We can’t do without the mean arterial pressure
The overall concept of mean arterial pressure or MAP for short is extremely important from the point of understanding of even ordinary people, who don’t have medical education. That’s because the vast majority of people associate the arterial pressure exactly with well-being. Respectively, any fluctuation in this crucial parameter can lead to emergency conditions.
In simple terms, the mean arterial pressure is just an average blood pressure recorded in a person. By the way, an alternative name of the mean arterial pressure is average arterial pressure.
As usual, the mean arterial pressure ranges from 70 to 110 mmHg. Obviously, a minimum mean arterial pressure of 60 mmHg is urgently required for proper blood flow to body organs, including the kidneys, coronary arteries, not to mention the brain. Therefore, the measure of the mean arterial pressure serves a health indicator of these organs.
These are the critical conditions, where the MAP should be monitored:
- Cardiac patients on vasodilator infusion.
- Head-injury patients.
- Patients with the septic shock.
- Patients with dissecting abdominal aneurysm.
Besides the categories of patients mentioned above, the mean arterial pressure control is actual for any person. Regular MAP checkups are crucial when it comes to evaluating the overall state of our body systems. By the way, if the MAP falls below the optimal range, the patient’s body tissues will be damaged.
The very essence of the MAP
In order to properly understand the whole nature of the MAP, we should have a closer look at blood pressure in general. Have you ever thought what blood pressure actually measures? Let’s make a thrilling trip into the depths of the human body.
The heart does a very important job inside of us. From the first day of our life or even before it, in the womb, till death, the heart keeps pumping huge amounts of blood throughout the body. Our blood is used to carrying nutrients and oxygen to the muscles as well as other organs.
Every day the heart pumps the blood through numerous arteries to the rest of our organism. Sure, the blood is getting back via veins to the heart. The process repeats over and over. The heart keeps sending the blood to the lungs to supply it with the oxygen and then send the oxygenated blood back to the body.
Before we proceed with blood pressure, we should make it clear that the blood is a liquid, and like all other liquids, it’s also used to exerting pressure on the vessels, through which it’s pumped. In this case, we’re talking about the blood vessels, of course. Ideally, we should call this pressure «hydrostatic pressure,» as it’s a typical properly of any liquid to exert pressure on anything on its way. That’s from the point of view of physics. Now, let’s get back to the medical point of view. The blood is pumped through the blood vessels. On its way, the blood hits against the walls of the blood vessels, thus exerting a definite pressure on them. That’s what we call «blood pressure».
When your blood pressure is taken by the doctor, he or she evaluates how much pressure is exerted on your blood vessels. It’s an extremely important measure, when it comes to assessing your health.
When the blood leaves the heart, it heads for the aorta, a large blood vessel. In this case, we’re mean exactly the descending aorta, as the blood goes down to the rest of the body. Keep in mind, the closer you’re to your heart, the more pressure you’ll feel. So, the farther the blood goes from the heart, the lower the pressure becomes.
When the blood leaves the heart, it does it via the aorta. From the aorta it goes through a series of smaller vessels, dubbed arteries. The next stage includes arterioles and then capillaries. From the capillaries the blood goes through the venules and then finds itself in the veins. Once the blood enters the venules and the veins, it starts its way back to the heart.
When your heart contracts, you have exactly ventricular contraction. It gives huge amounts of pressure. It’s because ventricles surpass the atria in terms of size. So, when they contract, sending your blood to the rest of your body, you’re on the way towards the systolic pressure, which is the greatest one. When the atria contracts, you get a liny bump. The entire process continues over and over again, as your heart beats.
Ok, we go away from the arteries and head for the arterioles. You might have already guessed that it’s a downward direction. On its way down, the blood faces a series of small fluctuations. As we approach the capillaries, blood pressure keeps going down and that’s quite logical, as we’re moving from the heart downward.
So, now you know for sure that we have different pressure close to the heart and far away from it. Let’s take a look at a common situation, when the doctor takes your blood pressure. You hear that you’re pressure is normal, and it’s…let’s assume 120/80, for instance. Of course, the bigger number is the systolic pressure, while the lowest value stands for the diastolic pressure. The systolic pressures is associated with the systole contraction, while the relaxation phase gives us the diastolic pressure.
Finally, we’ve approached to the number one point of our conversation, which is the mean arterial pressure. As the name suggests, we’re dealing with something average. Yes, we mean exactly the average pressure in the human arteries. Just forget about the fluctuations and focus on the average value.
