Arterial blood gas analysis

Arterial blood gas analysis

Hypoxaemia characterised by a Pa02 <>

Hypoxia refers to inadequate oxygenation of the tissues. Causes include hypoxemia, (hypoxemic hypoxia), blood cell disorders (anaemic hypoxia) and cellular disorders (circulatory hypoxia) and cellular poisoning (histoxic poisoning).

Respiratory failure refers to the failure of efficient gaseous exchange. It is characterised by a PaO2 <>2 > 50 mm Hg breathing atmospheric air in the absence of an intra-cardiac shunt and primary metabolic alkalosis.

Causes

Neuromuscular	Depressant drugs, CVA, spinal chord injury, Guillian Barre Syndrome, Tetanus
Skeletal	Flail chest
Lung parenchymal	ARDS, pneumonia, contusion
Cardiac	Pulmonary oedema
Airways	Upper airways obstruction, COAD

Alkalosis

Cause HC03- ¯ (patient attempting to compensate

Respiratory PaC02¯

HC03- normal (patient not compensating).

Pa C02 ­ (patient attempting to compensate)

Metabolic HC03-­

PaC02normal (patient not compensating).

Acidosis

Cause HC03-­ (patient attempting to compensate)

Respiratory PaC02­

HC03- normal (no patient compensation)

PaC02¯ (patient attempting to compensate)

Metabolic HC03-¯

PaC02 normal (no patient compensation)

Normal arterial blood gas values

Pa02= 80-100mmhg

PaC02= 35-45mmhg

PH = 7.35-7.45

Sa02=95-100%

HC03= 22-26mEq/litre

Analyzing a blood gas sample

· Start with pH

Blood’s hydrogen ion concentration ( H+ ).

A pH value > 7.45 reflects an H + deficit. (Alkalosis) and a pH value < 7.35 reflects an H+ excess (acidosis).

· Investigate further by checking the PaCO2 value

Known as the respiratory parameter it reflects how efficiently the lungs are eliminating carbon dioxide. Values of < 35mmHg indicatives hyperventilation, and > 35mmhg indicates hypoventilation.

· Investigate further, assessing the HC03 value

Known as the metabolic parameter. The kidneys regulate the bicarbonate (a base) according to the body’s needs.

· Investigate further by checking the PaO2 value

Normal value should be FiO2 x 5

When to do an ABG

· 20 minutes following arrival in cardio-thoracic recovery from theatre

· 20 minutes following any adjustments to ventilation parameters.

· prior to weaning the patient and prior to extubation

· following a deterioration of the patient’s condition (Sa02 £90%).

· at least once a shift for long-term ventilated patients

· at the team leaders discretion, if the patient’s clinical condition changes.

How to do an ABG

· prior to drawing the ABG, ensure that the patient has not had bronchodilators, hyperoxygenation or endotracheal suction for at least 30 min

· wash your hands thoroughly and glove

· turn stopcock off to the patient, place a 5cc syringe, turn stopcock off to the flush bag and withdraw 5cc blood

· turn stopcock half way between the aspiration port and off to the patient, place heparinized syringe insitu, turn stopcock off to the flush bag and withdraw heparinized sample using a blood gas syringe

· turn stopcock off to the patient, flush line using transducer flush mechanism

· turn stopcock off to the sampling port, flush intra arterial line and place fresh orange bung to stopcock