Arterial Lines- What Are They and Why Are They Used
There are times after a person in admitted to a hospital that a different type of measuring device is needed in order to obtain a more accurate blood pressure. You are all familiar with the standard blood pressure cuff, where the cuff is wrapped around the arm, inflated, and the doctor or nurse places a stethoscope on the arm and the blood pressure is obtained, but an arterial line works in a different manner.
When a standard blood pressure cuff is used, the measurements are measured in mm/hg. This means the measurement is in millimeters of mercury. As the cuff is inflated it creates pressure equivalent to the amount of pressure needed to make the mercury rise. So for example, if your blood pressure is 120/75, then it has taken this amount of pressure to raise the mercury 120 mm, ( the top number ), then the bottom number represents the least amount of pressure need. So what does this mean? When the stethoscope is placed on the arm, it is resting over the main artery in the arm. The cuff is inflated to about 200 mm/hg then slowly deflated. when the doctor or nurse first starts to hear the thump, this is the top number and when they stop hearing the thump, this is the bottom number. So to understand this a little deeper, the top number is the amount of pressure within the artery on the beat , and the bottom number is the amount of pressure in between the beats.
When a person is requiring the use of an arterial line, also known as an a-line, they are in fairly bad shape. A-lines are used in situations in which a standard pressure is either not obtainable, not may it be accurate. An example of why it may not be accurate may be in a case of PVD ( Peripheral Vascular Disease ) or often these lines are inserted while a person is in the operating room, in which due to the deliberate lowering of temperature, a standard cuff is in-accurate because of decreased blood flow. An arterial line works on a slightly different principle. It works on the basis of fluid or hydro dynamics. To insert an a-line, the physician positions the arm and wrist in such a way as to allow for the best and easiest access to the radial artery in the wrist. A long needle is inserted into the artery about 12-13 cm. As the needle is withdrawn, the catheter that remained is connected to a transducer, which is connected to a pressurized bag of heparin. This pressure bag is inflated to about 300mm/hg. This is a standard pressure rate within a main artery. Once the line is secured and connected to the transducer and measurement cables, it reads the pressure. This pressure also reads in mm/hg.
As the heart beats, it creates pressure within the artery and the blood flow around the catheter put pressure with the lumen ( narrow opening ) and it creates a pressure variant within the pressurized heparin, giving a signal to the transducer and displaying the differing readings to the monitor. It is normal for these numbers to fluctuate with every heart beat. This method is far more accurate than a standard blood pressure cuff.
So why is heparin used for this technique?. Heparin is a medication in the anti-coagulant class. It prevents blood from clotting by inhibiting vitamin k synthesis. vitamin k is a primary factor in blood clotting. Heparin is used to keep the arterial line from clotting off. Just enough heparin is used to prevent the clots. whenever there is a break in the integrity of the skin, it bleeds, vitamin k stimulated the clotting cascade in the form of fibrin and collagen. These two substances close and seal the wound, fairly rapidly.
Because of the nature of arterial lines, and their related use, they are most often found in the operating rooms and ICU's. Normally the lines are kept in 2-5 days, depending on the seriousness of the patient condition. as the patient improves, these lines are typically d/c'd ( discontinued ), and normal blood pressure monitoring can resume.