downregulation

I mentioned downregulation in the previous CCRN tip of the day. Downregulation is not a concept the CCRN requires you to understand, but it is a CMC and CSC concept. It’s also something anyone who cares for advanced heart failure patients will encounter again and again. The easiest way to explain downregulation is through the […]

I mentioned downregulation in the previous #CCRN tip of the day. Downregulation is not a concept the CCRN requires you to understand, but it is a #CMC and #CSC concept. It’s also something anyone who cares for advanced heart failure patients will encounter again and again.

The easiest way to explain downregulation is through the classic example of insulin resistance. Diabetics, due to the increased amount of glucose in their bloodstream, release more insulin. Over time this causes the insulin receptors on the surface of their liver cells to degrade. The degradation causes a decrease in the number of active receptors for the hormone. This mechanism is referred to as “downregulation.”

The pathophysiology of heart failure involves chronic exposure to high levels of circulating catecholamines. Consistent high blood serum levels of norepinephrine and epinephrine interact with the beta-1, beta-2, and alpha-1 receptor sites. The mechanism proceeds essentially as described in insulin resistance.

At the bedside, this evidences itself in the fact that our standard inotropic and vasoactive drips seem to be less effective, or require higher doses to take the standard effect. It also contributes to a high level of variability, depending on which receptor sites are downgraded and to what degree.

So, getting more specific, if your patient has beta-1 receptor site downregulation, it will look like this:


56-year-old male, PMH of nonischemic cardiomyopathy, s/p right heart cath and pulmonary-artery catheter placement, admitted to your ICU with the diagnosis of cardiogenic shock with a cardiac index of 1.8. Orders for dobutamine at 5-10 mcg/kg/min, titrate to keep CI greater than 2.1.


You admit your patient and do your assessment, initiating the dobutamine drip at 2.5 mcg/kg/min initially. After the patient’s nausea has subsided, you turn it up to 5 mcg/kg/min since his heart rate and blood pressure are stable, and you wait for another forty-five minutes. When you shoot your numbers, his cardiac index has dropped to 1.6.

You titrate the drip up to 7.5 mcg/kg/min and wait forty minutes. When you check numbers again, his index has dropped to 1.5. You increase the drip to 10 mcg/kg/min and recheck in thirty minutes because your patient…your patient looks worse. Sluggish capillary refill. Ashen nailbeds. Remains nauseated. Blood pressure has dropped to the 90s systolic, and your patient is more dyspneic than before. His index is now 1.3.

What’s happening? You are seeing the consequences of downregulation at work.

So what do you do? Turn the drip off. Yes, off.

Call the doctor, and using your best SBAR communication skills, tell him what happened as you uptitrated the drip. Suggest primacor, and possibly vasopressin if necessary, to maintain an adequate blood pressure. Why those drips, and why not norepinephrine? We will cover that in another episode.

dobutamine

Dobutamine is another inotropic agent we use in the ICU. It’s referred to as a catecholamine—our word for an organic compound that is released by the adrenal medulla during the fight-or-flight response—but dobutamine is synthetic. It’s actually a structural analogue of isoprenaline (Isuprel), and is administered as a racemic mixture (*important fact*).

This medication is administered primarily for patients in cardiogenic shock, but is also utilized heavily in the advanced heart failure population. It can be a home infusion for those patients. It’s also used for stress testing, interestingly enough. Make sure you hold the beta-blocker that morning.

Dobutamine acts on three different receptor sites on the surface of the myocardial cell. It directly stimulates the beta-1 receptors, the beta-2 receptors, and the alpha receptors. The fact it’s a racemic mixture ensures that the alpha activity of dobutamine is balanced. Because dobutamine (unlike dopamine) does not act on dopamine receptors, it doesn’t promote the release of norepinephrine. This means it won’t increase afterload despite its action on the sympathetic nervous system.

