ABG interpretation and arterial blood gases analysis

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Disorders of acid–base balance can lead to severe complications in many disease states, and occasionally the abnormality may be so severe as to become a life-threatening risk factor. The process of analysis and monitoring of arterial blood gas (ABG) is an essential part of diagnosing and managing the oxygenation status and acid–base balance of the high-risk patients, as well as in the care of critically ill patients in the Intensive Care Unit. Since both areas manifest sudden and life-threatening changes in all the systems concerned, a thorough understanding of acid–base balance is mandatory for any physician, and the anesthesiologist is no exception. However, the understanding of ABGs and their interpretation can sometimes be very confusing and also an arduous task. Many methods do exist in literature to guide the interpretation of the ABGs. The discussion in this article does not include all those methods, such as analysis of base excess or Stewart’s strong ion difference, but a logical and systematic approach is presented to enable us to make a much easier interpretation through them. The proper application of the concepts of acid–base balance will help the healthcare provider not only to follow the progress of a patient, but also to evaluate the effectiveness of care being provided.

Introduction

Arterial blood gas (ABG) analysis is an essential part of diagnosing and managing a patient’s oxygenation status and acid–base balance. The usefulness of this diagnostic tool is dependent on being able to correctly interpret the results. Disorders of acid–base balance can create complications in many disease states, and occasionally the abnormality may be so severe so as to become a life-threatening risk factor. A thorough understanding of acid–base balance is mandatory for any physician, and intensivist, and the anesthesiologist is no exception.

The three widely used approaches to acid–base physiology are the HCO₃^– (in the context of pCO₂), standard base excess (SBE), and strong ion difference (SID). It has been more than 20 years since the Stewart’s concept of SID was introduced, which is defined as the absolute difference between completely dissociated anions and cations. According to the principle of electrical neutrality, this difference is balanced by the weak acids and CO₂. The SID is defined in terms of weak acids and CO₂ subsequently has been re-designated as effective SID (SID_e) which is identical to “buffer base.” Similarly, Stewart’s original term for total weak acid concentration (A_TOT) is now defined as the dissociated (A^–) plus undissociated (AH) weak acid forms. This is familiarly known as anion gap (AG), when normal concentration is actually caused by A^–. Thus all the three methods yield virtually identical results when they are used to quantify acid–base status of a given blood sample.[1]

Why is it Necessary to Order an ABG Analysis?

The utilization of an ABG analysis becomes necessary in view of the following advantages:

• Aids in establishing diagnosis.
• Guides treatment plan.
• Aids in ventilator management.
• Improvement in acid/base management; allows for optimal function of medications.
• Acid/base status may alter electrolyte levels critical to a patient’s status.

Accurate results for an ABG depend on the proper manner of collecting, handling, and analyzing the specimen. Clinically important errors may occur at any of the above steps, but ABG measurements are particularly vulnerable to preanalytic errors. The most common problems that are encountered include nonarterial samples, air bubbles in the sample, inadequate or excessive anticoagulant in the sample, and delayed analysis of a noncooled sample.

Respiratory disorders

After the primary disorder is established as respiratory, then the following points will help us to approach further with regard to the respiratory disorder).[8]

• Ratio of rate of change in H⁺to change in paCO₂
• Alveolar arterial oxygen gradient
• Compensation

Ratio of rate of change in H⁺to change in paCO₂

The above ratio of rate of change in H⁺to change in paCO₂ helps in guiding us to conclude whether the respiratory disorder is acute, chronic, or acute on chronic. As we have seen, the hydrogen can be calculated from Table 1 and the change in H⁺ is calculated by subtracting the normal H⁺ from the calculated H⁺ ion.[9]

>0.8–acute

0.3–0.8–acute on chronic

Alveolar Arterial Oxygen Gradient

It is calculated as follows:

where PAO₂, alveolar partial pressure of oxygen; PiO₂, partial pressure of inspired oxygen; FiO₂, fraction of inspired oxygen; PB, barometric pressure (760 mmHg at sea level); PH₂O, water vapor pressure (47 mm Hg), PaCO₂, partial pressure of carbon dioxide in blood; R, respiratory quotient assumed to be 0.8.

