1.Monitoring of the Respiration:

a)Ventilation ( CO2 ).

b)Gas exchange in the lung ( PO2, SaO2 ).

2.Monitoring of Acid – Base status (pH, HCO3, BE).

How to do ABG

1) Sampling:

1.Preparation of the syringe with Heparin (fill the syringe dead space only)

2.Desinfection of the puncture aria(Radial, Dorsalis pedis, Posterior tibial, Femoral, Brachial)

Brachial and femoral arteries puncture are contraindicated in patients with abnormal haemostatic mechanism.

3.Puncture’s technique.

4.Press the puncture site firmly for 5 minutes.

5) How to treat the sample:

a)Remove all air bubbles.

b)Mix the blood with Heparin (rotate the syringe between two fingers).

c)Preferably the sample should be done immediately (accurate values ), otherwise put the sample in crushed ice ( properly ) till it has been done.

6) Complications :

a) Infection.

b) Bleeding.

c) Heamatom.

d) A-V fistula.

2) Capillary: (Indicated in babies and small children)

1.Take a special preheparinized tube.

2.Improve the skin circulation of the puncture site (Ear, Heel).

3.Technique. (Avoid air bobbles)


PaCO2 : increase Partial tension of Carbon Dioxide in Arterial Blood.

Normal Value 35 – 45 mmHg (40)

PaCO2Ý(Respiratory Acidosis )

1)Decrease of excretion :


b)Gas exchange ( COPD, ARDS ).

2)Over production ( Fever, TPN ).

PaCO2 ß ( Respiratory Alkalosis)


a)Metabolic acidosis.

b)Head injury.



2)Decrease of production:


b)Brian death.

PaO2: Partial Tension of Oxygen in Arterial blood .

Normal value : 70 – 100 mmHg

PaO2 = 102 – ( 0.33 X Age in years )

PaO2 Increase:

1)High FIO2.


PaO2 decrease:

1)Respiratory failure ( ARDS, COPD, Pulmonary edema, Lung collapse etc..)

2)Congenital heart disease.

3)Low cardiac output ( Chock ).


SaO2: Oxygen Saturation of Arterial blood.

The Percentage of Hemoglobin binds Oxygen

Normal value : 96 – 99 %

SaO2 increase

1)High FIO2.

SaO2 decrease

1)Lung diseases.

2)Low cardiac output.

3)Increase the amount of abnormal Hb (Met Hb, Carboxy Hb).

4)R – L Shunting diseases.

Acid – Base Balance


pH : Is the negative logarithm of Hydrogen Ions concentration.

pH = – Log( H+ )

Plasma pH = 7.35 – 7.45 ( 7.4 )

•Acids are H+ donors.

•Bases are H+ acceptors, or give up OH- in solution.

•Acids and bases can be:

–Strong – dissociate completely in solution

•HCl, NaOH

–Weak – dissociate only partially in solution

•Lactic acid, carbonic acid

The body produces more acids than bases

•Acids take in with foods

•Acids produced by metabolism of lipids and proteins

•Cellular metabolism produces CO2.

How can the body maintain his pH constant ?


A buffer : is a solution that resist changes in (H+)concentration upon addition of stronger acid or base.

A “ buffer pair “ is a combination of weak acid with his strong base ( as salt ) .

e.g. ( H2CO3 + NaHCO3 )


Buffering system in the body :





The aim of acid – base compensation is to keep the ratio PCO2 / HCO3 constant


HCO3 : Plasma Bicarbonate

Normal value : 23 – 28 mmol / L

HCO3 decrease

Metabolic Acidosis.


2)Diabetic ketoacidosis.

3)Intoxication .


5)Renal failure.

6)Intestinal obstruction.


HCO3 increase

Metabolic Alkalosis :



3)Loss of acid (vomiting, long standing NGT ).


•pH< 7.35 Acidemia

•pH > 7.45 Alkalemia

•The body response to acid-base imbalance is called compensation

•May be complete if brought back within normal limits

•Partial compensation if range is still outside norms.


•If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation.

•If problem is respiratory, renal mechanisms can bring about metabolic compensation.

Determine the degree of compensation


• pH is not within normal range

• The component that does not match the pH imbalance is still within its normal range


• pH is within the normal range and both components are either above or below normal range


• pH is not within normal range

• The component that does not match the pH disorder is above or below the normal range


Rates of correction

•Buffers function almost instantaneously

•Respiratory mechanisms take several minutes to hours

•Renal mechanisms may take several hours to days

Respiratory mechanisms

•Exhalation of carbon dioxide

•Powerful, but only works with volatile acids

•Doesn’t affect fixed acids like lactic acid

•CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-

•Body pH can be adjusted by changing rate and depth of breathing

Kidney excretion

•Can eliminate large amounts of acid

•Can also excrete base

•Can conserve and produce bicarb ions

•Most effective regulator of pH

•If kidneys fail, pH balance fails

Steps to Acid – Base Analysis


•– Acidotic, alkalotic, or normal?


– High ,or low , or normal ?

– Causing or compensating?


– High, low, or normal?

– Causing or compensating?

How To Read an ABG ?

pH : between 7.35 and 7.45 ( normal )

PCO2 > 45 R.Acidosis.

PCO2 < 35 R.Alkalosis.

HCO3 < 22 M.Acidosis.

HCO3 > 28 M.Alkalosis.

In most of these cases no need for correction .

If pH < 7.35 Acidemia

PCO2 > 45 mmHg respiratory .( correct ventilation )

HCO3 < 22 mmol metabolic ( NaHCO3 )

PCO2 > 45 mmhg + HCO3 < 22 Mixed

Sodium Bicarbonate ?

•Increase Acidosis intracellular (co2)

CO2 + H2O ↔ H2 CO3 ↔ H+ + HCO3-

•Increase Pco2

•Indication :

–Ph < 7.1

–BE > 10 mmol (Slow administration )

Caution : Hypokalemia

How To Read an ABG ?

If pH > 7.45 Alkalimia

PCO2 < 35 mmHg respiratory (correct hypocapenia)

HCO3 > 28 mmHg metabolic. (correct electrolyte imbalance, Diamox)

PCO2 <35 mmhg + HCO3 > 28 Mixed


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