1. Interpretation of the Arterial Blood Gas Mahler Beatrice, MD, PhD, University Assistant, Physiopathology and Immunology Chair I, UMF “Carol Davila”, UMF “Carol Davila”,

2. Introduction Arterial blood gas analysis is an essential part of diagnosing and managing a patient’s oxygenation status and acid- base balance.

3. pH The pH is a measurement of the acidity or alkalinity of the blood. It is inversely proportional to the number of hydrogen ions (H+) in the blood. – –The more H+ present, the lower the pH will be. – –Likewise, the fewer H+ present, the higher the pH will be

4. pH The normal blood pH range is 7.35 to 7.45. When the pH is below 7.35, the blood is said to be acidic, When the pH is above 7.45, the blood is said to be alkalotic. Significant changes in the blood pH above 7.8 or below 6.8 will interfere with cellular functioning, and if uncorrected, will lead to death.

5. Acidic state include Changes in body system functions – – in an a decreasein the force of cardiac contractions, – – a decrease in the vascular response to catecholamines, – – and a diminished response to the effects and actions of certain medications.

6. Alkalotic state interferes with tissue oxygenation, with normal neurological functioning, with normal muscular functioning.

7. Components of the Arterial Blood Gas The arterial blood gas provides the following values: – –pH-normal range is 7.35 to 7.45 – –PaO2 - The partial pressure of oxygen, – – The normal range is 80 to 100 mm Hg – –SaO2 - The arterial oxygen saturation. – –The normal range is 95% to 100%. – –PaCO2 - the carbon dioxide dissolved in arterial blood. – –The normal range is 35 to 45 mm Hg. – –HCO3 – bicarbonate – –The normal range is 22 to 26 mEq/liter – –B.E. - base excess – –The normal range is –2 to +2 mEq/liter. – –A negative base excess indicates a base deficit in the blood.

8. Steps to an Arterial Blood Gas Interpretation Acid-base evaluation requires a focus on three of the reported components: pH, PaCO2 and HCO3. This process involves three steps.

9. 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.

10. Step Two we now need to determine if it is caused primarily by a respiratory or metabolic problem, To do this, assess the PaCO2 level, 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. Remember that 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.

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

12. The relationships between pH, PaCO2 and HCO3. valuepHPaCO2HCO3 Respiratory Acidosis ↓↑ normal Respiratory Alkalosis ↑↓ normal Metabolic Acidosis ↓ normal ↓ Metabolic Alkalosis ↑ normal ↑

13. Acid-Base Disorders Respiratory Acidosis Respiratory Alkalosis Metabolic Acidosis Metabolic Alkalosis

14. Respiratory Acidosis pH less than 7.35 with a PaCO2 greater than 45 mm Hg, is caused by an accumulation of CO2 which combines with water in the body to produce carbonic acid, thus, lowering the pH of the blood, The hypoventilation can cause respiratory acidosis:   Central nervous system depression   Impaired respiratory muscle function   Pulmonary disorders   Massive pulmonary embolus   Hypoventilation due to pain, or abdominal distension.

15. The signs and symptoms of respiratory acidosis Pulmonary symptoms include dyspnea, respiratory distress, and/or shallow respirations. Nervous system manifestations include headache, restlessness, and confusion. If CO2 levels become extremely high, drowsiness and unresponsiveness may be noted. Cardiovascular symptoms include tachycardia and dysrhythmias.

16. Treatment Increasing ventilation will correct respiratory acidosis. The method for achieving this will vary with the cause of hypoventilation. If the patient is unstable, manual ventilation with a bagvalve-mask (BVM) is indicated until the underlying problem can be addressed. Causes that can be treated rapidly include pneumothorax, pain, and CNS depression related to medications. If the cause cannot be readily resolved, the patient may require mechanical ventilation while treatment is rendered. Although patients with hypoventilation often require supplemental oxygen, it is important to remember that oxygen alone will not correct the problem.