Hypoxemic respiratory failure can be associated with normal (10–15 mmHg) or increasedalveolar arterial oxygen gradient. Figure 2 shows the alogrithim for approach in a patient with hypoxemic respiratory failure. If this gradient is <20, then it indicates an extrapulmonary cause of respiratory failure.

Differentials of extrapulmonary causes of respiratory failure:

1. Central nervous system–Respiratory center depression due to causes such as drug overdose, primary alveolar hypoventilation, and myxedema.

2. Peripheral nervous system–Spinal cord diseases, Guillain-Barré syndrome, Amyotrophic lateral sclerosis.

3. Respiratory muscles–Hypophosphatemia, muscle fatigue, myasthenia gravis, and polymyositis.

4. Chest wall diseases–Ankylosing spondylitis, flail chest, thoracoplasty.

5. Pleural diseases–Restrictive pleuritis

6. Upper air way obstruction–Tracheal Stenosis, vocal cord tumor

Metabolic disorders

In patients with metabolic acidosis, an excess of acid or loss of base is present. This causes the HCO₃^–:H₂CO₃ratio and pH to fall while no change occurs in pCO₂–uncompensated metabolic acidosis.

As a result of compensatory mechanisms, the lungs in the form of CO₂ excrete H₂CO₃ and the kidneys retain HCO₃^–. pCO₂ falls and HCO₃^–: H₂CO₃ ratio and pH rise toward normal even though concentrations of HCO₃^–and H₂CO₃ are less than normal. This is called compensated metabolic acidosis and the expected paCO₂ is calculated as paCO₂ = [1.5 × HCO₃+ 8] ± 2.

Anion gap

For more than 40 years, the AG theory has been used by clinicians to exploit the concept of electroneutrality and has evolved as a major tool for evaluating the acid–base disorder. Anion gap is the difference between the charges of plasma anions and cations, calculated from the difference between the routinely measured concentration of the serum cations (Na⁺ and K⁺) and anions (Cl^– and HCO₃^–). Because electroneutrality must be maintained, the difference reflects the unmeasured ions. Normally, this difference or the gap is filled by the weak acids (A^–) principally albumin, and to a lesser extent phosphates, sulfates, and lactates.

When the AG is greater than that produced by the albumin and phosphate, other anions (e.g., lactates and ketones) must be present in higher than normal concentration.

Anion gap = (Na⁺ + K⁺) – [Cl^– + HCO₃^–]

Because of its low and narrow extracellular concentration, K⁺ is often omitted from the calculation The normal value ranges from 12 ± 4 when K⁺ is considered, and 8 ± 4 when K⁺ is omitted. Figure 3 shows the alogrithm for the approach to patients with normal AG acidosis.

The primary problem with AG is its reliance on the use of the normal range produced by the albumin and to a lesser extent phosphate, the level of which may be grossly abnormal in critically ill patients. Because these anions are not strong anions, their charges will be altered by changes in pH.[10,11]

Serum protein and phosphate

Normal AG = 2{albumin(gm/L)} + 0.5 {phosphate (mg/dL)}

Acid–base status

In Acidemic state – Anion gap decreases by 1-3

In Alkalemic state – Anion gap increases by 2-5

Major clinical uses of the anion gap

• For signaling, the presence of a metabolic acidosis and confirm other findings.
• Helping to differentiate between causes of metabolic acidosis: High AG versus normal AG metabolic acidosis. In an inorganic metabolic acidosis (e.g., due to HCl infusion), the infused Cl^– replaces HCO₃^–, and the AG remains normal. In an organic acidosis, the lost bicarbonate is replaced by the acid anion which is not normally measured. This means that the AG is increased.
• Providing assistance in assessing the biochemical severity of the acidosis and follow the response to treatment.

