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Arterial Blood Gases Dr James F Peerless March 2013 Slide 2 Objectives Indications Acid-Base Physiology Procedure Interpretation Case Studies Other Useful Information Slide 3 What is an ABG? Blood test which measures the acid-base status and oxygen levels in the blood Calculated values pH P a O 2 P a CO 2 Other values now calculated or derived Commonly used in: Acute care Critical care Pulmonary medicine Slide 4 Indications Clinical indications Any unexpected deterioration in a patient Acute exacerbation of a chronic disease Impaired consciousness Impaired respiratory effort To Determine the cause of the illness Determine the severity of a condition Monitor the progress of a patient Slide 5 Acid-Base Balance Tight regulation required to ensure Enzyme function Ion distribution Protein structure Body pH maintained by several buffer systems Bicarbonate/ carbonic acid Phosphate Hb and plasma proteins Slide 6 Acid-Base Relationships The equation below shows the relationship between protons (H + ), bicarbonate (HCO 3 - ), water, and CO 2 H + + HCO 3 - H 2 CO 3 H 2 O + CO 2 In any given system, the reaction will tend towards an equilibrium, and the ratio of the reagents can be determined by knowing the dissociation constant and the pH. Slide 7 HendersonHasselbalch Equation In any given system, the reaction will tend towards an equilibrium, and the ratio of the reagents can be determined by knowing the dissociation constant and the pH. Slide 8 Acidaemia Slide 9 Alkalaemia Slide 10 Acid-base Regulation 4 components Initially Bicarbonate/ carbonic acid buffer system (MINUTES) Respiratory compensation Hyper/hypoventilation (MINUTES) Renal compensation Changes to H + and HCO 3 - secretion/retention (HOURS) Hepatic Ureagenesis (HOURS) AA metabolism HCO 3 - + NH 4 + 2HCO 3 - + 2NH 4 + NH 2 CONH 2 + CO 2 + 3H 2 O Slide 11 Blood Gas Analysis Slide 12 Acid-base Physiology Blood has a normal pH of 7.40 The normal range is between 7.35 and 7.45 Any pH that is lower than 7.35 is considered acidotic Acidosis: a state of being acidotic Acidaemia: a condition of having acidic blood Any pH that is higher than 7.45 is considered alkalotic Alkalosis: a state of being alkalotic Alkalaemia: a condition of having alkaline blood Slide 13 Normal Values pH7.35 7.45 P a CO 2 4.7 6.0 kPa P a O 2 >10 kPa HCO 3 - 22 26 mmol L -1 Base excess 2 mmol L -1 Slide 14 Primary Disturbance When determining the cause of acid-base disturbances, look at what process is the primary component pH 7.28 / P a CO 2 5.0 / HCO 3 - 18 pH 7.55 / P a CO 2 5.0 / HCO 3 - 38 pH 7.28 / P a CO 2 10.0 / HCO 3 - 24 pH 7.55 / P a CO 2 2.0 / HCO 3 - 24 Metabolic Alkalosis Metabolic Acidosis Respiratory Acidosis Respiratory Alkalosis Slide 15 Compensatory/Secondary Mechanism When one abnormal mechanism starts to push the pH into the abnormal range of either acidosis or alkalosis, a second process will try to push the pH back toward a normal value. Important features of compensation: Compensation is always in the opposite direction if the primary disturbance is respiratory, the secondary compensatory mechanism must be metabolic if the primary disturbance is metabolic, the secondary compensatory mechanism must be respiratory The compensation process never over-corrects the primary disturbance. If the pH appears to be over-corrected, there is an additional mixed primary disturbance. Slide 16 Compensation Respiratory compensation starts within 30 minutes and is maximal within 12 hours. Metabolic compensation takes about 3-5 days for maximal compensation. Kidneys are slower than lungs to make changes. Slide 17 The Davenport Diagram Displays the relationship between pH, P a CO 2 and HCO 3 - Explains the compensatory mechanisms that occur in acid-base balance. Slide 18 The Davenport Diagram Slide 19 Procedure Explanation to pt. Equipment Sterile gloves Chlorhexidine Heparinised syringe Allens Test Withdraw 1-2 mls Remove air, and cap off; pressure over puncture Get it to the machine within 10 minutes (iced samples: 1-2 hrs) Slide 20 Slide 21 Interpretation 8 Steps 1.Assess the patient 2.Identify the source Arterial, venous, (or mixed venous) 3.Check the pH Normal, acidaemia, alkalaemia 4.Assess the respiratory part pCO 2 (4.5 6 kPa) 5.Assess the metabolic part HCO 3 - (std) (22-26 mmol/L) Slide 22 Interpretation 8 Steps 6.Check the base excess the difference between the patients standard bicarbonate level and 24 normal range +/- 2 mmol/L 7.Check for compensation Complete or partial 8.Check the pO 2