Oxygen and Oxygenation Maureen A. Seckel Crit Care Nurse 2014, 34:73-74. doi: 10.4037/ccn2014745 © 2014 American Association of Critical-Care Nurses Published online http://www.cconline.org
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Critical Care Nurse is the official peer-reviewed clinical journal of the American Association ofCritical-Care Nurses, published bi-monthly by The InnoVision Group 101 Columbia, Aliso Viejo, CA 92656. Telephone: (800) 899-1712, (949) 362-2050, ext. 532. Fax: (949) 362-2049. Copyright © 2011 by AACN. All rights reserved. Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
Ask the Experts Oxygen and Oxygenation
Q
My 70-year-old patient is sedated and synchronous with the ventilator; set rate of 12/min, patient rate of 12/min, tidal volume 500 mL, fraction of inspired oxygen (FIO2) 35%, and positive endexpiratory pressure (PEEP) 5 cm H2O. Arterial blood gas (ABG) analysis shows a pH of 7.4, PaCO2 40 mm Hg, PaO2 90 mm Hg, HCO3 24 mEq/L, and SaO2
Author Maureen A. Seckel is a clinical nurse specialist, medical critical care/pulmonary, at Christiana Care Health System in Newark, Delaware. Corresponding author: Maureen A. Seckel, RN, APN, ACNS-BC, CCNS, CCRN, Christiana Care Health System, 4755 Ogletown-Stanton Road, Newark, DE 19711 (e-mail: [email protected]). To purchase electronic and print reprints, contact the American Association of Critical-Care Nurses, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, [email protected]. ©2014 American Association of Critical-Care Nurses doi: http://dx.doi.org/10.4037/ccn2014745
www.ccnonline.org
100%. The FIO2 is mistakenly turned up and left on 100% for 2 hours and the repeat ABG results are the same except the PaO2 is now 180 mm Hg. What should the PaO2 be, and where is the extra oxygen going?
A
Maureen A. Seckel, RN, APN, ACNS-BC, CCNS, CCRN,
replies: We know that PaO2 is a measure of the partial pressure of oxygen dissolved in arterial blood and measured by ABG analysis. Normal PaO2 values on room air for adults are between 80 and 100 mm Hg.1-3 What should a PaO2 be when a patient is on oxygen? Two clinical tools that are used to look at whether gas exchange is normal or abnormal are the alveolar oxygen level (PAO2) to arterial oxygen level (PaO2) change, or A-a oxygen gradient and the PaO2/FIO2 or P/F ratio.2,4-6 PAO2 is the measure of the partial pressure of oxygen in the alveoli after patient ventilation and is responsible for the oxygen diffusion from a higher concentration in the alveoli to a lower concentration in the pulmonary capillaries. Because there is no direct clinical
measurement, the PAO2 is a calculated number and normal values are usually between 10 and 20 mm Hg higher than the PaO2 but are also affected by age and FIO2. The normal A-a gradient increases with age (age divided by 4 plus 4) and increases approximately 5 to 6 mm Hg for every 10% increase in FIO2.5 The P/F ratio is an easier calculation and more reliable measure at a higher FIO2 with a normal range between 300 and 500 mm Hg. Values less than 300 mm Hg are indicative of impaired gas exchange.2,4,6 Was the patient’s gas exchange at an FIO2 of 35% abnormal even though her PaO2 was normal at 90 mm Hg?
Calculating the A-a Gradient 1. Calculate the PAO2 by using the following formula for the patient on an FIO2 of 35%: PAO2 = FIO2 × (Pbar - PH O) 2 (PaCO2/RQ ) FIO2: Fraction of inspired oxygen, as a decimal Pbar: barometric pressure = 760 mm Hg at sea level PH O: partial pressure of H2O = 2 47 mm Hg at 37ºC PaCO2: arterial carbon dioxide from ABG analysis RQ: respiratory quotient, or the ratio of carbon dioxide production to oxygen uptake = 0.8
CriticalCareNurse
Vol 34, No. 5, OCTOBER 2014
Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
73
For an FIO2 of 35% or 0.35 PAO2 = [0.35 × (760 - 47)] - (40/0.8) PAO2 = 249 - 50 PAO2 = 199 mm Hg
2. Calculate the A-a gradient A-a gradient = PAO2 - PaO2 A-a gradient = 199-90 A-a gradient = 109 mm Hg
3. Calculate the normal A-a gradient for the patient’s age of 70 years Normal A-a gradient = (Age/4) + 4 Normal A-a gradient = (70/4) + 4 Normal A-a gradient = 21.5 mm Hg
So the A-a gradient of 109 mm Hg is greater than the normal A-a gradient for a 70-year-old patient, which is 21.5 mm Hg.
