Arterial blood gas analysis

In respiratory failure

Author : Dr. Krishan Chugh Head, Pediatric Intensive Care And Pulmonology Unit, Sir Ganga Ram Hospital New Delhi -110060 Ph Clinic 5418899, 5172202 Res 5931480, 5459988

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Introduction:

            Basic function of lungs is to exchange CO2 and O2 between blood and environment .  This function is extremely important for proper functioning of the body.  This function not only provides oxygen to body and removes CO2 from the body for normal metabolism, but also helps during abnormal functioning of kidneys by providing compensatory mechanism.  For gas exchange at alveolar level two components are involved - alveolar ventilation and perfusion of capillaries with blood.  Hence proper functioning of lungs requires matching of ventilation and perfusion.  Disturbance in ventilation / perfusion may affect the final gas exchange

Ventilation and perfusion of lungs:

            In an imaginary ideal situation all the alveoli of lungs are open and ventilated equally, at the same time the capillaries lining the alveoli are perfused with blood equally.  This does not occur even in a healthy person.  Generally, the perfusion of alveoli increases from craniocaudal direction while ventilation increase in reverse direction.  In disease states there may be decreased alveolar ventilation but perfusion remains same or vice versa.  Examples are pneumonia , collapse of alveoli, poliomyelitis or right to left shunt. If alveolar ventilation is maintained but perfusion of some of the alveoli is very poor or nil, no gas exchange will occur .  This is called dead space ventilation. This can be compensated by increasing the ventilation but ultimately decompensation sets in which then leads to respiratory failure.

Respiratory failure :

            Respiratory failure is defined as inability of lungs to deliver sufficient oxygen to meet the demand of body and / or to remove the carbon dioxide from the pulmonary circulation thereby leading to hypoxemia or hypercapnia or both.  It can be anticipated on clinical grounds but diagnosis is based on demonstration of paO2 < 60 mm Hg and / or paCO2 of > 50 mm of Hg on arterial blood gas analysis.  For consideration of intervention single blood gas values are not sufficient.  Clinical condition and serial blood gas values are of paramount importance for intervention.

            Traditionally, respiratory failure is classified into two types.  Type I is characterised by a low arterial pO2 with a normal or low arterial pCO2.  Type II failure is characterised by increased arterial pCO2 and concomitant hypoxemia.  It result from alveolar hypoventilation The difference in two types of respiratory failure is artificial, one may change to other.  Presence of hypoxemia / hypercapnia with a low    pH suggests acute respiratory failure. The body may try to compensate for acidosis.  Depending on the underlying disease, kidneys may be able to compensate for blood pH.  If inspite of hypercapnia blood pH is normal it suggests a compensated respiratory acidosis due to chronic respiratory failure.  A child with chronic respiratory failure may present with decompensation due to acute insult and needs immediate intervention.  For this purpose a pH < 7.3 in  a chronic respiratory failure indicates need for urgent intervention.

Terminologies in Arterial blood gas analysis and normal values :

1.  pH.  This indicates the acid base status of body.  Normal pH is between 7.35 - 7.45 .  A pH of < 7.3 indicates acidosis .  It may be because of increased CO2 (Respiratory)  or decreased bicarbonate ( Metabolic) or both ( mixed) . A pH of > 7.5 indicates alkalosis.  It may be because of washout of CO2 ( Respiratory ) or increased bicarbonates ( Metabolic) or both (Mixed).

2.  paCO2.  This indicates partial pressure of CO2 in arterial blood.  Normal values are between 35 - 45 mm of Hg .  A paCO2 value of < 30 indicates hyperventilation and is called hypocapnia .  A paCO2 values of > 50 indicates hypoventilation and is called hypercapnia.  An increase of 20 mm  in CO2 causes decrease of 0.1 unit in pH and a decrease of 10  mm of CO2 result in 0.1 unit in crease in pH.

3.  paO2.  This indicates partial pressure of oxygen in arterial blood.  Normal values depend on age.  For newborns it is between 50 - 70 mm of Hg, for older children 80  - 100 mm of Hg . A paO2 value of less than normal range is called hypoxemia.  Spurious hypoxemia is noticed in situations with increased cells, delay in processing, venous blood or in febrile patient.  Though for monitoring paO2 is universally used but it has limitations.  Traditionally, based on . paO2 values degree of hypoxia can be assesed. paO2 of 60 to 80 mm is labeled as mild hypoxia < 60 is moderate and < 40 mm of Hg is labeled as severe hypoxia.

4. Buffer Base ( BB) , Base Excess / Base deficit (EBE), Standard Base Excess (SBE), and Base Excess extracellular fluid (BE ecf)

Buffer base (BB) is the total buffers in the body which help in stabilising pH. Normal values of BB is 48 - 49  mmol / L . 50 % of BB is contributed by HCO3 , 25 % by Hb and rest 25 % by proteins, phosphates, sulphates etc.

