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Blood gas analyses are performed to evaluate the adequacy of ventilation, oxygenation, oxygen-carrying capacity of the blood and acid-base levels. The aims of doing a blood gas analysis are to detect 1. the presence and severity of hypoxemia and hyper(hypo)capnia and the amount of metabolic compensation. 2. changes in acid-base homeostasis, which might need further investigation and intervention.

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Important components of the Arterial Blood Gas Ph : Measurement of acidity or alkalinity, based on the hydrogen (H+) ions present. The normal range is 7.35 to 7.45 PO2 : The partial pressure of oxygen that is dissolved in arterial blood. The normal range is 80 to 100 mm Hg. SpO2 : The arterial oxygen saturation. The normal range is 95% to 100%. PaCO2 : The amount of carbon dioxide dissolved in arterial blood. The normal range is 35 to 45 mm Hg. HCO3 : The calculated value of the amount of bicarbonate in the bloodstream. The normal range is 22 to 26 mEq/liter


ACID – BASE DISORDERS METABOLIC ACIDOSIS Resulting from a primary deficit in the bicarbonate ion concentration or primary gain of strong acid, producing a lower pH and lower PaCO2. CAUSES : 1.Addition of excessive acid to ECF : DKA, lactic acidosis, methanol & salicylate poisoning 2.Failure to excrete normal acids : RF, distal RTA 3.Loss of bicarbonate : Prox RTA, diarrhoea.

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CLINICAL FEATURES : Resp : Hyperventillation (kussmaul’s respn), respiratory distress CVS : decreased CO & arterial BP, arrythmias Hyperkalemia Insulin resistance Fatigue & drowsy Increased bone resorption TREATMENT : Correction of underlying disease Dialysis for renal failure Bicarb correction

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METABOLIC ALKALOSIS Resulting from a promary gain in bicarb ion or loss of H+ ions resulting in elevated pH & elevated PaCO2 CAUSES : Vomiting, hypohalemia, diuretics, hyperaldosteronism, excessive ingestion of alkali. RESPIRATORY ACIDOSIS : Resulting from primary CO2 excess leading to decreases pH & increased PaCO2 CAUSE : head injury, COPD, Status asthmaticus,narcotics.

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RESPIRATORY ALKALOSIS : Excessive loss of CO2 due to overventillation resulting in increased pH and decreased PaCO2. Treatment : sedation, rebreathing from a paper bag, mechanical ventillation.


ANALYSIS OF ABG Step One Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. If it is above 7.45, the blood is alkalotic. If it is below 7.35, the blood is acidotic. Step Two If the blood is alkalotic or acidotic, we now need to determine if it is caused primarily by a respiratory or metabolic problem. To do this, assess the PaCO2 level. With a respiratory problem, as the pH decreases below 7.35, the PaCO2 should rise. If the pH rises above 7.45, the PaCO2 should fall. Compare the pH and the PaCO2 values. If pH and PaCO2 are indeed moving in opposite directions , then the problem is primarily respiratory in nature.

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Step three : Finally, assess the HCO3 value. Recall that with a metabolic problem, normally as the pH increases, the HCO3 should also increase. Likewise, as the pH decreases, so should the HCO3. Compare the two values. If they are moving in the same direction , then the problem is primarily metabolic in nature.

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Example 1 45-year-old female admitted to the casualty with a severe asthma attack. She has been experiencing increasing shortness of breath since admission three hours ago. Her arterial blood gas result is as follows: pH : 7.22 PaCO2 : 55 HCO3 : 25 ???????

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Respiratory acidosis

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A 55-year-old male admitted to your nursing unit with a recurring bowel obstruction. He has been experiencing intractable vomiting for the last several hours despite the use of antiemetics. Here is his arterial blood gas result: pH 7.50 PaCO2 42 HCO3 – 33 ????????????

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Metabolic alkalosis

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COMPENSATION …..How to identify????? Fully Compensated States : pH PaCO2 HCO3 Respiratory Acidosis normal, but <7.40 ↑ ↑ Respiratory Alkalosis normal, but >7.40 ↓ ↓ Metabolic Acidosis normal, but <7.40 ↓ ↓ Metabolic Alkalosis normal, but >7.40 ↑ ↑ Partially Compensated States : pH PaCO2 HCO3 Respiratory Acidosis ↓ ↑ ↑ Respiratory Alkalosis ↑ ↓ ↓ Metabolic Acidosis ↓ ↓ ↓ Metabolic Alkalosis ↑ ↑ ↑

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Example 3 A 59 yr old male is admitted to the hospital. He is a kidney dialysis patient who has missed his last two appointments at the dialysis center. His arterial blood gas values are reported as follows: pH : 7.32 PaCO2 : 32 HCO3 : 18 ??????????????

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Partially compensated metabolic acidosis

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A patient with chronic COPD being admitted for surgery. His admission labwork reveals an arterial blood gas with the following values: pH 7.35 PaCO2 48 HCO3 – 28 ????????

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Fully compensated respiratory acidosis

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John Doe is a trauma patient with an altered mental status. His initial arterial blood gas result is as follows: pH 7.33 PaC02 62 HC03 35 ????????

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Partially compensated respiratory acidosis

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A 54-year-old female admitted for an ileus. She had been experiencing nausea and vomiting. An NG tube has been in place for the last 24 hours. Here are the last ABG results: pH 7.43 PaC02 48 HC03 36 ??????

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Fully compensated metabolic alkalosis


CORRECTIONS Metabolic acidosis for every X decrease in HCO3, PCO2 decrease by acute : 1X chronic : 3X Metabolic alkalosis for every X raise in HCO3, PCO2 increase by acute : 2X chronic : 4X Resp acidosis 10mm Hg PCO2 inc = 1.2 mEq inc in HCO3 Resp alkalosis 10mm Hg PCO2 dec = 0.7 mEq dec in HCO3

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Thank You

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