24 INTRODUCTION TO ACID-BASE ABNORMALITIES
1 What four basic parameters are typically measured on a routine blood gas analysis?
2 How are the four basic parameters determined?
The pH, PaO2, and Pco2 are all directly measured using electrodes specific for the desired parameter. [HCO3−] is calculated based on the relationship between pH and PCO2. Many other calculated values might be reported, depending on the laboratory and the instrument used.
3 Is arterial or venous blood preferred for blood gas analysis?
Analyzing arterial or venous blood depends on the information the clinician is seeking. Arterial blood is needed for a meaningful interpretation of PO2. The arterial PO2 (PaO2) allows evaluation of the oxygenation of arterial blood, dependent only on respiratory function. Venous PO2 is affected by tissue utilization of oxygen as well as respiratory function. Thus, if an animal is in shock and has slow tissue perfusion, venous PO2 may be greatly decreased despite a normal PaO2. Similarly, an animal that is severely anemic has decreased oxygen-carrying capacity. This animal has a normal PaO2, but as blood passes through the tissue, oxygen is depleted and venous PO2 may be greatly reduced.
Venous blood is more easily collected and is acceptable for evaluation of pH, bicarbonate, and PCO2 concentrations. Because of changes similar to those described for PO2, measurement of pH, PCO2, and bicarbonate in venous blood may give a more accurate reflection of the acid base status of the tissues than do arterial samples. This is particularly true in situations such as cardiac arrest.
4 What is the proper method for collecting and handling samples for blood gas analysis?
Samples are collected in heparinized syringes. Heparin (1000 units/ml) is drawn into a 3-cc syringe and then expelled back into the bottle. This leaves a small amount of heparin coating the syringe barrel and in the dead space of the syringe. This small amount of heparin is sufficient to anticoagulate a 3-ml whole-blood sample. Heparin solution is acidic and will falsely lower pH and HCO3− if too much heparin is left in the syringe before collecting the sample.
Once the sample (arterial or venous) is drawn into the syringe, any air bubbles are removed, and the syringe is capped by sticking the needle into a rubber stopper to prevent equilibration with room air. The sample should be immediately analyzed or delivered to the laboratory for analysis. If the sample cannot be analyzed within about 15 minutes, it should be placed in an ice bath, and accurate results can be obtained for up to about 3 hours and possibly longer.
5 What changes would be expected from delayed analysis of a sample?
If the sample is not analyzed in a timely fashion, artifacts are induced, the result of metabolism by the cells in the blood. Cellular metabolism will utilize oxygen, thus lowering PO2, and will produce lactic acid, thus lowering both pH and bicarbonate.
6 What changes would be expected from excessive exposure to room air?
Exposure to room air may result from failure to cap the syringe in which the sample is collected or from the presence of large air bubbles in the sample that are not removed. Room air has a PO2 higher than and a PCO2 lower than blood. Thus an artificial increase in PO2 and an artificial decrease in PCO2 may be seen.
7 What is pH?
pH is a measurement of the hydrogen ion concentration [H+]. The notation of a lowercase p means the “power of,” so pH is the power of [H+].
By definition, pH = –log [H+]. Because of the negative log, there is an inverse relationship between pH and hydrogen ion concentration, which sometimes leads to confusion. As [H+] increases, the pH decreases (becomes more acidic), and as [H+] decreases, the pH increases (becomes more alkaline). The normal hydrogen ion concentration in biologic fluid is measured in nanoequivalents per liter (nEq/L), about one-millionth the concentration of most other electrolytes (e.g., Na+, Cl−, K+).
8 Why is [H+] measured as pH rather than as concentration?
The concept of pH was designed to “simplify” the representation of the wide range of H+ ions found in chemical systems. The range of [H+] in biologic systems that is compatible with life is much narrower, and the concept of pH is probably not needed. It would undoubtedly be simpler to refer to [H+] in nEq/L, but pH is now used because of tradition. Keep in mind that a change in pH of 1 unit (e.g., 7.4 to 6.4) represents a 10-fold change in [H+]. Similarly, a change of 0.3 pH units represents a twofold change in [H+].
9 If hydrogen ions are present in such low concentrations compared with other electrolytes, why are changes in pH so important?
Hydrogen ions are extremely reactive molecules. The proteins of the body, including enzymes, have many dissociable groups that can either bind or release hydrogen ions. Gain or loss of hydrogen ions can change the structure and function of these proteins. Therefore, changes in [H+] can have profound effects on the body.
10 What normal physiologic factors potentially alter pH?
Protein metabolism results in the daily generation of fixed acids. Carbohydrate metabolism results in the generation of CO2, a volatile acid. CO2 is a potential acid because in the presence of carbonic anhydrase, it can bind with water to form carbonic acid (H2CO3).
11 What are the definitions of an “acid” and a “base”?
An acid is a substance that can donate a hydrogen ion, whereas a base is an H+ acceptor.
In which A− acts as a base because it can bind to free H+ ion.
12 What are the normal body defenses that serve to protect against changes in pH?
The body contains many buffers, which blunt changes in pH. A buffer is a substance that can gain or lose hydrogen ions and thus minimize changes in pH. A buffer pair consists of a weak acid (HA) and its conjugate base (A−). If a large amount of hydrogen ions are added to a system, the buffer pair binds some of the H+ ion, thus minimizing the change in pH.
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