17. Respiratory Alkalosis Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 35 mm Hg. Any condition that causes hyperventilation can result in respiratory alkalosis. These conditions include: Psychological responses, such as anxiety or fear Pain Increased metabolic demands, such as fever, sepsis, pregnancy, or thyrotoxicosis Medications, such as respiratory

18. The signs and symptoms of respiratory alkalosis Nervous system alterations include light- headedness, numbness and tingling, confusion, inability to concentrate, and blurred vision. Cardiac symptoms include dysrhythmias and palpitations. Additionally, the patient may experience dry mouth, diaphoresis, and tetanic spasms of the arms and legs.

19. Treatment resolving the underlying problem, increased work of breathing and must be monitored closely for respiratory muscle fatigue.

20. Metabolic Acidosis Metabolic acidosis is defined as a bicarbonate level of less than 22 mEq/L with a pH of less than 7.35. Metabolic acidosis is caused by either a deficit of base in the bloodstream or an excess of acids, other than CO2. Diarrhea and intestinal fistulas may cause decreased levels of base. Causes of increased acids include: Renal failure Diabetic ketoacidosis Anaerobic metabolism Starvation Salicylate intoxication

21. Symptoms of metabolic acidosis Nervous system manifestations include headache, confusion, andrestlessness progressing to lethargy, then stupor or coma. Cardiac dysrhythmias are common Kussmaul respirations occur in an effort to compensate for the pH by blowing off more CO2. Warm, flushed skin, as well as nausea and vomiting are commonly noted.

22. Treatment is dependent upon the cause. The presence of metabolic acidosis should spur a search for hypoxic tissue somewhere in the body. Hypoxemia can lead to anaerobic metabolism system- wide, but hypoxia of any tissue bed will produce metabolic acids as a result of anaerobic metabolism even if the PaO2 is normal. The only appropriate way to treat this source of acidosis is to restore tissue perfusion to the hypoxic tissues. Other causes of metabolic acidosis should be considered after the possibility of tissue hypoxia has been addressed.

23. Metabolic Alkalosis Metabolic alkalosis is defined as a bicarbonate level greater than 26 mEq/liter with a pH greater than 7.45. Either an excess of base or a loss of acid within the body can cause metabolic alkalosis. Excess base occurs from ingestion of antacids, excess use of bicarbonate, or use of lactate in dialysis. Loss of acids can occur secondary to protracted vomiting, gastric suction, hypochloremia, excess administration of diuretics, or high levels of aldosterone.

24. Symptoms of metabolic alkalosis symptoms include dizziness, lethargy, disorientation, seizures and coma. Musculoskeletal symptoms include weakness, muscle twitching, muscle cramps and tetany. nausea, vomiting, respiratory depression.

25. Treatment Metabolic alkalosis is one of the most difficult acid-base imbalances to treat. Bicarbonate excretion through the kidneys can be stimulated with drugs such as acetazolamide,but resolution of the imbalance will be slow. It is significant to note that metabolic alkalosis in hospitalized patients is usually iatrogenic in nature.

26. Example 1 A.T.a 45-year-old female admitted to the nursing unit 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 Follow the steps: 1. Assess the pH. It is low (normal 7.35-7.45); therefore, we have acidosis. 2. Assess the PaCO2. It is high (normal 35-45) and in the opposite direction of the pH. 3. Assess the HCO3. It has remained within the normal range (22-26). Acidosis is present (decreased pH) with the PaCO3 being increased,reflecting a primary respiratory problem. For this patient, we need to improve the ventilation status by providing oxygen therapy, mechanical ventilation, pulmonary toilet or by administering bronchodilators.

27. Example 2 John Doe is 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, Follow the three steps again: 1. Assess the pH. It is high (normal 7.35-7.45), therefore, indicating alkalosis. 2. Assess the PaCO2. It is within the normal range (normal 35-45). 3. Assess the HCO3. It is high (normal 22-26) and moving in the same direction as the pH. Alkalosis is present (increased pH) with the HCO3 increased, reflecting a primary metabolic problem. Treatment of this patient might include the administration of I.V. fluids and measures to reduce the excess base.

