Blood Oxygen – Physiology of the Heart

00:01 Law of the heart: blood pressure equals heart rate… equals cardiac output times resistance.

00:01 Now, let’s talk a little bit about the energy metabolism of the heart. The heart’s just like a gasoline motor. It has to have fuel in order to contract just like the gasoline motor has to have gasoline in order to function. And you need to balance, the amount of energy that’s spent in causing the heart muscle to contract depending on the amount of work it has to do versus the amount of fuel that goes into it. And we usually do this by measuring something called the oxygen consumption of the myocardium that tells us how much fuel the heart is burning because you need oxygen to burn the fuel in the heart muscle cells and these two have to be balanced. You can see why. If the demand is more than the supply, the heart muscle’s going to run out of energy and it’s going to be hurting, so that, in fact, that’s exactly what happens in ischemic heart disease with atherosclerosis.

00:58 When there is narrowing in the blood vessels, it’s like narrowing in the fuel lines in your car. You have to go up a hill and suddenly, you step on the gas and guess what? The car doesn’t respond because not enough fuel is getting into the engine. Well, the same thing happens with the heart. You suddenly decide to run up a hill. If there is a restriction in the supply of blood to the heart muscle, it can’t function adequately. You have an imbalance between supply and demand- oxygen consumption and oxygen demand, and that results in a decrease in heart function just like the decrease in the motor when you don’t get enough gasoline into it when you are trying to go up a hill. We have calculated these numbers, we can calculate them in the cath lab. We know what normal values are, we can measure the amount of oxygen in the artery, we can measure the amount of oxygen in the veins. We can calculate all of these kinds of numbers and they are often done on critically ill patients or patients in the catheterization laboratory to give us a sense of… of how well the heart is functioning, given the amount of blood flow that it is receiving. We often do a very simple test in most patients. You can do this as an outpatient or inpatient.

02:09 This is a device that measures the oxygen saturation just with your fingertip.

02:14 You put your fingertip in a little device and it tells you how well saturated the blood is, that’s being pumped around the body. Now, we would like it most times for people to have oxygen saturations above 90. Of course, if you are on the top of Mount Everest without an oxygen mask, your oxygen tension is going to be much lower than 90. It would probably even get down into the 70s, which could be dangerous unless you are really a trained mountaineer and even in a trained mountaineer, sometimes, there is a little brain damage because of the very, very low oxygen at very, very high altitude. But, most normal people without heart or lung disease will have values well in the 90% saturation with oxygen measured at their fingertip and that’s often a measure of sort of integrated heart and lung function.

03:04 Is the oxygenation adequate? And often, for example, in my clinic, every time I see a patient, we measure that to make sure that the oxygen saturation in the periphery is adequate. These devices are very easily obtained and are in pretty much all Cardiology Clinics and inpatients. One other piece of equipment I wanted to bring to your attention: This is the balloon catheter that we use to measure the pressures on the right side of the heart.

03:34 the balloon is out at the tip, and as I said it’s like a sail on a sailboat. The circulation pulls the catheter through the right heart chambers and out into the pulmonary artery.

03:43 There is a number of extra little lumen so we can squirt material, drugs sometimes or just salt water to increase the blood volume in patients where we think the blood volume has become decreased. Finally, the last two slides are fairly complicated.

04:01 They will require you to sort of work your way through them and think about them.

04:06 They show you the integration of the cardiovascular system. The resistance in the periphery, the pumping chambers, the two ventricles and how they are all related. And in a normal person, these things are exquisitely balanced, so that the cardiac output, the peripheral resistance in the lung and the periphery all are balanced so that there is a normal cardiac output.

04:28 Let me give you an example of what happens. You start to exercise. Suddenly, with the exercise, the resistance in the peripheral vessels goes down, afterload goes down, the heart pumps more blood and you will increase your cardiac output. Trained athletes can increase their cardiac output by 4 and 5 fold. Most of us can usually do 2 and 3 fold without too much trouble. And a final slide, again shows the interrelationship of the resistance.

04:57 For example, you can see SVR that’s systemic vascular resistance. You see mean arterial pressure at the top, you see cardiac output and heart rate determined by stroke volume and heart rate. You see the whole circulatory system, all in relation to the various parameters that we can measure. In normal people, all of these things are exquisitely balanced both by the central nervous system which is constantly monitoring blood pressure and… and flow through the circulatory system, and also with activity from the kidney which puts out hormones that can increase or decrease the blood pressure. And we are going to talk a lot more of that as we get into various ways to diagnose and manage heart disease.

05:41 Thank you very much for going along with me in this series of discussions relating to the function of the heart. I know it’s a lot of material in a very short time.

05:54 Those of you who want to dig into it a little deeper, there is a number of references that will give you a lot more information than I can give you in the short time that I have with you for this topic. Thank you for being here today.