Blood Gas Analysis n TJ Berger MS, PA-C
ABG
Role of Acid Base Balance
NORMAL LABORATORY VALUES n Arterial Blood Gas: ¨ p. H ¨ PCO 2 ¨ [HCO 3 -] n 7. 40 7. 35 -7. 45 40 35 -45 mm Hg 24 22 -26 mmol/L or meq/L Plasma Electrolytes ¨ [Na+] ¨ [K+ ] ¨ [Cl-] ¨ [total CO 2] 135 -145 m. Eq/L 3. 5 -5. 0 m. Eq/L 96 -109 m. Eq/L 24 -30 m. Eq/L
Henderson-Hasselbalch Equation
Bicarbonate-Carbon Dioxide Buffer System
GENERAL ASPECTS OF ACID-BASE DISORDERS n n n A primary alteration in [H+], [HCO 3 -] or PCO 2 results in abnormal p. H The body has several mechanisms to correct p. H towards the normal range ¨ In the acute phase (minutes to hours), the extra- and intra-cellular buffer systems (most importantly the bicarbonate system) minimize the p. H changes ¨ In the chronic phase (hours to days), renal or respiratory compensation partially or completely restore p. H towards normal There are limits to both types of compensation (does not result in over correction of p. H)
Types of Acid-Base Disorders n Two types: respiratory and metabolic ¨ Primary respiratory disorders affect blood acidity by causing changes in PCO 2 ¨ Primary metabolic disorders are caused by disturbances in the HCO 3 - concentration n n Primary disturbances are usually accompanied by compensatory changes The presence of one disorder with its appropriate compensatory change is a simple disorder
Primary derangement with expected compensatory response PH PCO 2 HCO 3 Met Acidosis 2 1 Met Alkalosis 2 1 Resp Acidosis 1 2 Resp Alkalosis 1 2
Interpretation of Acid-Base n Metabolic acidosis ¨ n ¨ p. CO 2 “blown off” p. H high, bicarb high ¨ p. CO 2 retained ¨ Bicarb excreted Respiratory acidosis ¨ n Compensation Metabolic alkalosis ¨ n p. H low, bicarb low n p. H low, p. CO 2 high Respiratory alkalosis ¨ p. H high, p. CO 2 low
Interpretation n Look at p. H ¨ <7. 40 - acidosis n CO 2 increased- respiratory n CO 2 decreased- metabolic ¨ ¨ Bicarb follows CO 2 - partial compensation Bicarb diverges CO 2 - non compensated ¨ >7. 40 - alkalosis n CO 2 decreased-respiratory n CO 2 increased-metabolic ¨ ¨ Bicarb follows CO 2 - partial compensation Bicarb diverges CO 2 - non compensated
Acid-Base Survival Guide n p. H: ¨ Less than 7. 4= Acidosis ¨ Greater than 7. 4= Alkalosis. n p. CO 2 and Bicarb: ¨ Do they move in the same direction as the p. H, or the opposite direction? ¨ If they move in the sa. ME direction, it’s primary MEtabolic ¨ If they move in a diffe. REnt direction, it’s primary REspiratory n Is the non-primary system compensating appropriately?
