Hemodynamics Explained for ICU Nurses

Okay, foundational topic here.

Let's start with the difference between pressure and flow.

So imagine that your patient's MAP is 65.

That pressure may be acceptable, but it does not tell you how the patient created that number.

One patient may have a strong cardiac output, very relaxed blood vessels.

Another one may have a weak cardiac output while the blood vessels are tightly constriction.

They can land with the same MAP with completely different physiology.

That's why we never just stop at the blood pressure, right? We look at whether the blood is moving forward and whether that blood is carrying enough oxygen to the tissues.

Cardiac output is the amount of blood that the heart pumps in one minute.

It equals heart rate multiplied by stroke volume.

Stroke volume is the amount of blood ejected with each beat.

So if the heart rate is 100 and each beat ejects, say, 70 milliliters, the cardiac output is about 7 liters per minute.

If that same patient's stroke volume falls to 35 milliliters, the cardiac output falls to about 3.5 liters per minute, even though the heart rate didn't change.

Now let's talk about what changes stroke volume.

The big pieces are preload, afterload, continuity, and compliance.

Preload is the amount of myocardial fiber stretch at the end of filling, right before that ventricle contracts.

At the bedside, think about how much blood is returning to the ventricle and how that ventricle handles that incoming volume.

So picture a patient with a large GI bleed.

So venous return is going to fall, the ventricle fills with less blood, and the myocardial fibers start the next beat with less stretch.

That patient has a low preload.

Now picture a patient with severe right ventricular failure.

The right ventricle may be full, dilated, and struggling to eject.

The right-sided pressure may be high, but giving more fluid may only increase the backup into the venous system.

A high filling pressure is not the same thing as usable, useful preload.

So that brings us to right atrial pressure, which you usually see at the bedside as CVP, right? Right atrial pressure is the pressure inside the right atrium.