Articles on Medical Diseases and Conditions

Tag: Acid-Base

  • Interpretation of Acid-Base Data

    Acid-base data interpretation has always been one of the more difficult areas of laboratory medicine. In most uncomplicated untreated cases the diagnosis can be achieved with reasonable ease. There are several ways of approaching an acid-base problem. One way is to examine first the arterial PCO2 value. Since primary respiratory disorders result from hypoventilation or hyperventilation, which, in turn, are reflected by a change in arterial PCO2, normal PCO2 with abnormal pH is strong evidence against a primary respiratory disorder and should be an uncompensated metabolic disorder.

    If PCO2 is decreased, there are two major possibilities:

    1. The primary disorder could be respiratory alkalosis (hyperventilation). If so, pH should be increased in acute (uncompensated) cases or partially compensated cases. In fully compensated cases pH is within reference range, but frequently it is more than 7.40 even within the reference range.

    2. The primary disorder could be metabolic acidosis. If so, pH should be decreased in partially compensated cases. In fully compensated cases pH is within its reference range (similar to fully compensated respiratory alkalosis), but frequently it is less than 7.40 even within the reference range.

    If PCO2 is increased, there are also two major possibilities:

    1. The primary disorder could be respiratory acidosis (hypoventilation). If so, pH should be decreased in acute (uncompensated) cases or partially compensated cases. In fully compensated cases pH is within reference range, but frequently it is less than 7.40 even within reference range.

    2. The primary disorder could be metabolic alkalosis. If so, pH should be increased in partially compensated cases. In fully compensated cases pH is within its reference range (similar to fully compensated respiratory acidosis), but pH frequently is more than 7.40 even within the reference range.

    There is another way to interpret the data. If one first inspects pH, decreased pH means acidosis and increased pH means alkalosis. One then inspects the PCO2 value. If the PCO2 has changed in the same direction as the pH, the primary disorder is metabolic. If the PCO2 has changed in the opposite direction from that of the pH, the primary disorder is respiratory.

    Base excess analysis is not a vital part of this type of algorithm. However, base excess can sometimes be helpful, both as additional evidence for certain types of acid-base disorders or to help detect the presence of active acid-base disorder in fully compensated cases. A negative base excess is found in metabolic acidosis and, to a lesser extent, in respiratory alkalosis. A positive base excess is found in metabolic alkalosis and, to a lesser extent, in respiratory acidosis.

  • Acid-Base Compensation

    Compensation refers to the degree of PCO2 change when there is, or has been, an abnormality in pH.

    An uncompensated disorder is a primary metabolic or respiratory condition that has not been altered by any significant degree of correction. In the case of a primary metabolic condition the respiratory counterbalance (change in ventilation which is reflected by a change in arterial PCO2) is not evident; pH is abnormal but PCO2 remains normal. In the case of a primary respiratory condition, both PCO2 and pH are abnormal, and the degree of abnormality on both tests is relatively severe. In that case the renal counterbalance (increased bicarbonate formation to bring pH back toward normal) is not evident.

    A partially compensated disorder is present when both pH and PCO2 are outside their reference ranges. In primary respiratory disorders, the degree of pH abnormality is not as severe as in uncompensated cases.

    A fully compensated (sometimes referred to only as compensated) condition is a primary metabolic or respiratory disorder in which PCO2 is outside its reference range but pH has returned to its reference range.

  • Summary of Acid-Base Changes

    To summarize plasma pH problems, in metabolic acidosis there is eventual HCO–3 deficit, leading to decreased plasma pH and decreased CO2 content (or CO2 combining power). In respiratory acidosis there is primary H2CO3 excess, which causes decreased plasma pH, but the CO2 content is increased due to renal attempts at compensation. In metabolic alkalosis there is eventual bicarbonate excess leading to increased plasma pH and increased CO2 content. In respiratory alkalosis there is primary carbonic acid deficit, which causes increased plasma pH, but the CO2 content is decreased due to renal attempts at compensation. The urine pH usually reflects the status of the plasma pH except in hypokalemic alkalosis, where there is acid urine pH despite plasma alkalosis.

