Flashcards in Cardiac Anesthesia Deck (36)
Important things to know about protamine:
1- a polycationic protein used to neutralize heparin
2- 1 mg protamine : 100 U heparin
3- measure ACT 3 minutes after protamine administration
4- associated with a spectrum of hemodynamic effects
5- Most devastating complication: profound pulmonary hypertension accompanied by elevated CVP, flaccid distended RV, and systemic hypotension.
6- Heparin-protamine complex is removed by the reticuloendothelial system.
Why do we administer cardiovascular drugs?
Cardiovascular drugs allow us to pharmacologically modify all of the components of organ perfusion, including:
1- preload (end-diastolic volume)
4- heart rate
5- myocardial oxygen supply and demand
In layman's terms, describe the Frank-Starling principle.
Increased myocardial fiber length (i.e., end-diastolic volume) improves contractility up to a point of optimal contractile state, further stretching results in declining performance.
Discuss the limitations of drugs that alter vascular tone.
Preload can be altered with intravascular volume shifts and with drugs that change vascular tone, most notably the venous capacitance vessels.
Arterial vasodilators may shift failing myocardium to a more effective contractile state as a result of afterload reduction and decreased impedance to ventricular ejection.
The intrinsic contractile state is not improved by vasodilators, in contrast to the effect of positive inotropic agents.
What is the goal of inotropic support?
To optimize end-organ perfusion by increasing myocardial contractility and thus improve cardiac output.
Discuss the hemodynamic profile of the phosphodiesterase III inhibitors amrinone and milrinone.
Amrinone and milrinone are approximately equipotent to dopamine and dobutamine in increasing cardiac output through increased inotropy and improved lusitropy (myocardial relaxation).
What is the mechanism of action of amrinone and milrinone?
They are phosphodiesterase III inhibitors, which lead to a decrease in the breakdown of cAMP.
The resultant increase in cAMP will increase the heart's contractility and decrease afterload through vasodilation.
What adverse effects can result from use of phosphodiesterase inhibitors? How are these minimized?
Because the vasodilator effects may be profound, concurrent use of vasoconstrictors (e.g., epinephrine, norepinephrine, and phenylephrine) is often necessary, particularly after CPB.
Prolonged infusion of amrinone, but not milrinone, may cause significant thrombocytopenia through nonimmune-mediated peripheral platelet destruction.
How does increasing intracellular cAMP affect the cardiac myocyte?
Increased intracellular cAMP leads to elevated intracellular calcium, increased contractility, and improved lusitropy (myocardial relaxation). The end result is a restoration of the myofilaments to their resting state.
Describe hemodynamic profile of epinephrine.
Epinephrine has potent vasoconstrictive effects and ability to increase cardiac output.
Low dose: 0.12 mcg/kg/min, primarily alpha stimulation
Describe the hemodynamic profile of norepinephrine.
The potency of norepinephrine in stimulating b-adrenergic receptors is similar to that of
epinephrine, but it results in significant a-adrenergic stimulation at much lower doses. Typical dosage ranges are 0.02 to 0.25 mcg/kg/min
Describe the hemodynamic profile of dopamine.
Dopamine stimulates specific postjunctional dopaminergic receptors in renal, mesenteric, and coronary arterial beds to produce vasodilation. These dopaminergic effects occur at lower doses (0.5 to 1.0 mcg/kg/min), becoming maximal at 2 to 3 mcg/kg/min.
At intermediate doses (2 to 6 mcg/kg/min) b1-adrenergic stimulation is evident. (This is the dose used in the ACLS bradycardia protocol.)
Beginning at doses of about 10 mcg/kg/min (but as low as 5 mcg/kg/min), a-adrenergic stimulation is seen, which at higher doses overcomes dopaminergic effects, producing vasoconstriction.
Describe hemodynamic profile of isoproterenol.
Isoproterenol is an extremely potent b1- and b2- agonist that possesses no alpha-stimulating
properties. It increases heart rate, automaticity, and contractility and dilates both venous capacitance and arterial vessels. It may be a good choice for heart-rate maintenance in a denervated nonpaced transplanted heart.
Describe hemodynamic profile of dobutamine.
Dobutamine acts principally on b-adrenergic receptors, impacting b1-receptors in a relatively selective fashion. In addition, it has a mild indirect b1-stimulating effect that is secondary to
prevention of norepinephrine reuptake but is offset by slightly more potent b2-stimulation.
Generally at clinical doses minimal increases in heart rate, positive inotropy, increased cardiac output, and minimal or modest decreases in SVR and PVR occur. Because of the indirect b1-stimulating effect, patients concurrently
receiving b-blockers can exhibit marked increases in systemic vascular resistance without improvement in cardiac output. In addition, an occasional patient will display dose-related increases in heart rate.
Which characteristics of b-adrenergic agonists limit their effectiveness?
Positive chronotropic and arrhythmogenic effects (dose-dependent effect with epinephrine, isoproterenol, and dobutamine) result in increased myocardial oxygen consumption.
Vasoconstriction secondary to a1-activation (with norepinephrine, high-dose epinephrine,
and high-dose dopamine) results in increased afterload and subsequent increased
myocardial wall tension.
Isoproterenol and dobutamine to a lesser extent produce vasodilation caused by stimulation of vascular b2-receptors. Excessive vasodilation may decrease coronary perfusion.
How may the side effects and limitations of b-adrenergic agonists be minimized?
Side effects may be decreased by appropriate dosage adjustments and use of combinations of agents.
What are the mechanism and sites of action of nitrovasodilators?