BE  refers to the amount of variance from total buffer base (BB).  If BB is 40  mmol it means the buffer base is reduces by nearly 8 mmol / l or BE is 8.  (It is also called base deficit).  Since BE is an in vitro estimation of buffer base , it may not be true representative of buffer base in the body .  Because it is calculated by assumption that Hb is 15 gm / dl.  The latter may not be true for interstitial compartment.  Hence, the calculation of base excess / deficit taking in consideration the various body fluid compartments is called standard base excess(SBE).

5. HCO3 and Standard HCO3

HCO3 concentration depends on paCO2 values also.  An acute increase of 10 mm in paCO2 may cause increase in HCO3 by 1meq/L and an acute decrease of 10 mm in pCO2 may decrease HCO3 by 2 meq / L.

The TCO2 is sum of HCO3 and amount of CO2 dissolved in plasma.  For each mm Hg pCO2 0.03 ml CO2 is dissolved per 100 ml of plasma As HCO3 values change with CO2 level a standard HCO3 is used to denote the HCO3 independent of CO2 changes ( ie - HCO3 at pCO2 of 40 and temperature 37 degree C) . Normal values 22 - 26 meg /L

6. St pH:        It is the pH adjusted for temperature of 37 degree C and pCO2 of 40 mm of Hg.

O2 Saturation Proportion / percentage of Hb which is saturated with oxygen.  It can be known by blood gas analyser or by pulse oximeter. The pulse oximeter is noninvasive technique and measures peripheral hemoglobin O2 saturation (SaO2). Normal saturation is 95 - 98 % ( clinical cyanosis becomes evident if saturation is < 75 %).  At paO2 of 40 mm Hg 75 % of HbA is saturated , at 27 mm paO2 it is 50 % and at paO2 of 60 mm of Hg 90 % HbA is saturated .  Oxygen dissociation curve is sigmoid shaped which plateaus off at pO2 > 70 mm Hg.  Thus a patient with very high pO2 may have a saturation 97 - 99 % .  Hence SaO2 do not indicate hyperoxia.  The correlation of SaO2 and paO2 may be affected by change in type of Hb, pH, temperature and concentration of 2, 3 DPG.

AaDO2 : This is alveolar to arterial oxygen gradient Normal value is 5 - 15 mm of Hg.  It can be calculated as follows :-

            AaDO2           =          PAO2 - PaO2

                                                ( alveolar O2 - arterial O2)

            PaO2 is measured by ABG machine on blood while PAO2 is calculated by following formula:-

            PAO2                         =          (PB - PH2 O ) FiO2 - PaCO2

where PB       = Barometric pressure

            PH2O = Water vapour pressure

            FiO2   = O2 % in inspired O2  and

           paCO2= partial pressure of CO2 in blood

            Normally A-a Do2 is  < 10 mm and it is increased in RDS, meconium aspiration syndrome and persistent fetal circulation in newborns.  AaDo2 is about 200 mm Hg at Fio2 100 % .  Thus A-a Do2 helps in assessing the severity of disease and planning intervention.  The other method to use same value for assessment of severity is to calculate arterial to alveolar oxygen ratio (PaO2 / PAO2) which is about 0.8 in a healthy adult.  A value of < 0.6 indicates need for oxygen therapy while a value of < 0.15 indicates severe hypoxemia.

             Another calculation using AaDO2 and paO2 may be used for assessment of severity especially in newborn.  This is  Respiratory index ( RI)

            RI                     =          AaDO2 / PaO2

            RI > 1              =          Need for oxygen therapy

            RI > 1.8           =          Need for ventilation

            RI > 2              =          In a baby on ventilator contraindicates weaning

            RI > 5              =          Refractory hypoxemia.

INTERPRETATION OF ABG

For easy interpretation of ABG following steps are suggested

I           Step I - Look for pH.  Does it suggest acidosis or alkalosis

II           Step II - See paCO2.  Is it normal / Low / High

III          Step III -  See St BE Is it normal , low, high

IV         Step IV - See paO2 - Is it normal / Low / High  

(inc): Value Increases

(dec): Value Decreases

* Compensatory . Otherwise may be normal.  

(inc): Value Increases

(dec): Value Decreases

Collection of blood gas sample

            The arterial samples can be collected from radial , dorsalis pedis or  posterior tibial arteries.  Before puncturing radial artery it is advisable to perform `Allen Test’ to ensure collateral supply by ulnar artery.  Venous blood may be sampled for measurement of pH and HCO3 but is not suitable for pO2 and PCO2.  In newborns and young infants upto 8 - 10 weeks of age arterialised capillary samples can be taken from heel prick or ear lobules.

Precautions for collection of blood samples

1. Heparin is acidic and lowers pH. Use heparin of lower strength ( 1000 units/ ml instead of 5000 units / ml)

2. Use small volume of heparinised saline  just for lubricating syringe and plunger.  If volume is more , dissolved oxygen in heparinised saline may increase pO2.

3.  Avoid air bubble and let syringe fill spontaneously.

4. Previously it  was advised to use glass syringe as the old plastic syringes were permeable to air. However, the presently available plastic syringes can be used for arterial sampling.

5.  The sample should be processed immediately, preferably within 30 mintues.  The sample should be shaken, homogenised before putting in the machine .

Thank You.

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