In order to calculate the MAP, one needs to multiply cardiac output by peripheral resistance. Cardiac output can be defined as the amount of blood pumped by the human heart every minute, while peripheral resistance is the opposition to the blood flow. As for this resistance, we mean exactly friction between the walls of the blood vessels and the blood.
The History of blood pressure monitoring
Ok, we’ve just gotten familiar with the MAP. Do you want to know how humans discovered it? Obviously, they would have never done it without a well-organized system of blood monitoring. To get closer to the MAP, one needs to learn how to measure blood pressure in general. Let’s trace the way of a scientific thought.
Though blood pressure machines are widespread today, this crucial diagnostic instrument only arose in the late 19th century. It took more than two hundred years to develop this must-have medical stuff.
The measurement of blood pressure started in 1733, when bright Stephen Hales firstly demonstrated that the whole amount of pressure made by the human heart could be measured by means of the displacement of blood. For this purpose, Stephen used a horse. He inserted a pipe right into an artery and attached it to a glass tube, into which the blood would go and respectively could be measured. To our great regret, Stephen’s method had nothing to do with human being.
Then, Jean Leonard Marie Poiseuille invented a mercury manometer in 1828. The scientist dubbed this stuff the haemodynamometer. The first manometers arose in the 17th century. They were utilized to measure pressure. As for their appearance, they were U-shaped tubes with mercury or another liquid. During his experiment, Poiseuille inserted a hollow tube with retractable inner core into an artery and then attached to a manometer on the other end. The blood movement displaced mercury in the manometer. In this case, blood pressure was identified by simply measuring the overall amount of mercury displacement.
Carl Ludwig dared to improve this technique in 1847. He created the kymograph, which could represent data graphically. The entire system included the manometer attached to a float pen, which was connected to a revolving drum. The mercury moved with every pulse, the pen moved, thus making a wave chart of the blood movement on the drum.
Both those methods mentioned above shared one serious downside. They were bound to use invasive measures, so blood pressure couldn’t be measured without tubes inserted into the patient’s arteries. However, in 1855 non-invasive methods were invented. They were built around the idea of evaluating the counter-pressure required to stop circulation.
In 1860 Etienne Jules Marey managed to measure blood pressure by simply enclosing the arm in a water-filled glass chamber and gradually increasing the water pressure circulation stops. At this point the pressure was identified as the systolic pressure.
Then, in 1881, Samuel Siegfried Karl Ritter von Basch managed to improve the previous method. For this purpose Samuel placed a rubber bag around a manometer bulb and then started inflating the bag with water. As the water pressure grew, the mercury was displaced in the manometer, thus ensuring the measurement of blood pressure. The bag was positioned over the distal pulse and then inflated until the pulse couldn’t be recorded. The pressure at this point was the systolic pressure. Later water was replaced by air in the compression bag. It was Pierre Potain, who made this improvement in 1889.
In 1896 blood pressure measurement evolved again. It became more similar to current methods. Scipione Riva-Rocci appeared to be the first, who used a cuff placed around the patient’s arm and inflated with the help of an attached bulb. The doctor should keep increasing the pressure in the cuff, until the radial pulse turned unavailable. Right at this moment the doctor released the cuff’s pressure. In that case, the systolic pressure was the pressure at which the radial pulse came back.
Well, in the beginning of the 20th century, it was quite real to measure systolic pressure, but diastolic pressure still remained beyond the reach. Fortunately, in 1905 Nikolai Korotkoff firstly introduced the method to measure this kind of blood pressure. Korotkoff simply identified certain sounds, lately dubbed Korotkof sounds, produced by the blood in the arteries. He associated those sounds with changes in pressure. Today, this method is widely used all over the world. These sounds are what the doctor is listening when holding his stethoscope to the patient’s inner elbow. When the blood pressure cuff gets inflated and the blood flow is colluded, there aren’t any sounds. Once systolic pressure is reached the first sound is clearly heard. As the pressure in the cuff is released, the sounds disappear. The pressure reading during this short moment is the patient’s diastolic pressure.
We should be grateful that since ancient times researchers and physicians worked hard to provide us with up-to-date pressure measurement methods. Now every person has an excellent opportunity to have his or her blood pressure tested. Armed with this precious knowledge, you can avert many health disorders on time and make your lifestyle much safer.
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