...HISTORY of illness for ABG as to "How is the Patient" is the first fact needed to be known before jumping to interpret the ABG (arterial blood gas) or VBG (venous blood gas) result!
...First step, HISTORY; second, PaO2 for oxygenation; third, pH for acidosis or alkalosis; fourth, PaCO2 for respiratory component and finally fifth, HCO3 & BE for renal and net balance!
...if history is not understood first, the Primary cause might be misinterpreted as the Compensatory or vice versa.
...say, a diabetic patient is having uncontrolled sugar despite being on insulin and requires supplemental oxygen. This history itself gives the impression that the Primary cause is Metabolic and Compensatory might be Respiratory. Moreover, the disease process is Chronic rather than Acute.
...Therefore, history indicates that either the kidneys are going wrong or the lungs are going wrong primarily and so the other one is trying to compensate; and whether the process is Acute or Chronic.
...next PaO2 (normal 80-100 mm Hg and SpO2 93-100%) must be seen to ascertain the state of oxygenation. The rule of thumb is to minus 10 from the percentage of oxygen being given to get the expected PaO2. Say, 40% oxygen is being given, the expected PaO2 will be 30 Kpa (1 Kpa = 7.5 mmHg). But, the patient's ABG shows say 18 Kpa, definitely the oxygenation of the patient is affected and the lungs are also involved. But, the Primary cause from the history is Metabolic and it is Chronic.
...thereupon, pH needs to be seen. Say, the pH is 7.2. Therefore, there is a tentative impression of Metabolic Acidosis.
...next to be seen is the PaCO2 (normal 35-45 mm Hg), which is say, 61 mmHg. Clearly, there is Respiratory Acidosis, may be due to Respiratory infection because there is reduced oxygenation as well.
...Lastly, the HCO3 (normal 22-26 mmol/L) and BE (normal -2 to +2 mmol/L) need to be seen. Generally, PaO2 and HCO3 move in the same direction because of the equation: CO2 + H2O = H2CO3 = H+ + HCO3 under the influence of carbonic anhydrase. As it is apparent from this equation that when CO2 will rise HCO3 will also rise to compensate and vice versa. This patient has PaCO2 61 mm Hg (high), so the HCO3 as well will be high, say 30 mmol/L and the BE -5 mmol/L.
...interplay between the Lungs and the Kidneys maintains the Acid-Base balance through connecting flow of blood and buffers in between. The Lungs try to excrete out the acid as CO2 and the Kidneys try to excrete out in urine the acid as H+ ion as in the equation above; with generation of HCO3 to mop up the remaining H+.
... Therefore, in the above equation there is mixed Metabolic and Respiratory acidosis with Compensatory Metabolic Alkalosis but having primarily Metabolic Acidosis because the BE is -5 mmol/L and the History is of diabetes, a Metabolic process, which is Chronic. Superimposed Respiratory infection is causing Acute Respiratory Acidosis.
...the Lungs can compensate within few minutes to hours but the Kidneys take hours and days to compensate. In the given example, the Kidneys have been compensating for a long time because of the chronicity of the diabetic illness. The Acute Respiratory Acidosis is also likely to be compensated by the Kidneys if the Lungs would be unable to excrete out the CO2.
...therefore, having ascertained the direction of acid-base disturbance (mixed metabolic and respiratory acidosis with compensatory metabolic alkalosis), the magnitude of the problem is also understood that the pH has fallen markedly to 7.2 and the BE is -5 mmol/L, which might get corrected with prompt management of diabetic ketoacidosis and the acute respiratory acidosis with ventilatory support (BiPAP) besides other measures.
ABG Interpretation:
https://abg.ninja/abg
ABG (Arterial Blood Gas) Practice Quizzes with Tic Tac Toe Method:
http://www.registerednursern.com/abg-arterial-blood-gas-practice-quizzes-with-tic-tac-toe-method/
Arterial Blood Gas Case Questions and Answers: http://www.fammed.usouthal.edu/Pulmonology/Self-StudyAids/ABGs/ABGCaseQuestions&Answers.pdf
6 Easy Steps to ABG Analysis:
https://herzing.blackboard.com/bbcswebdav/pid-5299257-dt-content-rid-11795236_1/courses/06-2132-A-PN108-1/ABGebook.pdf
...First step, HISTORY; second, PaO2 for oxygenation; third, pH for acidosis or alkalosis; fourth, PaCO2 for respiratory component and finally fifth, HCO3 & BE for renal and net balance!
