ABG Analysis

Symbols used on ABG report and the Seven Step approach to ABG analysis    
Normal Arterial Blood Gas Values

 pH   7.35-7.45 

 PaCO2 35-45 mm Hg

 PaO2 70-100 mm Hg**

 SaO2 93-98%

 HCO3- 22-26 mEq/L

 MetHb  <2.0%

 COHb   <3.0%

 Base excess -2.0 to 2.0 mEq/L

 CaO2  16-22 ml O2/dl 

** Age-dependent
Symbols used on the ABG Reports


∎ pH(a)- 

pH is the negative logarithm of the hydrogen ion activity (pH = – logaH+)

pH stands for ‘potential of hydrogen’

Reference range (adult): 7.35 – 7.45 

Reference range (pediatric) 7.27-7.47 


Carbon dioxide partial pressure 

pCO2 in atmosphere 0.3 mmHg (can be considered as zero in comparison to expired air. Therefore, pCO2 is a direct reflection of the adequacy of alveolar ventilation in relation to the metabolic rate

pCO2 inversely proportional to the alveolar  ventilation 

PaCo2= VCO2x 0.863/ VA


Carbon Dioxide content of Blood

Normal Value Arterial Blood- 21.5mmol/L (48ml/dl)

Normal Value Mixed Venous Blood- 23.5mmol/L (52 ml/dl)

Only PCO2 is measured. HCO3 and CtCO2 is calculated from pCO2 and pH of the sample. 

ctO2 = (tHb x 1.36 x F O2Hb) + (Po2 x 0.003) 

mmHg to kPa conversion factor – multiply by 7.5

∎pO2- partial pressure of oxygen

Analytical methods for measuring oxygen saturation arterial blood gas analyzers  

1. pulse oximetry

2. CO-oximetry
Oxygen saturation

SpO2  noninvasively by pulse oximetry 

SaO2  Arterial blood gas analyzers calculate estimated oxygen saturation (O2sat) in a blood sample based on empirical equations using pH and PO2 values.

Co-Oximeter like the reports in the example directly measure SaO2 and therefore accurate.   


It is defined as the partial pressure of oxygen at which the oxygen carrying protein is 50% saturated

The P50 of normal adult haemoglobin is 26.6 mmHg.


cHCO3– is the concentration of bicarbonate (hydrogen carbonate) in the plasma of the sample. 

It is calculated using the measured pH and pCO2 values. 

The analyzer symbol cHCO3–(act).

What does cHCO3– tell you?

The actual bicarbonate is calculated by entering the measured values of pH and pCO2 in the Henderson-Hasselbalch equation.

Ka is the dissociation constant of the weak acid, pKa = log Ka, and [HA] and [A] are the molarities of the weak acid and its conjugate base.

∎cHCO3–(std) Standard bicarbonate

Concentration of hydrogen carbonate in plasma from blood which is equilibrated with a gas mixture with pCO2= 40 mmHg (5.3 kPa) and pO2 100 mmHg (13.3 kPa) at 37 °C

Eliminates the respiratory component in the acid base status

Low standard bicarbonate indicates a metabolic acidosis, and an elevated standard bicarbonate indicates a metabolic alkalosis.
∎Base Excess

Quantity (milliequivalent) of acid needed to titrate 1 L of blood to pH 7.4 at Temp 37C and PaCO2 40 mmHg. 

Assessment of the metabolic component of acid base disorders

In contrasted to the bicarbonate levels, the base excess is a calculated value intended to completely isolate the non-respiratory portion of the pH

Two types 

Blood – BE(b) 

Extra cellular fluid – BE(ecf)
∎ There are two calculations for base excess: 

BE(b) = (1 − 0.014 x hgb) x (cHCO3 − 24.8 + (1.43 x hgb + 7.7) x (pH −7.4)

BE(ecf) = cHCO3 − 24.8 +16.2 X (pH −7.4)

∎ BE (B)    [Blood] [Actual]

Actual base excess is the concentration of titratable base when the blood is titrated with a strong base or acid to a plasma pH of 7.40 at a pCO2 of 40 mmHg (5.3 kPa) and 37 °C at the actual oxygen saturation. 

Buffer base represents the Total Buffer Capacity in the blood, comprised of bicarbonate, hemoglobin, plasma proteins and phosphate

∎ BE (ecf) [Standard]

Standard base excess is an in vivo expression of base excess 

Standard base excess is the value when the Hemoglobin is at 5 g/dl

It refers to a model of the extra cellular fluid (one part of the blood is diluted by two parts of its own plasma) and is calculated using one third of the ctHb for blood in the formula
∎ What does cBase(Ecf) tell you?

cBase(Ecf) is the base excess in the total extracellular fluids, of which blood represents approximately one third. Buffering capacities differ in the extra-cellular compartments, which makes the cBase(Ecf) more representative of the in vivo base excess compared to actual BE(b).

Example: Base deficit with elevated anion gap indicates addition of acid (e.g., ketoacidosis)

Base deficit with normal anion gap indicates loss of bicarbonate (e.g., diarrhea). The anion gap is maintained because bicarbonate is exchanged for Chloride during excretion.

∎ pO2(A-a) Alveolar/Arterial Gradient

Alveolar/Arterial Gradient Alveolar Po2 = [(barometric pressure – water vapor pressure) × Fio2] – [1.25 × Pco2]- PaO2

∎ pO2 a/A Arterial/Alveolar Oxygen Ratio

The arterial/alveolar (a/A) ratio= PaO 2 /PA o 2 

The A/a gradient increases as the concentration of oxygen the patient inspires increases.