    As noted, CO2 content or combining capacity essentially constitutes the numerator of the Henderson-Hasselbalch equation. PCO2 is essentially a measurement of the equation denominator and can be used in conjunction with pH to indicate acid-base changes. This is the system popularized by Astrup and Siggaard-Anderson. PCO2 follows the same direction as the CO2 content in classic acid-base syndromes. In metabolic acidosis, PCO2 is decreased, because acids other than H2CO3 accumulate, and CO2 is blown off by the lungs in attempts to decrease body fluid acidity. In metabolic alkalosis, PCO2 is increased if the lungs compensate by hypoventilation; in mild or acute cases, PCO2 may remain normal. In respiratory alkalosis, PCO2 is decreased because increased ventilation blows off more CO2. In respiratory acidosis, PCO2 is increased because of CO2 retention due to decreased ventilation.

  • Acid-Base Test Specimens

    In the early days, acid-base studies were performed on venous blood. Venous specimens are nearly as accurate as arterial blood for pH and HCO3 (or PCO2) measurements if blood is obtained anaerobically from a motionless hand or arm before the tourniquet is released. Nevertheless, arterial specimens have mostly replaced venous ones because venous blood provides less accurate data in some conditions such as decreased tissue perfusion due to shock. Even more important, one can also obtain blood oxygen measurements (PO2) with arterial samples. Arterial blood is most often drawn from the radial artery in the wrist. Arterial puncture is little more difficult than venipuncture, and there is a small but definite risk of extravasation and hematoma formation that could compress the artery. Although glass syringes have some technical advantages over plastic syringes or tubes (such as a slightly smaller chance of specimen contamination with room air than when using plastic syringes), most hospitals use only plastic. It is officially recommended that the specimen tube or syringe should be placed in ice immediately for transport to the laboratory, both to prevent artifact from blood cell metabolism and to diminish gas exchange between the syringe and room air. The blood must be rewarmed before analysis. Actually, it is not absolutely necessary to ice the specimen in most cases if the analysis takes place less than 15 minutes after the specimen is obtained. Icing the specimen in plastic tubes can elevate PO2 values a little if they are already over 80 mm Hg (10.7 kPa). One investigator found that at 100 mm Hg (13.3 kPa), the false elevation averages 8 mm Hg (1.06 kPa). Also, icing in plastic tubes increases plasma viscosity over time and interferes with resuspension of the RBC, which affects hemoglobin assay in those instruments that calculate O2 content from PO2 and total hemoglobin. If mixing before assay is not thorough, hemoglobin values will be falsely decreased somewhat. In addition, if electrolytes are assayed on the arterial specimen, potassium may be falsely increased somewhat.

    Capillary blood specimens from heelstick are often used in newborn or neonatal patients because of their small blood vessels. Warming the heel produces a semiarterial (“arterialized”) specimen. However, PO2 is not reliable and PCO2 sometimes differs from umbilical artery specimens. The majority of reports do not indicate substantial differences in pH; however, one investigator found a mean decrease in PCO2 of 1.3 mm Hg (0.17 kPa), a mean pH increase of 0.02 units, and a mean decrease of 24.2 mm Hg (3.2 kPa) in PO2 from heelstick blood compared to simultaneously drawn umbilical artery blood.

    Heparin is the preferred anticoagulant for blood gas specimens. The usual method is to wash the syringe with a heparin solution and then expel the heparin (which leaves about 0.2 ml of heparin in the dead space of the syringe and needle). If too much heparin remains or the blood sample size is too small (usually when the sample is <3 ml), there is a disproportionate amount of heparin for the amount of blood. This frequently causes a significant decrease in PCO2 (and bicarbonate) and hemoglobin values, with a much smaller (often negligible) decrease in pH. These artifactual decreases in PCO2 are especially apt to occur when the sample is obtained from indwelling catheters flushed with heparin.

  • Acid-Base and pH Measurements

    Fluid and electrolyte problems are common in hospitalized patients. Most of these problems are secondary to other diseases or are undesirable side effects of therapy. There are a few diseases regularly associated with certain pH or electrolyte alterations that can help suggest the diagnosis and can be used to monitor therapy.

    Fluid and electrolytes in one form or another make up nearly all of the human body. It is useful to think of these constituents as though they were contained in three separate compartments between which are variable degrees of communication: individual cells, containing intracellular fluid; vascular channels, containing blood or lymph; and the extracellular nonvascular tissue containing the interstitial fluid. Shifts of fluid and electrolytes between and within these compartments take place continually as the various activities concerned with homeostasis, cell metabolism, and organ function go on. To some degree, these changes can be monitored clinically by their effects on certain measurable parameters, including the pH and concentration of certain ions (electrolytes) in a fairly accessible substance, the blood. This chapter covers blood pH and its disturbances; the next chapter discusses certain electrolyte and fluid disorders.