...if history is not understood first, the Primary cause might be misinterpreted as the Compensatory or vice versa.
...say, a diabetic patient is having uncontrolled sugar despite being on insulin and requires supplemental oxygen. This history itself gives the impression that the Primary cause is Metabolic and Compensatory might be Respiratory. Moreover, the disease process is Chronic rather than Acute.
...Therefore, history indicates that either the kidneys are going wrong or the lungs are going wrong primarily and so the other one is trying to compensate; and whether the process is Acute or Chronic.
...next PaO2 (normal 80-100 mm Hg and SpO2 93-100%) must be seen to ascertain the state of oxygenation. The rule of thumb is to minus 10 from the percentage of oxygen being given to get the expected PaO2. Say, 40% oxygen is being given, the expected PaO2 will be 30 Kpa (1 Kpa = 7.5 mmHg). But, the patient's ABG shows say 18 Kpa, definitely the oxygenation of the patient is affected and the lungs are also involved. But, the Primary cause from the history is Metabolic and it is Chronic.
...thereupon, pH needs to be seen. Say, the pH is 7.2. Therefore, there is a tentative impression of Metabolic Acidosis.
...next to be seen is the PaCO2 (normal 35-45 mm Hg), which is say, 61 mmHg. Clearly, there is Respiratory Acidosis, may be due to Respiratory infection because there is reduced oxygenation as well.
...Lastly, the HCO3 (normal 22-26 mmol/L) and BE (normal -2 to +2 mmol/L) need to be seen. Generally, PaO2 and HCO3 move in the same direction because of the equation: CO2 + H2O = H2CO3 = H+ + HCO3 under the influence of carbonic anhydrase. As it is apparent from this equation that when CO2 will rise HCO3 will also rise to compensate and vice versa. This patient has PaCO2 61 mm Hg (high), so the HCO3 as well will be high, say 30 mmol/L and the BE -5 mmol/L.
...interplay between the Lungs and the Kidneys maintains the Acid-Base balance through connecting flow of blood and buffers in between. The Lungs try to excrete out the acid as CO2 and the Kidneys try to excrete out in urine the acid as H+ ion as in the equation above; with generation of HCO3 to mop up the remaining H+.
... Therefore, in the above equation there is mixed Metabolic and Respiratory acidosis with Compensatory Metabolic Alkalosis but having primarily Metabolic Acidosis because the BE is -5 mmol/L and the History is of diabetes, a Metabolic process, which is Chronic. Superimposed Respiratory infection is causing Acute Respiratory Acidosis.
...the Lungs can compensate within few minutes to hours but the Kidneys take hours and days to compensate. In the given example, the Kidneys have been compensating for a long time because of the chronicity of the diabetic illness. The Acute Respiratory Acidosis is also likely to be compensated by the Kidneys if the Lungs would be unable to excrete out the CO2.
...therefore, having ascertained the direction of acid-base disturbance (mixed metabolic and respiratory acidosis with compensatory metabolic alkalosis), the magnitude of the problem is also understood that the pH has fallen markedly to 7.2 and the BE is -5 mmol/L, which might get corrected with prompt management of diabetic ketoacidosis and the acute respiratory acidosis with ventilatory support (BiPAP) besides other measures.
ABG Interpretation:
https://abg.ninja/abg
ABG (Arterial Blood Gas) Practice Quizzes with Tic Tac Toe Method:
http://www.registerednursern.com/abg-arterial-blood-gas-practice-quizzes-with-tic-tac-toe-method/
Arterial Blood Gas Case Questions and Answers: http://www.fammed.usouthal.edu/Pulmonology/Self-StudyAids/ABGs/ABGCaseQuestions&Answers.pdf
6 Easy Steps to ABG Analysis:
https://herzing.blackboard.com/bbcswebdav/pid-5299257-dt-content-rid-11795236_1/courses/06-2132-A-PN108-1/ABGebook.pdf
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