Approach to Arterial Blood Gases S Sunanda
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IntroductionChapter 1

2
Arterial blood gases reflect the interaction between cardiovascular and pulmonary system. Arterial blood gases are an important diagnostic tool for the evaluation of critical patients. They are useful for determining the oxygenation status, Ventilatory status as well as the acid-base balance of the patient. The processes that alter the acid-base status of a patient can be divided into metabolic or respiratory processes.
 
NORMAL VALUES
(Of arterial blood gases at sea level and breathing room air)
pH
7.35 to 7.45
pCO2
35 to 45 mm Hg
pO2
Adult
80–100 mm Hg
Newborn
60–70 mm Hg
HCO3
22–26 mEq/lit
SaO2
Adult
> 95%
Newborn
40–90%
Base excess
−2 to +2 mmol/L
A – aO2 gradient
< 3.3 kPa (<25 mm Hg) if
(Alveolar-arterial pO2) FiO2 is 0.21 (i.e. on room air).
Tests include pO2, pCO2, pH and various derived parameters like…Base excess, Most benchtop analysers also measure hemoglobin, oxygen saturation, and met-and 3carboxy-hemoglobin. The concentration of oxygen being breathed, called the fractional concentration of inspired O2 (FiO2) should also be specified on request form.
Acidemia and alkalemia refer to alterations in blood pH metabolic processes are those that primarily alter the bicarbonate concentration (Alkali or base) in the blood. A decrease in serum bicarbonate leads to metabolic acidosis, while an increase in serum bicarbonate leads to a metabolic alkalosis.
Respiratory processes alter the pH of the serum by changing the carbon dioxide levels. CO2 accumulation causes an acid state in the blood, through carbonic acid (H2CO3). As the respiratory rate or tidal volume increases (Hyperventilation), CO2 elimination from the body also increases leading to respiratory alkalosis, while decrease in ventilation (Hypoventilation) leads to retention of CO2 in the form of H2CO3 (Carbonic acid) in the body leading to respiratory acidosis.
In conclusion, pH altering processes can be one of the four types. metabolic acidosis, metabolic alkalosis, respiratory acidosis and respiratory alkalosis.
A single disorder may account for observed acidemia or alkalemia, but often more than one disorder occur concurrently. These are referred to as mixed or complex acid-base disorders. For example, an alkalemic ABG may exhibit a mixed respiratory acidosis and a metabolic 4alkalosis. Identifying the simple as well as the complex acid-base disorders will be possible by applying the stepwise approach outlined in the next sections.
 
INDICATIONS FOR ABG ANALYSIS
  • For diagnoses of acute and chronic respiratory failure associated with COPD, severe ILD or sleep apnea, etc.
  • Diagnosis of Type I and Type II respiratory failure is made based on ABG analysis.
    TYPE I
    TYPE II
    Hypoxia PaO2 < 60
    Hypoxia PaO2 < 60
    With PaCO2 normal or <40
    Hypercarbia PaCO2 >50
  • For evaluation of central nervous system dysfunction, cardiovascular disorders, acid-base disturbances and for evaluation of dyspnea.
  • In management of patients on mechanical ventilators and during the weaning process from the ventilators.
  • In identifying the need for long-term oxygen therapy and to determine the effectiveness of therapy in patients on oxygen, on ventilators with supplemental oxygen.
  • Oxygen is toxic, high concentrations can damage lungs and eyes resulting in worsening of ARDS and retrolental fibroplasias in premature infants. Repeated ABG analysis helps in supplying the lowest possible inhaled oxygen concentration to a patient to maintain blood oxygen pressure (PaO2) at a level that keeps the patient safe.5
  • To measure the acid-base level in cardiac or renal failure, uncontrolled DM, drug overdose, or a severe infection.
  • To detect exposure to carbon monoxide and other chemicals.
 
SAMPLE COLLECTION
The test is performed by collecting a sample of blood from an artery. Site can be anesthetized if you desire. A small needle is inserted perpendicularly into the artery. Sample may be collected from the radial artery in the wrist, the femoral artery in the groin, or the brachial artery in the arm. If repeated samples are necessary, an “arterial line” is left inside the artery. In newborns, sample would be obtained through the umbilical artery catheter. If radial pulse is unobtainable (BP <60 systole), the sample is obtained from the femoral artery in the groin.
 
Allen's Test
Before blood is drawn, the circulation to the hand is tested if the wrist is the site.
Allen's test is a test used to determine the integrity of the blood supply to the hand.
With the hand elevated both the ulnar and the radial arteries are occluded, which leads to blanching of the hand. Then, one of the arteries is released and, in the normal case the blanching disappears over the whole of 6the hand. If the blanching persists it indicates inadequate blood supply. This is repeated with both arteries, in theory the whole of the blood supply of the hand can come from either artery.
The skin over the artery is cleaned first. Next, a small needle attached to a syringe is inserted through the skin and into the artery. Syringe contains heparin to prevent clotting of the blood. After the blood is drawn, pressure must be applied to the puncture site for at least 5 minutes (or little longer if patients are on anticoagulants) to completely stop the bleeding. Bandage can be applied if required. Ensure that there are no air bubbles in the blood sample taken and should be corked off and kept in ice to prevent exposure to air. The sample must be analyzed as immediately as possible.
 
PREREQUISITES FOR THE TEST
  • Enquire if the patient has any bleeding or clotting problems.
  • Enquire about use of medications such as aspirin, warfarin.
  • If the test is to be performed without oxygen, the oxygen must be turned off for 20 minutes before the sample is taken to ensure accurate test results.
  • If the patient is started on oxygen to know the effectiveness of therapy, sample should be drawn after 20 minutes.