

The medical student asks you to explain the pulmonary capillary wedge tracing changes that occur with acute severe mitral regurgitation.

What would you say?

(A 67-year-old, 128 kg, 5’8”, male presents for quadruple CABG and mitral valve replacement. He complains of worsening chest pain, is an insulin dependent diabetic, has chronic hypertension, and smokes cigarettes. He reports sleeping in the sitting position and takes prevacid for severe acid reflux.)

Answer 6

In the presence of mitral regurgitation, the pulmonary capillary wedge tracing usually exhibits a prominent v wave, an absent x descent, and a rapid y descent.

In the healthy heart, the c wave reflects elevation of the mitral valve during early ventircular systole, the v wave reflects venous return against a closed mitral valve, the x descent occurs with the downward displacement of the atrium during ventricular contraction, and the y descent represents the decline in atrial pressure as the mitral valve opens during diastole.

Acute volume overload of the relatively noncompliant left atrium leads to markedly increased left atrial pressures with a prominent v wave (sometimes referred to as a cv wave) and an abolished x descent on the pulmonary capillary wedge tracing.