28. Compensation Now see what happens when an acid-base imbalance exists over a period of time. When a patient develops an acid-base imbalance, the body attempts to compensate, The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the pH into the normal range.

29. The Respiratory Buffer Response A normal by-product of cellular metabolism is carbon dioxide (CO2). CO2 is carried in the blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid (H2CO3). The blood pH will change according to the level of carbonic acid present. This triggers the lungs to either increase or decrease the rate and depth of ventilation until the appropriate amount of CO2 has been re- established. Activation of the lungs to compensate for an imbalance starts to occur within 1 to 3 minutes.

30. The Renal Buffer Response In an effort to maintain the pH of the blood within its normal range, the kidneys excrete or retain bicarbonate (HCO3 -). As the blood pH decreases, the kidneys will compensate by retaining HCO3 -, and as the pH rises, the kidneys excrete HCO3- through the urine. Although the kidneys provide an excellent means of regulating acid-base balance, the system may take from hours to days to correct the imbalance. When the respiratory and renal systems are working together, they are able to keep the blood pH balanced by maintaining 1 part acid to 20 parts base.

31. Compensation A patient can be – –uncompensated, – –partially compensated, – –Or fully compensated.

32. Example H.T. 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, the pH - It is low the PaCO2 - It is low-Because the pH and PaCO2 are moving in thesame direction, it indicates that the acid-base disorder is primarily metabolic. In this case,the lungs, acting as the primary acid-base buffer, are now attempting to compensate by“blowing off excessive C02”, and therefore increasing the pH. the HCO3- It is low (normal 22-26). We would expect the pH and the HCO3- to move in the same direction, confirming that the primary problem is metabolic.

33. Steps one to Interpretation Assess the pH. This step remains the same and allows us to determine if an acidotic or alkalotic state exists.

34. Steps two Assess the PaCO2. In an uncompensated state, we have already seen that the pH and PaCO2 move in opposite directions when indicating that the primary problem is respiratory. But what if the pH and PaCO2 are moving in the same direction? That is not what we would expect to see happen. We would then conclude that the primary problem was metabolic. In this case, the decreasing PaCO2 indicates that the lungs, acting as a buffer response, are attempting to correct the pH back into its normal range by decreasing the PaCO2 (“blowing off the excess CO2”). If evidence of compensation is present, but the pH has not yet been corrected to within its normal range, this would be described as a metabolic disorder with a partial respiratory compensation.

35. Steps three Assess the HCO3. The pH and HCO3 move in the same direction, indicating that the primary problem was metabolic. But what if our results show the pH and HCO3 moving in opposite directions? That is not what we would expect to see. We would conclude that the primary acid-base disorder is respiratory, and that the kidneys, again acting as a buffer response system, are compensating by retaining HCO3, ultimately attempting to return the pH back towards the normal range.

36. Partially Compensated States valuepHPaCO2HCO3 Respiratory Acidosis ↓↑↑ Respiratory Alkalosis ↑↓↓ Metabolic Acidosis ↓↓↓ Metabolic Alkalosis ↑↑↑

37. Fully Compensated States valuepHPaCO2HCO3 Respiratory Acidosis normal, but <7.40 ↑↑ Respiratory Alkalosis normal, but >7.40 ↓↓ Metabolic Acidosis normal, but <7.40 ↓↓ Metabolic Alkalosis normal, but >7.40 ↑↑

38. REMEMBER! The only difference between partially and fully compensated states is whether or notthe pH has returned to within the normal range. In compensated acid-base disorders, the pH will frequently fall either on the low or high side of neutral (7.40). where the pH falls within the normal range is helpful in determining if the original acid-base disorder was acidosis or alkalosis.