Example 1 n 7. 32 p. CO 2= 30 HCO 3 - =16 Pa. O 2=100
Example 1 n Look at the p. H 7. 32 30 16 100 ¨ The p. H is low, therefore by definition the patient is acidemic
Example 1 n n What is the process? 7. 32 30 16 100 PCO 2 and HCO 3 - are abnormal in the same direction ¨ n Less likely a mixed acid base disorder Need to distinguish the initial change from the compensatory response A low HCO 3 - represents acidosis and is consistent with the p. H and is, therefore, the initial change ¨ The low PCO 2 must be the compensatory response ¨ Since the primary change involves HCO 3 -, this is a metabolic process (Metabolic Acidosis) ¨
Example 2 n 7. 24 60 27 87
Example 2 n Look at the p. H 7. 24 60 27 87 ¨ The p. H is low, therefore by definition the patient is acidemic
Example 2 n n What is the process? 7. 24 60 27 87 PCO 2 and HCO 3 - are abnormal in the same direction ¨ n Less likely a mixed disorder Need to distinguish the initial change from the compensatory response A high p. CO 2 represents acidosis and is consistent with the p. H and is, therefore, the initial change ¨ The high HCO 3 - represents the compensatory change ¨ Since the primary change involves p. CO 2, this is a respiratory process (Respiratory Acidosis) ¨
Metabolic Acidosis n n Either an increase in acid accumulation or a decrease in extracellular bicarbonate Resulting decrease in both serum p. H and [HCO 3 -] As a compensatory response, there is an increase in alveolar ventilation leading to a decrease in Pa. CO 2 + H 2 O H 2 CO 3 H+ + HCO 3 -
Metabolic Acidosis Symptoms n Most symptoms are caused by the disease or condition that is causing the metabolic acidosis n Metabolic acidosis itself usually causes rapid breathing n Confusion or lethargy may also occur
Metabolic Acidosis Signs and tests n Blood tests to diagnose metabolic acidosis may include: ¨ An arterial blood gas to assess the severity of the metabolic acidosis ¨ A metabolic panel to reveal the cause and severity of the metabolic acidosis ¨ A complete blood count (CBC) to assess possible causes of metabolic acidosis n n Infection Severe anemia leading to hypoxia
Metabolic Acidosis Treatment n Aimed at the underlying disorder n Restoration of a normal extracellular p. H ¨ Raise the systemic p. H to above 7. 20; this is a level at which the major consequences of severe acidemia should not be observed n Correction of the acidemia can be achieved rapidly by the administration of Na. HCO 3 but should only be given if needed
Metabolic Acidosis n The expected compensatory Pa. CO 2 can be calculated using the Winters equation ¨ n Pa. CO 2 = 1. 5 x [HCO 3 -] + 8 (+/- 2) Metabolic acidosis can be separated on the basis of whethere is an anion gap ¨ AG: [Na]-([Cl]+[HCO 3 -])= 12 (+/- 4 m. Eq/L)
Anion Gap Metabolic Acidosis MUDPILERS n Methanol n Uremia n DKA/Alcoholic KA n Paraldehyde n Isoniazid n Lactic Acidosis n Etoh/Ethylene Glycol n Rhabdo/Renal Failure n Salicylates
Non-Anion Gap Metabolic Acidosis HARDUPS n Hyperalimentation n Acetazolamide n Renal Tubular Acidosis n Diarrhea n Uretero-Pelvic Shunt n Post-Hypocapnia n Spironolactone
NORMAL ANION GAP METABOLIC ACIDOSIS n n Metabolic acidosis with a normal anion gap is also referred to as hyperchloremic metabolic acidosis This can be due to renal and non-renal causes A renal cause may be present when there is an abnormality in the reabsorption of filtered HCO 3 - by the proximal tubule, or an abnormality in the excretion of net acid by the collecting duct Tests of the minimum urine p. H, the urine net charge and/or HCO 3 - reabsorption are used to distinguish renal and non-renal causes
Metabolic Alkalosis n n n An increase in serum [HCO 3 -] or a loss of acid with a relative increase in serum [HCO 3 -] CO 2 + H 2 O H 2 CO 3 H+ + HCO 3 Resulting increase in both serum p. H and [HCO 3 -] Compensation includes a decrease in alveolar ventilation and an increase in the Pa. CO 2 Calculated by the equation ¨ Pa. CO 2 = 0. 7 x change [HCO 3 -]