Calculating the P/F ratio The P/F ratio is the ratio of the PaO2 to the FIO2. The ABG analysis showed that the patient’s PaO2 was 90 mm Hg at an FIO2 of 35% (0.35 in decimal form). Therefore the P/F ratio is 90/0.35, which equals 257. Discussion The patient’s A-a gradient was 109 mm Hg on an FIO2 of 35%, which is larger than the expected gradient of 21.5 for the patient’s age of 70 years. The P/F ratio is also lower at 257 than expected for an FIO2 of 35%. Although the patient has a normal PaO2 of 90 mm Hg at an FIO2 of 35%, her A-a gradient was clearly much larger than you would anticipate and her P/F ratio also was abnormal. So why was this? An abnormal A-a gradient and P/F ratio may be due to diffusion defects, ventilation-perfusion mismatch, or a shunt.1,5 Chronic disease
74
CriticalCareNurse
states such as interstitial lung disease impair alveolar oxygen diffusion across a thickened alveolar-capillary membrane. This impaired diffusion leads to chronic hypoxemia as not all the inspired oxygen is able to diffuse to the capillary bed. Diffusion defects usually respond to supplemental oxygen and are less likely to be the cause of an abnormal A-a gradient or abnormal P/F ratio.1,5,7 Ventilation-perfusion mismatch can occur when there is an imbalance or alteration between adequately ventilated and adequately perfused alveoli. There are multiple causes; for example, pathologic conditions that cause decreased ventilation such as atelectasis, pneumonia, or pulmonary edema. Examples of clinical conditions that lead to decreased perfusion include pulmonary embolism or cardiogenic shock.1,3 Shunt occurs when blood bypasses the alveoli and does not participate in gas exchange. Shunting can be due to cardiac or large vessel abnormalities, chronic obstructive pulmonary disease, acute respiratory distress syndrome, atelectasis, and pneumonia.1 The patient had abnormal gas exchange even before the FIO2 was inadvertently set too high for 2 hours. Higher than needed oxygen levels can lead to absorption atelectasis and oxygen toxicity, and in general, goals of oxygen therapy should be to keep PaO2 levels between 50 and 60 mm Hg.1 Additional assessment and review of the patient’s medical history and of the current hospitalization
(including indication for mechanical ventilation) and discussion with the provider team needs to take place to determine the clinical conditions leading to the patient’s abnormal A-a gradient and P/F ratio. CCN Financial Disclosures None reported.
References 1. Ellstrom K. The pulmonary system. In: Alspach JG, ed. AACN Core Curriculum for Critical Care Nurses. 6th ed. St Louis, MO: Saunders Elsevier; 2006:45-183. 2. Burns SM. Indices of oxygenation. In: Lynn-McHale Wiegand DJ, ed. AACN Procedure Manual for Critical Care. 6th ed. St Louis, MO: Elsevier Saunders; 2011:255-258. 3. St. John RE, Seckel MA. Airway and ventilatory management. In: Burns SM, ed. AACN Essentials of Critical Care Nursing. 3rd ed. New York, NY: McGraw-Hill; 2014:119-157. 4. Johnson KL. Diagnostic measures to evaluate oxygenation in critically ill adults. AACN Clin Issues. 2004;15:506-524. 5. Marino PL. The ICU Book. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2012. 6. Hennessey IAM, Japp AG. Arterial Blood Gases Made Easy. Philadelphia, PA: Elsevier; 2007. 7. Bartter TC, Pratter MR, Abourzgheib W, Irwin RS. Respiratory failure, part I: a physiologic approach to respiratory failure. In: Irwin RS, Rippe JM, eds. Intensive Care Medicine. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2011.
Ask the Experts Do you have a clinical, practical, or legal question you’d like to have answered? Send it to us and we’ll pass it on to our Ask the Experts panel. Questions may be mailed to Ask the Experts, Critical Care Nurse, 101 Columbia, Aliso Viejo, CA 92656; or sent by e-mail to [email protected]. Questions of the greatest general interest will be answered in this department each and every issue.