Disorders that are associated with a low or negative serum AG are listed in Table 2.

Table 2

Disorders associated with low serum anion gap

Cause	Comments
Laboratory error	Most frequent cause of low anion gap
Hypoalbuminemia	Second most common cause of low serum anion gap
Multiple myeloma	Level of anion gap correlates with serum concentration of paraprotein
Halide intoxication	Anion gap depends on serum halide concentration
(bromide, lithium, iodide)	(low anion gap with lithium ≥4 mEq/L)
Hypercalcemia	more likely in hypercalcemia associated with 10 hyperparathyroidism
Hypermagnesemia	Theoretical cause but not documented in literature
Polymyxin B	Anion gap depends on serum level; occurs with preparation with chloride
Underestimation of serum sodium	Most frequent with hypernatremia or hypertriglyceridemia
Overestimation of serum chloride	Rare with ion selective electrodes
Overestimation of serum bicarbonate	Spurious in serum HCO3 if cells not separated from sera

Table 3 elaborates the species of the unaccounted anions along with their sources of origin and diagnostic adjunts in case of high AG metabolic acidosis.

Table 3

Description of the species of unmeasured anions, source of origin, and diagnostic adjuncts in case of high anion gap metabolic acidosis

Cause	High serum anion gap
	Comments
	Species	Origin	Diagnostic adjuncts
Renal failure	Phosphates, sulphates	Protein metabolism	BUN/creatinine
Ketocidosis	Ketoacids	Fatty acid metabolism	Serum/urine ketones
Diabetic	β Hydroxybutyrate
Alcoholic
Starvation	Acetoacetate
Lactic acidosis	Lactate		Lactate levels
Exogenous poisoning	Salicylate	Salicylate	Concomitant
	Lactate	Respiratory and metabolic alkalosis
	ketoacids

In the patients with metabolic alkalosis, there is an excess of base or a loss of acid which causes the HCO₃^–:H₂CO₃ ratio and pH to rise, but with no change occurring in pCO₂, which is called uncompensated metabolic alkalosis. However, the kidney has a large capacity to excrete excess bicarbonate and so, for sustaining the metabolic alkalosis, the elevated HCO₃^–concentration must be maintained through an abnormal renal retention of HCO₃^–.

Compensatory respiratory acidosis may be so marked that pCO₂ may rise higher than 55 mmHg. Expected paCO₂is calculated as paCO₂ = [0.7 × HCO₃^–+ 21] ± 2 or 40 + [0.7 ΔHCO₃]. This is called compensated metabolic alkalosis.

Most of the patients with metabolic alkalosis can be treated with chloride ions in the form of NaCl (saline responsive) rather than KCl (which is preferable). When NaCl is given, Cl^–ions are supplied, and so the blood volume increases and the secretion of aldosterone in excess decreases. Thus, excessive urinary loss of K⁺and excessive reabsorption of HCO₃^– stops. When metabolic alkalosis is due to the effects of excessive aldosterone or other mineralocorticoids, the patient does not respond to NaCl (saline resistant) and requires KCl.

Based on the urinary chloride, metabolic alkalosis is divided into:

Chloride responsive or extracellular volume depletion (urinary chloride < 20)

• Vomiting
• Diuretic
• Post hypercapnic
• Chronic diarrhea
• Chloride resistant (urinary chloride > 20)
• Severe potassium depletion
• Mineralocorticoid excess–Primary hypealdosteronism, Cushing’s Syndrome, Ectopic ACTH
• Secondary hypereldosteronism–Renovascular disease, malignant hypertension, CHF, cirrhosis

Aproach to mixed disorder

Mixed metabolic disturbances (e.g., high AG from diabetic ketoacidosis plus normal AG from diarrhea) can be identified using the relationship between AG and HCO₃^–, which is called the gap–gap ratio. It is the ratio of change in anion gap (ΔAG) to change in HCO₃^– (ΔHCO₃^–).