Metabolic Alkalosis CLEVER PD n Contraction n Licorice n Associated with High Urine Cl levels n Endo: Conn’s/Cushing’s/Bartter’s n Vomiting n Excess Alkali n Refeeding Alkalosis n Post-hypercapnia n Diuretics
Metabolic Alkalosis Classification n Saline responsive (UCl<10 meq/d) ¨ A sign of extracellular volume contraction ¨ By far more common n Saline unresponsive (UCl>10 meq/d) ¨ A volume expanded state ¨ May be associated with HTN
Metabolic Alkalosis - Causes n Chloride responsive Renal Cl loss (diuretics) ¨ GI Cl loss (vomiting, gastric sxn) ¨ Alkali administration ¨ n Chloride resistant Mineralocorticoid excess (Cushing’s, aldosteronism) ¨ Bartter’s syndrome ¨ Licorice ¨
Metabolic Alkalosis Symptoms and signs n None are characteristic n Orthostasis in saline responsive state n Weakness and hyporeflexia may occur with associated hypokalemia
Metabolic Alkalosis Treatment n Saline responsive ¨ 0. 9% Na. Cl and KCl are administered ¨ Acetazolamide useful in patients who will not tolerate volume expansion ¨ HCL, 0. 1 mol/L IV in severe cases ¨ Hemodialysis n Saline unresponsive ¨ Surgical removal of a mineralocorticoid producing tumor ¨ ACEI to block aldosterone effects
Respiratory Acidosis n Normally, alveolar ventilation removes the metabolically produced CO 2 and maintains a normal Pa. CO 2 of 40 mm. Hg n CO 2 + H 2 O H 2 CO 3 H+ + HCO 3 n Disorder associated with a low p. H and a high PCO 2
Respiratory Acidosis Acute n CNS Depression (drugs/CVA) n Airway Obstruction n Pneumonia n Pulmonary Edema n Hemo/Pneumothorax Chronic n COPD/RLD: any hypoventilation state n Myopathy
Respiratory Acidosis Symptoms and signs n Somnolence and confusion n Myoclonus with asterixis n Coma from CO 2 narcosis n Increased ICP ¨ Papilledema, pseudotumor cerebri, HTN/bradycardia
Respiratory Acidosis Treatment n Aimed at underlying cause ¨ Remove airway obstruction ¨ Naloxone n Improve ventilation ¨ Bi. PAP ¨ Mechanical ventilation
Respiratory Alkalosis n n n Develops because of an increase in alveolar ventilation resulting in a decreased CO 2 tension in the body CO 2 + H 2 O H 2 CO 3 H+ + HCO 3 - Metabolic compensation consists of: Acute utilization of nonbicarbonate buffers ¨ Chronic renal loss of [HCO 3 -] ¨
Respiratory Alkalosis - Causes n n n Fever Pain Anxiety Pneumonia Severe anemia n n Pulmonary embolism Drugs Sepsis Pregnancy
Respiratory Alkalosis CHAMPS (think speed up breathing) n CNS disease n Hypoxia n Anxiety n Mechanical ventilation n Progesterone n Salicylates/Sepsis
Respiratory Alkalosis Symptoms and signs n Light-headedness n Anxiety n Paresthesias n Peri-oral numbness n Tetany (severe)
Respiratory Alkalosis Treatment n Directed toward the underlying cause n Re-breathing (paper bag) n Sedation
Example 3 n 7. 52 25 20 71
Example 3 n Look at the p. H 7. 52 25 20 71 ¨ The p. H is high, therefore by definition the patient is alkalemic
Example 3 n n What is the process? 7. 52 25 20 71 PCO 2 and HCO 3 - are abnormal in the same direction ¨ n Less likely a mixed disorder Need to distinguish the initial change from the compensatory response A low p. CO 2 represents alkalosis and is consistent with the p. H and is, therefore, the initial change ¨ The low HCO 3 - represents the compensatory change ¨ Since the primary change involves p. CO 2, this is a respiratory process (Respiratory Alkalosis) ¨
Step-By-Step Analysis of Acid-Base Status 1. Determine the primary disturbance by evaluating p. H, HCO 3 - and PCO 2 values 2. Calculate the anion gap 3. Calculate the HCO 3 - concentration if the anion gap is increased 4. Examine the patient to determine whether the clinical signs are compatible with the analysis
Acid-Base Survival Guide n Is there an anion gap? (Na - Cl - bicarb > 12) ¨ If no, you’re done ¨ If yes, take the anion gap - 12. Add that to the bicarb level ¨ If it’s greater than 26, you’ve got a metabolic alkalosis as well ¨ If it’s less than 22, you’ve got a non-anion gap metabolic / respiratory acidosis, too Classic pimping: You can have 3 disorders coexisting, but not 4 (Your lungs can either be making respiratory acidosis or alkalosis, not both)