Vol 34, No. 5, OCTOBER 2014
Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
www.ccnonline.org
Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
Subscription Information http://ccn.aacnjournals.org/subscriptions Information for authors http://ccn.aacnjournals.org/misc/ifora.xhtml
Submit Manuscript www.editorialmanager.com/ccn
E-mail alerts http://ccn.aacnjournals.org/subscriptions/etoc.xhtml
Critical Care Nurse is the official peer-reviewed clinical journal of the American Association ofCritical-Care Nurses, published bi-monthly by The InnoVision Group 101 Columbia, Aliso Viejo, CA 92656. Telephone: (800) 899-1712, (949) 362-2050, ext. 532. Fax: (949) 362-2049. Copyright © 2011 by AACN. All rights reserved. Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
Ask the Experts Oxygen and Oxygenation
Q
My 70-year-old patient is sedated and synchronous with the ventilator; set rate of 12/min, patient rate of 12/min, tidal volume 500 mL, fraction of inspired oxygen (FIO2) 35%, and positive endexpiratory pressure (PEEP) 5 cm H2O. Arterial blood gas (ABG) analysis shows a pH of 7.4, PaCO2 40 mm Hg, PaO2 90 mm Hg, HCO3 24 mEq/L, and SaO2
Author Maureen A. Seckel is a clinical nurse specialist, medical critical care/pulmonary, at Christiana Care Health System in Newark, Delaware. Corresponding author: Maureen A. Seckel, RN, APN, ACNS-BC, CCNS, CCRN, Christiana Care Health System, 4755 Ogletown-Stanton Road, Newark, DE 19711 (e-mail: [email protected]). To purchase electronic and print reprints, contact the American Association of Critical-Care Nurses, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, [email protected]. ©2014 American Association of Critical-Care Nurses doi: http://dx.doi.org/10.4037/ccn2014745
www.ccnonline.org
100%. The FIO2 is mistakenly turned up and left on 100% for 2 hours and the repeat ABG results are the same except the PaO2 is now 180 mm Hg. What should the PaO2 be, and where is the extra oxygen going?
A
Maureen A. Seckel, RN, APN, ACNS-BC, CCNS, CCRN,
replies: We know that PaO2 is a measure of the partial pressure of oxygen dissolved in arterial blood and measured by ABG analysis. Normal PaO2 values on room air for adults are between 80 and 100 mm Hg.1-3 What should a PaO2 be when a patient is on oxygen? Two clinical tools that are used to look at whether gas exchange is normal or abnormal are the alveolar oxygen level (PAO2) to arterial oxygen level (PaO2) change, or A-a oxygen gradient and the PaO2/FIO2 or P/F ratio.2,4-6 PAO2 is the measure of the partial pressure of oxygen in the alveoli after patient ventilation and is responsible for the oxygen diffusion from a higher concentration in the alveoli to a lower concentration in the pulmonary capillaries. Because there is no direct clinical
measurement, the PAO2 is a calculated number and normal values are usually between 10 and 20 mm Hg higher than the PaO2 but are also affected by age and FIO2. The normal A-a gradient increases with age (age divided by 4 plus 4) and increases approximately 5 to 6 mm Hg for every 10% increase in FIO2.5 The P/F ratio is an easier calculation and more reliable measure at a higher FIO2 with a normal range between 300 and 500 mm Hg. Values less than 300 mm Hg are indicative of impaired gas exchange.2,4,6 Was the patient’s gas exchange at an FIO2 of 35% abnormal even though her PaO2 was normal at 90 mm Hg?
Calculating the A-a Gradient 1. Calculate the PAO2 by using the following formula for the patient on an FIO2 of 35%: PAO2 = FIO2 × (Pbar - PH O) 2 (PaCO2/RQ ) FIO2: Fraction of inspired oxygen, as a decimal Pbar: barometric pressure = 760 mm Hg at sea level PH O: partial pressure of H2O = 2 47 mm Hg at 37ºC PaCO2: arterial carbon dioxide from ABG analysis RQ: respiratory quotient, or the ratio of carbon dioxide production to oxygen uptake = 0.8
CriticalCareNurse
Vol 34, No. 5, OCTOBER 2014
Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
73
For an FIO2 of 35% or 0.35 PAO2 = [0.35 × (760 - 47)] - (40/0.8) PAO2 = 249 - 50 PAO2 = 199 mm Hg
2. Calculate the A-a gradient A-a gradient = PAO2 - PaO2 A-a gradient = 199-90 A-a gradient = 109 mm Hg
3. Calculate the normal A-a gradient for the patient’s age of 70 years Normal A-a gradient = (Age/4) + 4 Normal A-a gradient = (70/4) + 4 Normal A-a gradient = 21.5 mm Hg
So the A-a gradient of 109 mm Hg is greater than the normal A-a gradient for a 70-year-old patient, which is 21.5 mm Hg.
