Vascular Resistance – Determinants of Blood Pressure (Nursing)

00:01 Now let's go on to our vascular side of our four parts of blood pressure.

00:05 Remember, we have vascular resistance and fluid volume status.

00:09 With vascular resistance, we're measuring the afterload or the resistance to the blood flow coming out of the left ventricle.

00:16 This is measured by systemic vascular resistance on your bedside monitor.

00:20 Normal values is 800-1200 dynes, when you go towards 800, that's vasodilating.

00:26 And when you're going higher up to 1200, that's vasoconstriction.

00:31 Now, when you're increasing your systemic vascular resistance, so you're going higher up towards 1200 dynes.

00:37 This will increase your MAP or it should increase your MAP.

00:41 But also increasing your SVR could decrease your cardiac output.

00:45 Let's dive into this a little bit more so we can understand it.

00:50 Now, when you have a normal size blood vessel, and you have normal blood volume going through it, you're going to have a normal blood pressure.

00:58 But if we try to increase blood pressure by vasoconstricting, we narrow that blood vessel down, we have a normal amount of blood still going through it, we get a higher blood pressure at the end of it.

01:10 I call this the garden hose theory.

01:13 So think of a garden hose.

01:14 When you were a kid, you'd have a garden hose, you turn the faucet on, what would you do to spray everybody? You would put your thumb over it.

01:22 Keep that visual in your head.

01:24 Now, let's say that the garden hose is your aorta.

01:29 The faucet is your heart.

01:31 And the water going through it is the blood flow.

01:34 Now, when you turn on the faucet all the way, and you just have the hose, you're not putting your thumb or anything over it.

01:41 What's happening? You're getting a lot of blood volume out but the pressure is at a moderate amount.

01:47 Now, if you want to increase that blood pressure, you want to increase the pressure of the water, you put your thumb over it.

01:53 What you're doing is effectively decreasing that size of that aorta which is an increasing the pressure coming up.

02:03 But how much water is coming out? Do we have more water, the same amount of water or less water? Well, when you're putting your thumb over it, you actually increased pressure, but you decrease the amount of cardiac output, you decrease the amount of blood flow coming out.

02:19 That's how systemic vascular resistance can reduce cardiac output.

02:24 Now your body can overcome this.

02:26 So how does your body overcome vasoconstriction so that we get the same amount of cardiac output? Well, it has to increase our heart rate and our contractility so that we get more cardiac output to overcome the vasoconstriction.

02:42 Let me show you this diagram here.

02:44 We have a normal amount of blood flow coming out of a normal vessel.

02:48 When we vasoconstrict, we have less blood flow coming out of it.

02:52 So the heart needs to increase contractility and increase rate so that we get more cardiac output and more blood flow coming through a smaller vessel.