Types of Shock and Pressors

While on my Anesthesiology rotation, I notice that we always have different types of pressors handy to control the pt's MAP during surgery. When the patient had a low MAP but a higher HR, we would give the patient phenylephrine because it acts only on the alpha receptors and would therefore, not cause tachycardia. If the patient had a low MAP and a low HR, we would give Levophed (norepinephrine) because it has a side affect of increasing the HR. Choosing the correct pressor reminded me about my ICU rotation way back in the fall and I wanted to brush up on the different types of shock and the medications used to treat. There are four categories of shock: Hypovolemic, cardiogenic, distributive and combined. To help distinguish between the different types, you look at a few different measurements: SVR (systemic vascular resistance meaning the afterload), PCWP (Pulmonary Capillary Wedge Pressure meaning the preload) and CO (cardiac output).

Hypovolemic

Induced when a cause for decreased preload to the heart occurs which causes a decrease in cardiac output. Cardiac output = HR x SV. When you have a decrease in preload, the SV (stroke volume) will go down thus, decreasing the CO. The body reacts to the decreased CO by increasing the SVR to compensate for the diminished perfusion to vital organs. The PCWP will, of course, be low. 

Causes for hypovolemic shock can be divided into 2 categories

1. Hemorrhage-induced – trauma, GI bleed, ruptured hematoma, hemorrhagic pancreatitis, fractures, or a ruptured aneurysm

2. Fluid loss-induced – diarrhea, vomiting, heat stroke, insensible losses such as burns and "third spacing". Third-space losses are common postoperatively and in patients who have intestinal obstruction, pancreatitis, or cirrhosis. 

Treatment - FLUIDS!! There are three classes of fluids: crystalloids, colloids and blood products. If the patient is in hypovolemic shock secondary to blood loss, you will obviously give PRBCs. Crystalloids are usually used before colloids as they are less expensive and effective. 

Cardiogenic

Cardiogenic shock occurs when there is cardiac pump failure.When the cardiac pump fails the CO decreases and again, the SVR increases in an effort to compensate for the diminished perfusion to the vital organs. This looks very similar to hypovolemic shock except in this case the PCWP is increased because there is plenty of fluid but it is stopping up in the heart causing an increase in pressure in the pulmonary artery. 

Causes can be divided into 4 categories: myopathic, arrhythmic, mechanical, or extracardiac

Cardiomyopathies- In a sum, all of these causes are due to pathology of the cardiac muscle severe enough to affect the pumping ability of the heart. Severe MIs, dilated cardiomyopathies, stunned myocardium following prolonged ischemia or cardiopulmonary bypass and myocarditis

Arrhythmias - Both atrial and ventricular arrhythmias can produce cardiogenic shock. When the atrium are not contracting correctly, there will be improper filling of the ventricles and therefore a decreased CO. If the ventricles are not contracting correctly, then there will be a poor stroke volume which will also decrease the CO. Ventricular fibrillation will completely abolish CO while other arrythmias of the ventricles will decrease the CO. 

Mechanical - Causes include valvular defects, ventricular septal defects or rupture, atrial myxomas, and a ruptured ventricular free wall aneurysm. An atrial myxoma can reduce ventricular filling by obstructing the flow of blood. A ruptured left ventricular free wall aneurysm can produce pump failure as well.

Extracardiac – Extracardiac (obstructive) causes of cardiogenic shock include anything that would cause strain on the heart and decrease cardiac output such as a pulmonary embolism, tension pneumothorax, severe constrictive pericarditis and tamponade.

Treamtent - I am not going to go into detail here. You could guess what the treatment may be for many of these (fix the underlying problem). I do want to talk about pressors and what type would be appropriate for cardiogenic shock. 

Dobutamine - has a greater action on B1 than B2. Acting on the B1 receptors will improve cardiac output by increasing pumping function of the heart. 

Norephinephrine - acts on alpha and beta 1 receptors but alpha action is > than beta. Will vasoconstrict and improve pumping function of the heart. 

Epinephrine - also acts on alpha and beta receptors but when compared to norepinephrine, alpha action = beta action. Thus, it improves cardiac contractility even more than norepinephrine.

Phosphodiesterase Inhibitors - will cause vasoconstriction

Distributive

Distributive (aka vasodilatory) shock is a consequence of severely decreased SVR (will see hypotension). In this type of shock, the cardiac output is increased (will see tachycardia) to compensate for the decreased SVR. Pt's skin will feel warm due to the decreased SVR.

Causes - anything that will cause vasodilation of the blood vessels such as sepsis, toxic shock syndrome, SIRS, Addison's crisis, drug toxins and anaphylaxis.

Treatment - the main problem here is systemic vasodilation. You will want to have a pressor that acts strongly on the alpha receptors. You can use epinephrine, norepinephrine or phenylephrine. 

Combination

Any of the above types of shocks can coexsist. I recall a pt I had in the ICU who had sepsis as well as necrotizing pancreatitis so he had a combination of hypovolemic shock and distributive shock.