Mixed Acid-Base Disorders Presence of more than one simple disorder n Double or triple disorders can coexist n ¨ May be difficult to ascertain ¨ Helpful to determine if the degree of compensation differs from what is expected
Mixed Acid-Base Disorders n Acute respiratory acidosis ¨ HCO 3 - 1. 0 meq/L per 10 mm. Hg PCO 2 n Chronic respiratory acidosis ¨ HCO 3 - ≈5 meq/L per 10 mm. Hg PCO 2 n Respiratory acidosis ¨ p. H will decrease by 0. 8 for every increase in 10 mm. Hg of PCO 2 n Example PCO 2 50 will drive p. H down to 7. 32
∆/∆ The delta ratio is sometimes used in the assessment of elevated anion gap metabolic acidosis to determine if a mixed acid base disorder is present n Knowledge of the relationship between the rise in AG (delta AG) and the fall in bicarbonate (delta HCO 3) is important in understanding mixed acid-based disorders n
∆/∆ n Delta ratio = ∆ Anion gap/∆ [HCO 3 -] or ↑anion gap/ ↓ [HCO 3 -] Measured anion gap – Normal anion gap Normal [HCO 3 -] – Measured [HCO 3 -] n (AG – 12) (24 - [HCO 3 -]) n
∆/∆ n n n A delta-delta value below 1: 1 indicates a greater fall in [HCO 3 -] than one would expect given the increase in the anion gap This can be explained by a mixed metabolic acidosis ¨ i. e a combined elevated anion gap acidosis and a normal anion gap acidosis, as might occur when lactic acidosis is superimposed on severe diarrhea In this situation, the additional fall in HCO 3 - is due to further buffering of an acid that does not contribute to the anion gap ¨ i. e addition of HCl to the body as a result of diarrhea
∆/∆ n A value between 1 and 2 ¨ n Pure Anion Gap Acidosis A value greater than 2 ¨ High AG acidosis and a concurrent metabolic alkalosis or a pre-existing compensated respiratory acidosis
Alveolar-arterial gradient n n A measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen The equation for calculating the A–a gradient is:
A-a gradient Patm = 760 PH 2 O = 47 A normal A–a gradient for a young adult non-smoker breathing air, is between 5– 10 mm. Hg Age/4 +4
Causes of elevated A-a gradient n 1. Diffusion defect (rare) 2. V/Q mismatch 3. Right-to-Left shunt (intrapulmonary or cardiac) 4. Increased O 2 extraction (Ca. O 2 -Cv. O 2)
Case n Case X ¨ Labs: Na 132 , K 6. 0, Cl 93, HCO 3 - 11 glucose 720, BUN 38, Cr 2. 6. ¨ UA: p. H 5, SG 1. 010, ketones negative, glucose positive. Plasma ketones trace. ¨ ABG: p. H 7. 27 p. CO 2 23 HCO 3 - 10
Case n Look at the p. H 7. 27 23 10 ¨ The p. H is low, therefore by definition the patient is acidemic
Case n n What is the process? 7. 27 23 10 PCO 2 and HCO 3 - are abnormal in the same direction ¨ n Less likely a mixed acid base disorder Need to distinguish the initial change from the compensatory response A low HCO 3 - represents acidosis and is consistent with the p. H and is, therefore, the initial change ¨ The low PCO 2 must be the compensatory response ¨ Since the primary change involves HCO 3 -, this is a metabolic process (Metabolic Acidosis) ¨
Case Labs: Na 132 , K 6. 0, Cl 93, HCO 3 - 11 glucose 720, BUN 38, Cr 2. 6. ¨ Calculate the Anion Gap ¨ n ¨ Is the compensation adequate? n ¨ AG: [Na]-([Cl]+[HCO 3 -])= 28 Pa. CO 2 = 1. 5 x [HCO 3 -] + 8 (+/- 2)= 24. 5 Since the actual PCO 2 falls within the estimated range, we can deduce that the compensation is adequate and there is no separate respiratory disorder present
Case Since the anion gap is elevated, calculate the delta-ratio to rule out concurrent metabolic alkalosis n Measured anion gap – Normal anion gap Normal [HCO 3 -] – Measured [HCO 3 -] n 28 -12 / 24 -11= 16/13= 1. 23 n Since the delta gap is between 1 and 2, we can deduce that this is a pure metabolic acidosis n
Case n Assessment ¨ Compensated elevated anion gap acidosis most likely secondary to DKA