Diagnostic Tests

On this page we will try to explain in Non-Technical terms, about two very useful and important diagnostic tests. You may have had one or both of them performed in the past.

Before we get into the discussion of these tests, I would like to encourage you to ask questions. You may e-mail a question to us at rtoldguy@mtnisp.com in the subject line use.... SRC Question. We will try through the SRC Staff's own knowledge and our other resources to answer your question. Please allow one week for your answer to be e-mailed to you.

Arterial Blood Gases - Arterial Blood Gases are just what the term implies. The Blood for the test is drawn from an artery. The arteries routinely used are the radial artery found in your wrist where your pulse is normally taken. The other artery is the brachial artery found in your arm at the bend of your arm. The bracial artery is the one heard by stethescope or palpated by an automated machine when taking your blood pressure.

Arterial blood must be used for accurate blood gas measurement. Arterial blood is the blood going from the heart to take oxygen out to the body and pick up carbon dioxide for its' return trip through the veins. Venous blood is drawn for all other laboratory blood work. The arterial blood is drawn using a small needle attached to a small syringe or to capillary tubes (very small glass tubes). Blood Gas Machines usually perform the test on a small amount of blood. The blood is kept from clotting by heparin (a drug to prevent blood from clotting) coating the inside of the syringe or capillary tube, for the test the blood must remain fluid and as whole blood.

Now let us talk about Blood Gases. Three results are measured directly pH - PCO2 - PO2 these are the primary values and the ones we will talk about.

pH - stands for potential of hydrogen and is used to measure the acidity or alkalinity of a substance. Pure water has a pH of 7.0 which is termed neutral. A substance with a pH less than 7 is considered to be acid and any substance with a pH greater than 7 is considered to be alkaline. The normal range for Arterial Blood ph is 7.35 - 7.45 which means that arterial blood is slightly alkaline. When an arterial blood pH is less than 7.35 a condition of acidosis exists and a pH above 7.45 then a condition of alkalosis exists.

PCO2 - stands for partial pressure of the gas Carbon Dioxide dissolved in the arterial blood. The normal range for PCO2 is 35 - 45 mmhg (millimeters of mercury) or the measurement for barometric pressure, which you see on the weather reports as inches of mercury.

PO2 - stands for partial pressure of oxygen dissolved in the blood. The normal range
is 80 - 100 mmhg.

pH indicates the bodies acid base balance. Your body being the marvelous mechanism it is tries to maintain this normal balance of pH.

PCO2 is an indication of how well you are getting air in and out of your lungs, termed Ventilation.

PO2 on the other hand is an indication of how well the oxygen is passing out of your lungs and into the blood, termed respiration or perfusion.

A rise in PCO2 above normal causes the pH to go down below 7.35 termed Respiratory Acidosis. A drop in PCO2 below normal causes a rise in pH above 7.45 termed Respiratory Alkalosis. If the pH adjusts in this manner then the condition would be Acute or of recent or short term duration. If the PCO2 is high or low and the pH is within the normal range, then the condition would be chronic or of duration long enough for the body to adjust. Remember, your body tries to maintain the pH balance.

Pulse Oximetry Pulse Oximetry is a non-invasive method of measuring the amount of oxygen dissolved in the blood. It was found that Blood Hemoglobin (the red blood cells) absorb light at different levels according to the amount of oxygen carried or absorbed by the hemoglobin. A sensor that clips to or is wrapped around a finger or toe has a light emitting diode on one side and a light receiver on the other side. The instrument knows how much light is emitted and how much light is absorbed the machine calculates and gives a reading of oxygen saturation in a percentage 95-100% being normal.

Pulmonary Function Tests Now we will discuss PFT's Pulmonary Function Tests. You may hear people refer to a PFT as a breathing test, and it is. The PFT actually tests and evaluates your breathing mechanics.

The PFT measures volumes of air and movement of air in timed studies, to evaluate and determine a breathing problem and its' severity. Now let us look at some of the things tested.

FVC (Forced Vital Capacity)... is the maximum amount of air that can be exhaled forcefully after taking in as deep a breath as possible. The result of this tested breath is compared to normals collected by testing a large numbe of individuals with out a history of lung disease. All ages and sizes of adults were tested. If your FVC is lower than the predicted normal then a restrictive problem is present. This means that a problem exists that prevents you from taking in and exhaling the amount of air you should be able to exhale.

When the FVC test is performed the flow of air coming out of the lungs is also looked at in relation to speed and time. The Volumes in this part of the test are FEV1 (Forced Expiratory Volume for one second) also FEV3 and sometimes FEV2 (Forced expiratory volume for 3 seconds and 2 seconds respectfully). MMEF (Maximum Mid-Expiratory Flow) Each of these values are calculated by the Pulmonary Function machine you are breathing into.

Now what do these timed air flow and speed of flow studies show. FEV1 shows how well the air moves out of the large airways. At FEV3 at least 90% of the FVC should be out. MMEF is the measurement of the middle part of the FVC. The first 25% and the last 25% of the FVC are not taken into account. A lower than normal result in any of the timed flow studies is indicative of an obstructive problem.

Obstructive problems are just what the term implies, something is causing an obstruction to the air leaving the lungs. Chronic Bronchitis, Emphysema also termed COPD (Chronic Obstructive Pulmonary Disease) or COLD (Chronic Obstructive Lung Disease) or the newest term CLD (Chronic Lung Disease). This obstruction could be excessive secretions or mucous in the airways as in Chronic Bronchitis. Airway constriction or collapse as air leaves the airways as in COPD is another cause. Two other tests we will briefly discuss done when a comprehensive PFT is ordered.

RV (Residual Volume) Residual Volume is the amount of air left in the lungs after the FVC is performed. Your lungs do not completely empty after the FVC. The Residual Volume is the air left in the lungs to keep them from collapsing. In Chronic Pulmonary Disease the Residual Volume increases above normal and although the FVC is below normal the lungs are stretched from the disease, and the total amount of air in the lungs is increased. The Residual Volume is what increases. Stretching causes the lungs to lose their elasticity and causes the airway collapse as air leaves the lungs an obstructive problem. The increase in the Residual Volume increases and causes an increase in CO2 This rise in CO2 causes a rise in PCO2 that is seen in the ABG Test.

A diffusion test can also be performed which is a measurement of a gas moving out of the lungs. Remember that PO2 in the ABG test is also a measurement of the way gas diffuses out of the lungs except the ABG measures the oxygen diffusion.

We hope this has explained enough about the tests we have discussed so that you are better informed.