39. How to evaluate the blood oxygenation? the arterial blood gas can also be used to evaluate blood oxygenation, this is the PaO2, The normal blood PaO2 value is 80-100 mm Hg. the blood oxygenation can evaluated by pulse oximetry

40. Oxyhemoglobin Dissociation Curve

41. Oxyhemoglobin dissociation curve The oxyhemoglobin dissociation curve is a tool used to show the relationship between oxygen saturation and the PaO2. The oxyhemoglobin dissociation curve can be used to estimate the PaO2 if the oxygen saturation is known. The illustration demonstrates that if the curve is not shifted, an oxygen saturation of 82% is equivalent to a PaO2 of about 60 mm Hg. With a left shift, the same saturation is equivalent to a much lower PaO2.

42. The evaluation of blood oxygenation Step Four Assess the PaO2. A value below 80 mm Hg can indicate hypoxemia, depending on the age of the patient. Correction of a patient’s blood oxygenation level may be accomplished through a combination of augmenting the means of oxygen delivery and correcting existing conditionsthat are shifting the oxyhemoglobin curve.

43. Pulse oximetry noninvasive method for the continuous measurement of oxygen saturation in arterial blood, noninvasive method for the continuous measurement of oxygen saturation in arterial blood, can be used in primary care to help diagnose cardiac and pulmonary disease, can be used in primary care to help diagnose cardiac and pulmonary disease,

44. Is used to Is used to to detect hypoxia before it is clinically obvious during procedures such as endoscopy, bronchoscopy, cardiac catheterization, and exercise and sleep testing. to detect hypoxia before it is clinically obvious during procedures such as endoscopy, bronchoscopy, cardiac catheterization, and exercise and sleep testing. PO is also used to monitor ICU patients on mechanical ventilation; PO is also used to monitor ICU patients on mechanical ventilation; in surgical units for all aspects of care, including anesthesia and postoperative recovery; in surgical units for all aspects of care, including anesthesia and postoperative recovery; and in labor and delivery departments for maternal and fetal monitoring. and in labor and delivery departments for maternal and fetal monitoring. to be a valuable tool during the transport of emergency patients, as well as in the emergency department. to be a valuable tool during the transport of emergency patients, as well as in the emergency department.

45. Principles of Pulse Oximetry The pulse oximeter probe, attached by wires to the monitoring device, can be designed to fit a variety of application sites, including the finger, nose, or earlobe. The pulse oximeter probe, attached by wires to the monitoring device, can be designed to fit a variety of application sites, including the finger, nose, or earlobe. On one side of the probe are two light-emitting diodes (LEDs) that alternately produce two specific wavelengths many times each second: Red light is transmitted at about 660 nm; and infrared light, at approximately 940 nm. A photo detector located on the opposite side of the probe measures the intensity of light transmitted through the tissue. The unique light absorption of hemoglobin in pulsating arteries can be distinguished from that in nonpulsating venous, capillary, and tissue components. On one side of the probe are two light-emitting diodes (LEDs) that alternately produce two specific wavelengths many times each second: Red light is transmitted at about 660 nm; and infrared light, at approximately 940 nm. A photo detector located on the opposite side of the probe measures the intensity of light transmitted through the tissue. The unique light absorption of hemoglobin in pulsating arteries can be distinguished from that in nonpulsating venous, capillary, and tissue components. Hemoglobin absorbs light according to the degree to which it is oxygenated. Oxyhemoglobin absorbs less red light and more infrared light than does deoxyhemoglobin. Hemoglobin absorbs light according to the degree to which it is oxygenated. Oxyhemoglobin absorbs less red light and more infrared light than does deoxyhemoglobin. Pulse oximeters are calibrated by their manufacturers with stored values using empiric data from healthy human volunteers. In the clinical setting, the absorption ratios of patients' oxygenated and deoxygenated hemoglobin wavelengths are compared with these stored values. In addition to pulse rate, the oxyhemoglobin saturation percentage as measured by PO (SpO2) is calculated and digitally displayed. Pulse oximeters are calibrated by their manufacturers with stored values using empiric data from healthy human volunteers. In the clinical setting, the absorption ratios of patients' oxygenated and deoxygenated hemoglobin wavelengths are compared with these stored values. In addition to pulse rate, the oxyhemoglobin saturation percentage as measured by PO (SpO2) is calculated and digitally displayed.