The a/A ratio is not dependent on Fio2 (Normal Range 0.8 to 0.9,    0.75 in the elderly)

Low a/A ˂ 0.6 indicates Shunt, V/Q mismatch or diffusion defect, 

˂ 0.35 weaning failure 

˂ 0.15 Refractory hypoxemia 


Hct  Fraction of the volume of erythrocytes in the volume of whole blood

tHb Total hemoglobin 

sO2  Oxygen saturation of hemoglobin

FO2Hb  Fraction of oxyhemoglobin in total hemoglobin in blood

FCOHb Fractional carboxyhemoglobin

FMetHb  Fractional Methemoglobin

FHHb  Fractional Deoxyhemoglobin


∎ BO2 (Oxygen capacity of hemoglobin) The maximum concentration of oxygen bound to hemoglobin in blood, saturated so that all deoxyhemoglobin is converted to oxyhemoglobin
ctO2(a) (Total content of oxygen in the arterial blood)

Na+ Sodium concentration

K+ Potassium concentration

Ca++ Calcium concentration 

Ca++(7.4) Calculated value of ionized calcium at pH 7.4

Cl- Chloride concentration 

AnGap Unmeasured anions in the plasma

Anion GapC  Na- Cl-HCo3

Anion Gap(K+) Na+K-Cl-HCO3

∎ Glucose(Concentration of glucose in plasma)

As both hyper and hypoglycemia can produce neurological damage, aggressive treatment of deviations in cGlu is warranted.

Lactate (L-Lactate)

Lactate(P) is the concentration of lactate in plasma

D-lactic acidosis Isomer produced by gut flora occur in patients with short bowel syndrome

D-lactate are not found in healthy individuals 

L-lactic acid  Isomer produced by humans and the acid responsible for lactic acidosis

ABG machines measure only L Lactate 
Lactate- A marker for sepsis and Trauma

Lactate is used as an indicator of impaired metabolism in trauma and sepsis patients may help emergency caregiver’s further diagnosis, risk stratify, and treat patients in the ED

Rising levels of lactic acid are associated with increased mortality independent of degree of organ dysfunction and shock at presentation

Lactic Acid Clearance= 

Initial lactate – subsequent lactate/initial lactate × 100 


Plasma osmolality & osmolarity

Osmolality- osmolality (with an “ℓ”) is a measure of the osmoles (Osm) of solute per kilogram of solvent (osmol/kg or Osm/kg)

Osmolarity (with an “r”) is defined as the number of osmoles of solute per liter (L) of solution (osmol/L or Osm/L).

Mnemonic- Remember the word R for Liter 

Osmolarity is affected by changes in water content, as well as temperature and pressure

In practice, there is almost negligible difference between the absolute values of the different measurements.

Both terms are often used interchangeably

Calculated osmolarity = 2 Na + 2 K + Glucose + Urea (all in mmol/L).

To calculate plasma osmolarity use the following equation : = 2[Na+] + [Glucose]/18 + [ BUN ]/2.8 where [Glucose] and [BUN] are measured in mg/dL.

Seven Step Approach To The Diagnosis Of An ABG

First Check whether the report is reliable 

H+ =24X CO2/HCO3
pH Approximate H+ concentration

7.60 25

7.55 28

7.50 32

7.45 35

7.40 40

7.35 45

7.30 50

7.25 56

7.20 63

7.15 71
Acid Base Status 

Step 1  Look for pH  Acidemic pH < 7.4

Alkalemic pH > 7.4

Step 2  Look for CO2 and HCO3

Is main disturbance metabolic or respiratory?

Step 3  If the primary disorder is respiratory, is it acute or chronic

 ΔpH= 0.008 x Δ PCO2 (Acute)

 ΔpH= 0.003 x Δ PCO2 (chronic)

Sometimes pH can be markedly altered due to other disorders

Acute hypercapnia      Δ H+ / ΔPaCO2 > 0.7

Chronic hypercapnia  Δ H+ / Δ PaCO2 < 0.3
Step 4  Calculate renal compensation to determine the metabolic deviation due to respiratory disorder 

For acidosis 

Early (6-24 hours) ΔHCO3= 1/10 X ΔPCO2

Late (1-4 days) ΔHCO3= 4/10 x  ΔPCO2

For alkalosis 

Early ΔHCO3= 2/10 X ΔPCO2

Late (2days) ΔHCO3= 5/10 x  ΔPCO2 

Decide- If not there is superimposed metabolic disturbances or the compensation is not yet complete
Step 5  If there is a metabolic acidosis is the Anion gap increased. 


 Determine the AG and narrow down your diagnosis
Step 6 If there is a metabolic disturbance is there appropriate respiratory  compensation

Metabolic acidosis (Winters formula)

Expected PaCo2= 1.5(HCO3)+ 8

Metabolic alkalosis

Expected PaCO2= 0.7(HCO3)+21

Step 7  Diagnosis of mixed acid base disorder

If anion gap is increased >20 there is an high anion gap acidosis
Is there a non anion gap acidosis?
1. Gap-Gap Δ AG/ Δ HCO3

If less than 1 there is a superimposed non anion gap metabolic acidosis

If more than 1.6 there is a superimposed metabolic alkalosis
2. Calculate Excess Anion Gap [HCO3 + (Anion gap-12)] = 24

If this value is less than 24 there is a hidden non anion gap acidosis 

If this value is more than 24 then there is a metabolic alkalosis hidden in the numbers
3. Bicarbonate gap

Δ AG- ΔHCO3= (Na-Cl-HCO3)-12—(27-HCO3)


If Bicarbonate gap is more than +6 there is a metabolic alkalosis hidden in the numbers

If Bicarbonate gap is less than – 6 then there is a metabolic acidosis hidden in the numbers 







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