3CAPILLARY BLOOD GASESpH: Same as arterial or slightly lower (Normal = )pCO2: Same as arterial or slightly higher (Normal = 40-45)pO2: Lower than arterial (Normal = 45-60)O2 Saturation: >70% is acceptable.
4CAPILLARY BLOOD GASESSaturation is probably more useful than the pO2 itself when interpreting a CBG.The heel is the most commonly used siteThe CBG is often used for pediatric patients because it is easier to obtain than the ABGless traumatic (no risk of arterial thrombosis, hemorrhage).
5blood gas machinesThe blood gas machines in most labs actually measure the pH ,the pCO2 and the pO2.The [HCO3-] and the base difference are calculated values using the Henderson-Hasselbalch equation
6For a rough estimate of [H+] [H+] = (7.80 -pH) x100.This is accurate from a pH 7.25 to 7.48;40 mEq/L = [H+] at the normal pH of 7.40.pH is a log scale, and for every change of 0.3 in pH from 7.40 the [H+] doubles or halves.For pH 7.10 the [H+] = 2 x 40, or 80 nmol/L, and for pH 7.70 the [H+] = 1/2 x40, or 20 nmol/L.
7pCO2 and pHA change in pCO2 up or down 10 mm Hg is associated with an increase or decrease in pH of 0.08 units.As the pCO2 decreases, the pH increases; as the pCO2 increases, the pH decreases.
8base deficit and base excess A pH change of 0.15 is equivalent to a base change of 10 mEq/L.A decrease in base (i.e, [HCO3-]) is termed a base deficit, and an increase in base is termed a base excess.
9Acidosis and alkalosis Acid-base disorders are very common clinical problems.Acidemia is a pH <7.37, andalkalemia is a pH >7.44.Acidosis and alkalosis are used to describe how the pH changes.The primary causes of acid-base disturbances are abnormalities in the respiratory system and in the metabolic or renal system.
10normal compensatory response Any primary disturbance in acid-base homeostasis invokes a normal compensatory response.A primary metabolic disorder leads to respiratory compensation, and a primary respiratory disorder leads to an acute metabolic response due to the buffering capacity of body fluids.A more chronic compensation (1-2 days) due to alterations in renal function.
11Mixed acid-base disorder Most acid-base disorders result from a single primary disturbance with the normal physiologic compensatory response and are called simple acid-base disorders.In certain cases, however, particularly in seriously ill patients, two or more different primary disorders may occur simultaneously, resulting in a mixed acid-base disorder.The net effect of mixed disorders may be additive (eg, metabolic acidosis and respiratory acidosis) and result in extreme alteration of pH;or they may be opposite (eg, metabolic acidosis and respiratory alkalosis) and nullify each other’s effects on the pH.
14INTERPRETATION OF BLOOD GASES Step 1:Determine if the numbers fit.The right side of the equation should be within about 10% of the left side.If the numbers do not fit, you need to obtain another ABG
15INTERPRETATION OF BLOOD GASES Step 2:determine if an acidemia (pH <7.37)or an alkalemia (pH >7.44) is present.
16Step 3: Identify the primary disturbance as metabolic or respiratory. For example, if acidemia is present, is the pCO2 >44 mm Hg (respiratory acidosis), or is the [HCO3 -] <22 mmol/L (metabolic acidosis).In other words, identify which component, respiratory or metabolic, is altered in the same direction as the pH abnormality.If both components act in the same direction (eg, both respiratory [pCO2 > 44 mm Hg] and metabolic [HCO3 - <22 mmol/L] acidosis are present), then this is a mixed acid-base problem.The primary disturbance will be the one that varies from normal the greatest, that is, with a [HCO3 -] = 6 mmol/L and pCO2 = 50 mm Hg, the primary disturbance would be a metabolic acidosis, the [HCO3 -] is about one-quarter normal, whereas the increase in pCO2 is only 25%.