Calculating the P/F ratio The P/F ratio is the ratio of the PaO2 to the FIO2. The ABG analysis showed that the patient’s PaO2 was 90 mm Hg at an FIO2 of 35% (0.35 in decimal form). Therefore the P/F ratio is 90/0.35, which equals 257. Discussion The patient’s A-a gradient was 109 mm Hg on an FIO2 of 35%, which is larger than the expected gradient of 21.5 for the patient’s age of 70 years. The P/F ratio is also lower at 257 than expected for an FIO2 of 35%. Although the patient has a normal PaO2 of 90 mm Hg at an FIO2 of 35%, her A-a gradient was clearly much larger than you would anticipate and her P/F ratio also was abnormal. So why was this? An abnormal A-a gradient and P/F ratio may be due to diffusion defects, ventilation-perfusion mismatch, or a shunt.1,5 Chronic disease
74
CriticalCareNurse
states such as interstitial lung disease impair alveolar oxygen diffusion across a thickened alveolar-capillary membrane. This impaired diffusion leads to chronic hypoxemia as not all the inspired oxygen is able to diffuse to the capillary bed. Diffusion defects usually respond to supplemental oxygen and are less likely to be the cause of an abnormal A-a gradient or abnormal P/F ratio.1,5,7 Ventilation-perfusion mismatch can occur when there is an imbalance or alteration between adequately ventilated and adequately perfused alveoli. There are multiple causes; for example, pathologic conditions that cause decreased ventilation such as atelectasis, pneumonia, or pulmonary edema. Examples of clinical conditions that lead to decreased perfusion include pulmonary embolism or cardiogenic shock.1,3 Shunt occurs when blood bypasses the alveoli and does not participate in gas exchange. Shunting can be due to cardiac or large vessel abnormalities, chronic obstructive pulmonary disease, acute respiratory distress syndrome, atelectasis, and pneumonia.1 The patient had abnormal gas exchange even before the FIO2 was inadvertently set too high for 2 hours. Higher than needed oxygen levels can lead to absorption atelectasis and oxygen toxicity, and in general, goals of oxygen therapy should be to keep PaO2 levels between 50 and 60 mm Hg.1 Additional assessment and review of the patient’s medical history and of the current hospitalization
(including indication for mechanical ventilation) and discussion with the provider team needs to take place to determine the clinical conditions leading to the patient’s abnormal A-a gradient and P/F ratio. CCN Financial Disclosures None reported.
References 1. Ellstrom K. The pulmonary system. In: Alspach JG, ed. AACN Core Curriculum for Critical Care Nurses. 6th ed. St Louis, MO: Saunders Elsevier; 2006:45-183. 2. Burns SM. Indices of oxygenation. In: Lynn-McHale Wiegand DJ, ed. AACN Procedure Manual for Critical Care. 6th ed. St Louis, MO: Elsevier Saunders; 2011:255-258. 3. St. John RE, Seckel MA. Airway and ventilatory management. In: Burns SM, ed. AACN Essentials of Critical Care Nursing. 3rd ed. New York, NY: McGraw-Hill; 2014:119-157. 4. Johnson KL. Diagnostic measures to evaluate oxygenation in critically ill adults. AACN Clin Issues. 2004;15:506-524. 5. Marino PL. The ICU Book. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2012. 6. Hennessey IAM, Japp AG. Arterial Blood Gases Made Easy. Philadelphia, PA: Elsevier; 2007. 7. Bartter TC, Pratter MR, Abourzgheib W, Irwin RS. Respiratory failure, part I: a physiologic approach to respiratory failure. In: Irwin RS, Rippe JM, eds. Intensive Care Medicine. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2011.
Ask the Experts Do you have a clinical, practical, or legal question you’d like to have answered? Send it to us and we’ll pass it on to our Ask the Experts panel. Questions may be mailed to Ask the Experts, Critical Care Nurse, 101 Columbia, Aliso Viejo, CA 92656; or sent by e-mail to [email protected]. Questions of the greatest general interest will be answered in this department each and every issue.
Vol 34, No. 5, OCTOBER 2014
Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
www.ccnonline.org
Downloaded from http://ccn.aacnjournals.org/ at University of Pittsburgh, HSLS on November 11, 2014
