Arterial Blood Gases Monitoring & Interpretation for Anesthesi

Abstract

Arterial blood gas (ABG) analysis is now common in perioperative and acute-care settings and is used to aid diagnosis and to monitor the progress of the patient and the response to any interventions. pH range compatible with life is (6.8–7.8). Body defenses against changes in [H+] concentration includes The chemical acid-base buffer system of the body fluids (bicarbonate buffer system, phosphate buffer system and proteins), Respiratory regulation and renal control. Acid-Base Disorders Respiratory acidosis: When PaCO2 is above 6kPa (45mmHg) decreasing pH below 7.35. Causes may be respiratory as pneumonia, pulmonary embolus, pulmonary edema, COPD. Or non-respiratory as altered level of consciousness, opiates, spinal cord injury. Symptoms include altered mental state, tachycardia, peripheral vasodilation, muscle twitching and cardiac arrhythmias. Respiratory alkalosis: When PaCO2 is below 4.5kPa (35mmHg) rising pH above 7.45. Causes include hyperventilation due to hypoxia, some neurological conditions and Over-ventilation with a mechanical ventilator. Symptoms are paraesthesia, impaired consciousness, arrhythmias and seizures. Metabolic acidosis: The pH falls below 7.35 and plasma bicarbonate level below 22mmol/L. Causes include excessive loss of [HCO3-], ingestion of acid or excess acid production as lactic acid, cardiac arrest, sepsis, liver failure and diabetic ketoacidosis. Symptoms are headache, fatigue, reduced level of consciousness, arrhythmias and Rapid-deep respiration. Metabolic alkalosis: the pH rises above 7.45 and plasma bicarbonate is above 26mmol/L. Causes include severe vomiting, Thiazide or loop diuretics or too much alkaline antacid medication. Symptoms are weakness, confusion, convulsions and reducing respiratory rate and depth. ABG sampling Blood gas sampling problems include air in the blood sample, venous sampling or admixture, anticoagulant effect, metabolism and effects of temperature. Continuous In-Vivo techniques is useful for patients requiring serial monitoring of arterial blood gases, an arterial catheter is inserted into the radial or femoral artery. It also allows for direct blood pressure monitoring. ABG Analyzers Considerations for purchasing ABG analyzers: - Technical considerations include size & weight of the machine, reagent & sensor electrodes, calibration and analytical quality control. - Operational considerations include facilities & resources, training, safety and connectivity. - Economic considerations: purchasers may buy analyzers from a single supplier for benefit of a single contract, larger price discounts, same training for all staff, and simplifying consumable orders. Also indirect costs for space, heating, electricity, waste disposal, syringes and gloves should be considered by purchasers. Anesthetic indications of arterial blood gas sampling: - Pre-operative indications include old patient, morbid obesity, smokers, pulmonary diseases, chronic renal insufficiency and gastrointestinal diseases. - Intraoperative indications include neuro-anesthesia, thoracic surgery of tier ii and tier iii, laparoscopic surgery, newborn and liver transplantation surgery. - Post-operative indications include delayed awakening, massive blood transfusion, septic patient, diagnosis of pulmonary embolism and diagnosis of TURP syndrome. Interpretation: - Assess pulmonary gas exchange and acid-base status independently. - Assessing pulmonary gas exchange through assessing PaO2 ([on room air] >10KPa or >80mmHg, [on supplemental O2] FiO2-PaO2<10KPa), PaCO2 (4.7–6KPa or 35-45mmHg) and [HCO3-] (22-28mmol/l). Classify gas exchange into one of the four possible categories, normal