17Step 4: Calculate the anion gap. Anion gap = Na+ - (Cl- + HCO3 -). Normal anion gap is 8-12 mmol.
18Step 5: If the anion gap is elevated Then compare the changes from normal between the anion gap and [HCO3 -].If the change in the anion gap is greater than the change in the [HCO3 -] from normal, then a metabolic alkalosis is present in addition to a gap metabolic acidosis.If the change in the anion gap is less than the change in the [HCO3 -] from normal, then a non gap metabolic acidosis is present in addition to a gap metabolic acidosis.
19METABOLIC ACIDOSIS: DIAGNOSIS AND TREATMENT Metabolic acidosis represents an increase in acid in body fluids .Reflected by a decrease in [HCO3 -] and a compensatory decrease in pCO2.
20Anion Gap Acidosis:Anion gap >12 mmol/L; caused by a decrease in [HCO3 -]balanced by an increase in an unmeasured acid ion from either endogenous production or exogenous ingestion (normochloremic acidosis).
21Non anion Gap Acidosis: Anion gap = 8-12 mmol/L; caused by a decrease in [HCO3 -] balanced by an increase in chloride (hyperchloremic acidosis). Renal tubular acidosis is a type of non gap acidosisThe anion gap is helpful in identifying metabolic gap acidosis, non gap acidosis, mixed metabolic gap and non gap acidosis. If an elevated anion gap is present, a closer look at the anion gap and the bicarbonate helps differentiate among(a) a pure metabolic gap acidosis(b) a metabolic non gap acidosis(c) mixed metabolic gap and non gap acidosis, and(d) a metabolic gap acidosis and metabolic alkalosis.
23Treatment of Metabolic Acidosis 1. Correct any underlying disorder (control diarrhea, etc).2. Treatment with bicarbonate should be reserved for severe metabolic gap acidosis.If the pH <7.20, correct with sodium bicarbonate.The total replacement dose of [HCO3 -] can be calculated as follows:3. Replace with one-half the total amount of bicarbonate over 8-12 h and reevaluate.Be aware of sodium and volume overload during replacement.Normal or isotonic bicarbonate drip is made with 3 ampules NaHCO3 (50 mmol NaHCO3/ampule) in 1 L D5W.
24METABOLIC ALKALOSIS:Metabolic alkalosis represents an increase in [HCO3 -] with a compensatory rise in pCO2.
25Differential Diagnosis In two basic categories of diseases the kidneys retain [HCO3 -]They can be differentiated in terms of response to treatment with sodium chlorideand also by the level of urinary [Cl-] as determined by ordering a Spot,or random urinalysis for chloride (UCl).
28Chloride-Sensitive (Responsive) Metabolic Alkalosis: The initial problem is a sustained loss of chloride out of proportion to the loss of sodium (either by renal or GI )This chloride depletion results in renal sodium conservation leading to a corresponding reabsorption of [HCO3 -] by the kidney.In this category of metabolic alkalosis, the urinary [Cl-] is <10 mEq/L,and the disorders respond to treatment with intravenous NaCl.
29Chloride-Insensitive (Resistant) Metabolic Alkalosis: The pathogenesis in this category is direct stimulation of the kidneys to retain bicarbonate irrespective of electrolyte intake and losses.The urinary [Cl-] >10 mEq/L, and these disorders do not respond to NaCl administration.
30Treatment of Metabolic Alkalosis Correct the underlying disorder.1. Chloride-responsivea. Replace volume with NaCl if depleted.b. Correct hypokalemia if present.c. NH4Cl and HCl should be reserved for extreme cases.2. Chloride-resistanta. Treat underlying problem, such as stopping exogenous steroids.
31RESPIRATORY ACIDOSIS: DIAGNOSIS AND TREATMENT Respiratory acidosis is a primary rise in pCO2 with a compensatory rise in plasma [HCO3 -].Increased pCO2 occurs in clinical situations in which decreased alveolar ventilation occurs.