A 45-year-old alcoholic male patient was admitted to the hospital for 2 weeks. He was being treated for pyogenic lung abscess. He seemed to be improving but became unwell again. A blood gas analysis showed pH 7.31, PaCO2 30 mmHg, PaO2 106 mmHg (0.3 FiO2), HCO3− 14 mmol/L, standard base excess (SBE) 15 mmol/L, Na+ 131 mmol/L, K+ 5 mmol/L, Cl− 96 mmol/L, osmolar gap 8 mmol/Kg, lactate 2 mmol/L, and albumin 3 g/dL.

Arterial blood gas analysis is an essential component of diagnosing and managing critically ill patients in the ICU. The proper understanding and application of the concepts of acid–base balance will help the clinician to follow the progress of the patient and also to evaluate the effectiveness of the treatment provided to them.

Step 1: Take an arterial blood sample

•If possible, take an arterial blood gas (ABG) sample at room air and start oxygen supplementation immediately.

•The radial artery is preferred for collecting the sample.

•Prefer to use 22-gauge needle.

•Avoid air bubbles.

•Cool the sample immediately. A. Potential sampling error.

•Air contamination—spurious increase in PO2.

•Duration of exposure is more important than volume of air bubbles.

•Expel air immediately.

•Discard the sample if froth is present.

R. Pandit, M.D., F.C.I.C.M. (*)

Department of Critical Care, Seven Hills Hospital, Mumbai, India e-mail: dr_rapandit@yahoo.com

B. Venous sample—absence of flash of blood on entry into the vessel and pulsations during syringe filling and absence of autofilling of the syringe.

•Venous admixture.

•Cross-check with pulse oximetry and clinical status.

Anticoagulant effects: Dilution error—drop in PCO2 and PO2. pH usually remains unchanged.

D.Metabolism: Blood cells consume O2, produce CO2, and lower pH. Magnitude of changes depends on the initial level.

Step 2: Take detailed history and do proper clinical examination

•Very often, it is the presenting symptom or signs which are a clue to the interpretation of the acid–base status.

•For example in a patient with vomiting, the primary acid–base problem could be metabolic alkalosis (due to loss of hydrochloric acid) as opposed to someone with diarrhea, whose primary problem could be metabolic acidosis (due to loss of bicarbonate ions).

•The important aspect to remember is that it is the underlying disorder of the patient which determines the acid–base status and not just the pH of the blood.

•A stepwise approach helps to interpret ABG correctly.

•Interpretation of serum electrolytes is also important for an accurate estimation of mixed acid–base disorders.

Step 3: Know the normal values

Step 4: Assess oxygenation (Ref Appendix 2)

•Look at oxygenation (PaO2 and SaO2).

•Look at the PaO2/FiO2 ratio.

–Normally, the ratio is around 1:400–1:500 given the fact that at 0.21 FiO2, the PaO2 is approximately 100 mmHg.

–Less than 1:400—suggestive of V–Q mismatch or diffusion defect or intracardiac shunt.

–Less than 300 with bilateral lung infiltrate in chest skiagram—ARDS.

•A-a gradient

–A-a gradient = PAO2 − PaO2

Here, PAO2 is alveolar PO2 (calculated from the alveolar gas equation) and PaO2

On room air and at sea level, the FiO2 is 0.21, the Patm is 760 mmHg, and PH2O is 47 mmHg.

– On room air, PAO2 can be calculated by:

150 − PaCO2 / 0.8

•Normal A-a gradient in a 20-year-old person is 5 mmHg, which increases to 10 mmHg in a 35-year-old person. If A-a gradient is 20 mmHg at any age, it is abnormal. If FiO2 is above 0.21, it is unreliable.

Step 5: Assess acid–base disorder (Ref Appendix 2)

I. Look at the pH—is there acidemia or alkalemia?

A normal pH would suggest a mixed disorder or a normal acid–base status.

II.Check CO2 and HCO3− to determine whether the primary problem is metabolic or respiratory in origin.

III.If the primary disorder is respiratory, determine whether it is an acute disorder or a chronic disorder.

IV. Apply the rules of compensation to know if it is a simple or a mixed disorder. V. Mind the gaps—anion gap, delta gap, and osmolar gap.

I.Look at the pH

The pH is actually the −log [H+]. By altering either the PCO2 or the HCO3−, [H+] will change, and so will pH.

•An acidemia (low pH) can result from either a low HCO3 or a high CO2.

•An alkalemia (high pH) can result from either a high HCO3 or a low CO2.

II. Look at the CO2 and HCO3− to determine if the primary problem is metabolic or respiratory in origin

The primary acid–base disturbances include the following:

•Low HCO3—metabolic acidosis

•High HCO3—metabolic alkalosis

•High PCO2—respiratory acidosis

•Low PCO2—respiratory alkalosis A. Metabolic acidosis

•Metabolic acidosis results from a primary decrease in plasma [HCO3−].

•It is due to either an excretion of bicarbonate-containing fluids or by utilization of bicarbonate.

•It is very important to calculate the anion gap (AG) if the primary disorder is metabolic acidosis.

•AG = Na+ − (Cl− + HCO3−); the normal AG is 12 ±2 mEq/L.

•A higher gap usually denotes the presence of unmeasured anions in the body (Table 58.1).

•In non-AG metabolic acidosis, the bicarbonate losses are accompanied by cation loss, hence no change in AG (Table 58.2).

460 R. Pandit

V.Mind the gaps

A1. Calculate AG in case of metabolic acidosis.

High denotes raised AG metabolic acidosis, and normal or narrow denotes nonAG acidosis.

A2. Calculate adjusted AG.

Adjusted AG = calculated AG + 2.5 × (4 − serum albumin in gm%)

B. In less obvious cases, the coexistence of two metabolic acid–base disorders may be apparent by calculating the difference between the change in AG

(delta AG) and the change in serum HCO3− (delta HCO3−). This calculation is called the bicarbonate gap or the delta gap:

•Delta gap = delta AG − delta HCO3−

•Where delta AG = patient’s AG − 12 mEq/L{normal AG}

•Delta HCO3− = 24 mEq/L{normal HCO3−} − patient’s HCO3−

•Normally, the delta gap is zero if there is only AG acidosis. A positive raised delta gap or a decreased delta gap denotes presence of mixed lesion.

•A positive delta gap of more than 6 mEq/L is suggestive of presence of metabolic alkalosis and/or HCO3− retention.

•The delta gap of less than 6 mEq/L is suggestive of presence of hyperchloremic acidosis and/or HCO3− excretion.