When hydrogen ions accumulate in blood, the decrease in serum HCO₃^– is equivalent to the increase in AG and the increase in AG excess/HCO₃^– deficit ratio is unity, i.e., pure increase in AG metabolic acidosis. When a normal AG acidosis is present, the ratio approaches zero.

When a mixed acidosis is present (high AG + normal AG), the gap–gap ratio indicates the relative contribution of each type to the acidosis. If it is <1, then it suggests that there is a normal AG metabolic acidosis associated with it and if >2 it suggests that there is associated metabolic acidosis.

Rules for rapid clinical interpretation of ABG

When required to make a proper approach towards the evaluation of blood gas and acid–base disturbances in the body, the following scheme is suggested:

1. Look at pH – < 7.40 – Acidosis; > 7.40 – Alkalosis

2. If pH indicates acidosis, then look at paCO₂and HCO₃^–

3. If paCO₂is ↑, then it is primary respiratory acidosis

   1. To determine whether it is acute or chronic

      ΔH⁺ / ΔpaCO₂ <0.3–chronic

      >0.8–acute

      0.3-0.8–acute on chronic

   2. Calculate compensation by the respective methods

      Acute: [HCO₃^–] ↑ by 1 mEq/L for every 10 mmHg ↑ in paCO2 above 40.

      Chronic: [HCO₃^–] ↑ by 3.5 mEq/L for every 10 mmHg ↑ in paCO2 above 4

4. If paCO₂↓ and HCO₃^– is also ↓→ primary metabolic acidosis

   Calculate expected paCO₂as follows:

   paCO₂ = [1.5 × HCO₃+ 8] ± 2 metabolic acidosis only

   paCO₂ < expected paCO₂→ concomitant respiratory alkalosis.

   paCO₂ > expected paCO₂→ concomitant respiratory acidosis

5. If HCO₃^–is ↓, then AG should be examined.

6. If AG is unchanged → then it is hyperchloremic metabolic acidosis.

7. If AG is ↑ → then it is wide AG acidosis.

8. Check gap-gap ratio

   ΔAG/Δ HCO3^– = 1, pure increased AG metabolic acidosis

   <1 normal anion gap metabolic acidosis

   >2 associated metabolic acidosis.

9. If pH indicates alkalosis, then look at HCO₃^– and paCO₂.

10. If paCO₂is ↓ → then it is primary respiratory alkalosis.

    1. Whether it is acute or chronic (with the same formula as above)

    2. Calculate compensation by the respective methods:

       Acute: [HCO₃^–]↓ by 2 mEq/L for every 10 mmHg

       ↓ in paCO₂below 40.

       Chronic: [HCO₃^–] ↓ by 5 mEq/L for every

       10mmHg ↓ in paCO₂ below 40.

11. If paCO₂ ↑ and HCO₃^– also ↑ → then it is primary metabolic alkalosis.

    Calculate the expected paCO₂

    paCO₂ = [0.7 × HCO3^–+ 21] ± 2 Or 40 + [0.7 ΔHCO3] → metabolic alkalosis only

    paCO₂ < expected paCO2 → concomitant respiratory alkalosis.

    paCO₂ > expected paCO2 → concomitant respiratory acidosis

12. Check urinary chloride

    if urinary chloride < 20 → chloride responsive or ECV depletion

    if urinary chloride > 20→ chloride resistant

13. If pH is normal ABG may be normal or mixed disorder

    1. ↑paCO₂ and ↓HCO₃^–→ respiratory and metabolic acidosis

    2. (b) ↓paCO₂ and↑ HCO₃^–→ respiratory and metabolic alkalosis.

       Calculate % difference (ΔHCO₃^–/HCO₃^–and ΔpaCO₂/paCO₂) to see which is dominant disorder.

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