Tropes and Pressors - part 1 of 3

Shock  is a state of oxygen deficit (hypoxia) to tissues; all tissues – and of particular concern, vital organs (heart, brain, lungs, kidneys).  Shock is global cellular hypoxia.  Whilst shock can be caused by a lack of oxygen in the blood itself, most shock is actually a drop in perfusing pressure of what blood is available.  Shock video

MAP is Mean Arterial Pressure.  It is, in its most basic of descriptors the average blood pressure (mean) that perfuses vital organs.  A low MAP will therefore be consistent with shock, irrespective of how oxygenated the blood is.  Magic MAP number to aim for is 60 mmHg.  Below 60, and we struggle to perfuse our Kidneys.

MAP is determined by two things - Cardiac output, and Systemic vascular resistance.

Cardiac output is the volume of blood you pump out in any given minute (normally 4200-7000ml).  It is made up of how much you heart pumps in one beat (Stroke volume (SV)) x the number of times/in that you beat (Heart Rate (HR)).  

The maths looks like this: CO = SV x HR. MAP video

Stroke Volume - Your ventricles are a muscular bag of blood.  When they contract (systole), blood is ejected in a volume called a stroke volume (SV).  We don’t eject all of our blood with every contraction, infact your heart fills with about 100ml, and pumps out 70ml. 

This stroke volume represents 70% of the ventricle’s filling volume.  That percentage (70%) is called an ejection fraction (EF). 

Stroke volume (SV) is determined by a few factors: 

* strength of your heart beat (Inotropy)
* how much blood was in your heart (Preload)
* the pressure inside the artery that your heart is pumping into (Systemic Vascular Resistance – also called  (Afterload)
* the stiffness of the ventricle (Compliance)

Cardiac output and stroke volume video

Inotropy - This is a concept that refers to the strength of a heart’s contraction. 

So what if I could give you a drug that increased the force of your contraction?  Then you’d fill with 100ml, but pump out a greater SV by increasing your EF.  That drug is therefore referred to as a positive Inotrope.

Positive Inotropes are hormones or drugs that increase the strength (force) of the heart contraction (Inotropy).   Typical Inotropes include Dopamine, digoxin, adrenaline, dobutamine and others; we will discuss in detail in Part 2.

Loss of inotropy occurs after infaction, ischaemia, or with an aging stiff heart.  Collectively this is called Heart Failure.

Preload - Fundamentally, preload is about the filling and the stretching of your ventricles.  Passively the left ventricle fills up to about 70 ml, then the atria contracts squeezing (pushing ) in another 25-30 ml.  Like, you’re your suit case is full, but some muppet says, “hey Dad, can you fit my jacket in?” so you are there squeezing a jacket into an already full case.  The case isn’t full it's now preloaded.   It is stretched and bulging.  Preload in the heart is determined by a couple of factors.  The force of an atrial contraction (lost in AF or flutter), and the volume of blood returning to the left heart from the lungs.  So, atrial arrythmias, and hypovolaemia can dramatically lead to a reduced SV, CO, MAP and subsequently shock. 

Systemic Vascular Resistance (Afterload) - As a pipe carrying fluid is compressed (narrowed), the pressure inside the pipe increases. Thus, squeezing blood through a narrower artery leads to increased in pressure.  This is called systemic vascular resistance (SVR).  Increasing the SVR leads to increased blood pressure, mean arterial pressure (MAP) and increased perfusion to organs.  

Vasopressors are natural hormones, or drugs that cause an increase vasoconstriction.  Commonly these are just referred to as “pressors”.  Common drugs that fall into these categories include Noradrenaline(norepinephrine), Metaraminol, phenylephrine, vasopressin, and good old adrenaline