  • Beta-1 activity: agonist, increases contractility and heart rate
  • Beta-2 activity: agonist, resulting in vasodilation of blood vessels in skeletal muscle tissue, as well as dilating bronchioles
  • Alpha-1 activity: balanced agonist and antagonist activity via the (+) and (-) isomers

The effects of dobutamine are dose-dependent. The primary hemodynamic factor followed for titration of the drip is cardiac index. Standard dose range is 2.5-20 mcg/kg/min, and may be titrated by 2.5 mcgs every 15 minutes or so. Increase in heart rate is more marked starting around 12 mcg/kg/min.

Dobutamine increases myocardial metabolism and oxygen consumption. This mechanism results in worsened myocardial ischemia and angina may occur. Hence its use during stress testing…

Effects on other systems

Nausea and vomiting can be quite pronounced, as activation of the beta-2 receptor sites slows gastric motility. This effect can be worse in patients who are diabetic, as well as those who have received anesthetic medications.

Dobutamine stimulates glycogenolysis in skeletal muscle and gluconeogenesis in the liver, which raises blood sugar, as well as insulin secretion in the pancreas, to drive said serum glucose into the cell. If your patient is a diabetic, however…

It increases renin secretion from the kidney. This results in activation of the RAAS, which is a mechanism for hypertension.

It inhibits histamine release.

PRIMARY CONSIDERATIONS: Administration of dobutamine can contribute to down-regulation of the beta-receptors. It’s also proarrythmogenic, and can precipitate ventricular and supraventricular arrhythmias, as well as cause chest pain from angina.

dopamine

I’m not talking about the dopamine already present in your system that allows you to like things and enjoy life. I’m talking about the intravenous drip used as an inotrope in the intensive care unit.

Dopamine is a primitive drug.

I mean that literally. Also, it smells bad. Seriously. Open the bag and take a whiff–sulfur, yum.

Dopamine is the precursor to norepinephrine and epinephrine. It’s a neurotransmitter that acts centrally and peripherally, once metabolized, on the sympathetic nervous system. The sympathetic nervous system is that fight-or-flight response we all remember from the last time we did CPR.

We used dopamine all the time when I was growing up as a nurse. The doctors would write for “renal dose dopamine” and we knew that meant 1-3 mcg/kg/min, titrate to urine output greater than 50cc/hr.

We ran it in septic patients all the time, based on the theory it would protect the kidneys and the gut from the low perfusion state induced by sepsis. We would run it before levophed–back then, levophed was the last drug you reached for. We’d use it to give our fresh hearts a little kick to get them extubated and make their numbers look good, help them have adequate urine output without lasix.

So how does dopamine induce diuresis?

It causes “diuresis” by decreasing aldosterone secretion in the adrenal cortex. Aldosterone is what encourages your kidneys to retain sodium at the distal renal tubule. This causes water retention. If you block aldosterone, you prevent the retention of sodium and water. It also, at low doses, increases renal blood flow & GFR, promoting the excretion of sodium. As we know, H2O can’t think for itself but merely follows sodium every where it goes.

It also inhibits insulin secretion. This is one reason why patients (particularly type 2 diabetics or patients who have received anesthetic medications) on dopamine drips will have a higher serum glucose. We didn’t worry about it much back then, because all of our patients were on insulin drips anyway.

Dopamine has another strange effect: it affects the release of thyroid-stimulating hormone and inhibits prolactin release. Consider hypo- or hyperthyroidism if your patient begins exhibiting unexplained heart rate/blood pressure changes while on a dopamine drip that hasn’t been titrated.

The effects of dopamine are dose-dependent.

  • Low dose effects (1-2 mcg/kg/min) include vasodilation of the renal and mesenteric blood vessels by acting on the dopaminergic 1 & 2 receptors.
  • Moderate dose effects (2-10 mcg/kg/min) are primarily beta-1, enhancing contractility and heart rate.
  • Higher doses than 10 mcg/kg/min act overwhelmingly on the alpha-1 receptors, causing vasoconstriction.

PRIMARY CONSIDERATIONS: causes tachyarrhythmias, particularly in hypovolemic patients. At high doses, profound vasoconstriction will occur, which can cause necrosis, gangrene, and even compartment syndrome if infused through a peripheral IV.

#CCRN #CSC #CMC #AACN