Nitrates (ie nitroglycerin and sodium nitroprusside) are pro-drugs that penetrate the vascular endothelium and become reduced to nitric oxide (NO).
Nitroglycerin requires intact endothelial enzymatic activity, which is not present in the smallest or damaged vessels, whereas nitroprusside nonenzymatically degrades into NO and cyanide. NO then stimulates production of cyclic guanosine monophosphate (cGMP), resulting in decreased cellular calcium levels and thus vascular smooth muscle relaxation.
Sodium nitroprusside acts primarily on the arterial vasculature whereas nitroglycerin has its
most prominent effect on venous capacitance vessels.
NO can also be delivered directly to the pulmonary circulation by inhalation, thereby reaching the pulmonary vascular smooth muscle by diffusion across the alveolar-capillary membrane. Direct delivery to the pulmonary circulation reduces potentially undesirable systemic hypotension and causes specific pulmonary vasodilation.
What is fenoldopam?
A potent short-acting dopamine-1 agonist that decreases SVR and increase renal blood flow via DA-1 receptor-mediated vasodilation.
Increases renal cortical and outer medullary blood flow.
T or F: ARF is a frequent complication of cardiac surgery, with a reported incidence of 30-50% in more complicated procedures.
T or F: The current evidence-based opinion is that dopamine does not prevent ARF and should not be used for this purpose, and the use of dopamine may be harmful by causing tachycardia and MI due to its adrenergic effects.
Describe the mechanism of action of labetalol. What is the onset and duration of action?
Mechanism of action: alpha and beta blocker
Onset: 2-5 minutes
Duration of action: 2-4 hours
Describe the mechanism of action of esmolol. What is the onset and duration of action?
Mechanism of action: ultra-short acting beta blocker
Onset: 60 seconds
Duration of action: 10-20 minutes
Describe the mechanism of action of nicardipine. What is the onset and duration of action?
Mechanism of action: calcium channel blocker
Onset: 5-15 minutes
Duration of action: 4-6 hours
Describe the mechanism of action of nitroprusside. What is the onset and duration of action?
Mechanism of action: arterial and venous vasodilator
Onset: within seconds
Duration of action: 1-2 minutes
Can result in cyanide toxicity
Describe the mechanism of action of hydralazine. What is the onset and duration of action?
Mechanism of action: direct vasodilator
Onset: 5-15 minutes
Duration of action: 12 hours
Describe the mechanism of action of fenoldopam. What is the onset and duration of action?
Mechanism of action: short acting dopamine agonist that increases renal perfusion and sodium excretion
Onset: 5-15 minutes
Duration of action: 30-60 minutes
Noninvasive blood pressure monitoring and pulse oximetry may be difficult because of the narrow pulse pressure occurring in patients with a continuous-flow LVAD (eg, HeartMate II). Patients often have no palpable pulse. Patients with a pulsatile LVAD (eg, HeartMate XVE) should have a palpable pulse, and a noninvasive blood pressure device should be able to obtain a blood pressure. Physical examination of a patient with a normally functioning LVAD should reveal a well-perfused, neurologically intact patient in no distress.
Under anesthesia, an intraarterial catheter is useful for monitoring blood pressure. Normal mean blood pressure should remain between approximately 70?80 mm Hg. Increases in systemic afterload may result in decreased pump flow and should be avoided. Increasing the pump flow rate will increase diastolic pressure with minimal changes in systolic pressure. Decreased ejection of blood from the native left ventricle increases the probability of thrombosis. This decreased ejection of blood would also lower pulse pressure further, making monitoring more difficult. While decreased pulse pressures are normal, a flat arterial tracing may herald hypovolemia or an obstructed ventricular cannula.
The battery life in a fully charged battery pack in some LVAD models is 3 hours. When available the LVAD should be connected to a power supply in the operating room. Extra batteries should be identified in the preoperative setting in case of power failure.
What is 1 MET?
One MET is equal to the amount of energy expended during 1 minute of rest, which is roughly 3.5 mL of oxygen per kg of bodyweight per minute (3.5 mL/kg/min).
For a 70 kg individual, one MET equals 250 mL oxygen/min.
Recall that 3 METs are equivalent of climbing two flights of stairs at a reasonable rate without stopping. 10 METs are equivalent to participating in strenuous activity.
What is pH stat?
Refers to the practice of temperature correcting gas tensions by adding carbon dioxide and maintaining a "normal" carbon dioxide tension of 40 mmHg and a pH of 7.4 during hypothermia.
What is alpha-stat?
Refers to the use of uncorrected gas tensions during hypotehermia. This does not require addition of carbon dioxide and has been shown to preserve cerebral autoregulation and improve myocardial preservation.
T or F: pH stat management is commonly used in pediatric cardiac surgery, but alpha- stat management is more commonly used in adult cardiac surgery.
T or F: Perioperative MI risk-reduction therapy with medications such as beta-blockers, clonidine, statins, and aspirin are superior to invasive testing, angioplasty, and CABG.
T or F: Following an MI, elective surgery should be delayed to 6-8 weeks per ACC/AHA guidelines.
T or F: Elective noncardiac surgery should be delayed 14 days after balloon angioplasty and 30 days after bare metal stent implantation.
T or F: Elective noncardiac surgery should be delayed 365 days after drug eluting stent placement.
True- In patients in whom noncardiac surgery is required, a consensus decision among treating clinicians as to the relative risks of surgery and discontinuation or continuation of antiplatelet therapy can be useful.
Elective noncardiac surgery after DES implantation may be considered after 180 days (6 months) if the risk of further delay is greater than the expected risks of ischemia and stent thrombosis