Eric Dryver Emergency Department Skåne s University Hospital Lund Sweden pH log H Normal pH 738 742 High H leads to low pH pH lt 738 is called acidemia ID: 1034109
Download Presentation The PPT/PDF document "Acid-Base Interpretation" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
1. Acid-Base InterpretationEric DryverEmergency DepartmentSkåne’s University Hospital, Lund Sweden
2. pH = -log [H+]Normal pH: 7.38 – 7.42High [H+] leads to low pHpH < 7.38 is called “acidemia”High [H+] leads to high pHpH > 7.42 is called “alkalemia”
3. [H+] is determined by pCO2 and [HCO3-][H+] = 180 x pCO2 / [HCO3-]**Henderson-Hasselbalch Equation, pCO2 in kPa
4. [H+], CO2 and [HCO3-] are in a state of equilibrium[H+] + [HCO3-] ↔ [H2CO3] ↔ CO2 + H2OWhen one of the three values changes,it affects the other two
5. Law of Mass ActionChangeShiftResult↑ [HCO3-][H+] + [HCO3-] → [H2CO3] → CO2 + H2O↓ [H+]↓ [HCO3-][H+] + [HCO3-] ← [H2CO3] ← CO2 + H2O↑ [H+]↑ CO2[H+] + [HCO3-] ← [H2CO3] ← CO2 + H2O↑ [H+]↓ CO2[H+] + [HCO3-] → [H2CO3] → CO2 + H2O↓ [H+]
6. Metabolic DisordersIncrease in [HCO3-] leads to drop in [H+] (alkalemia)Alkalemia caused by increased [HCO3-] is called “metabolic alkalosis”Decrease in [HCO3-] leads to increase in [H+] (acidemia)Acidemia caused by decreased [HCO3-] is called “metabolic acidosis”
7. Respiratory DisordersIncrease in pCO2 leads to increased in [H+] (acidemia)Acidemia caused by increased pCO2 is called “respiratory acidosis”Decrease in pCO2 leads to decrease in [H+] (alkalemia)Alkalemia caused by decreased pCO2 is called “respiratory alkalosis”
8. Law of Mass ActionChangeShiftResult↑ [HCO3-][H+] + [HCO3-] → [H2CO3] → CO2 + H2O↓ [H+]↓ [HCO3-][H+] + [HCO3-] ← [H2CO3] ← CO2 + H2O↑ [H+]↑ CO2[H+] + [HCO3-] ← [H2CO3] ← CO2 + H2O↑ [H+]↓ CO2[H+] + [HCO3-] → [H2CO3] → CO2 + H2O↓ [H+]pH < 7.38[HCO3-] < 22Metabolic acidosispH < 7,38pCO2 > 5.7Respiratory acidosispH > 7.42[HCO3-] > 27Metabolic alkalosispH > 7,42pCO2 < 5.0Respiratory alkalosis
9. TerminologyAcidemia: pH < 7.38Alkalemia: pH > 7.42Acidosis: process that decreases pHAlkalosis: process that raises pH
10. Normal Arterial ValuespH7.38 - 7.42pCO25.0 - 5.7 kPaHCO3-22 - 26 mmol/LLactate< 2 mmol/L
11. Venous Blood GaspHAdd 0.03 to approximate arterial pHpCO2Remove 0.6 kPa to approximate arterial pCO2
12. Systematic Interpretation If venous values: + 0.03 to pH and - 0.6 from pCO2ADominant Disorder:Acidosis or alkalosis?Metabolic or respiratory?CID
13. Step 1: Dominant Disorder?pH < 7.38 +HCO3 < 22 mmol/L: metabolic acidosis pCO2 > 5.7 kPa: respiratory acidosispH > 7.42 +HCO3 > 27 mmol/L: metabolic alkalosispCO2 < 5.0 kPa: respiratory alkalosis
14. Replace Values?Venous blood gas (pO2 2.9)pH 7.21 arterialpCO2 4.8 arterialStep 1: Dominant Disorder?pH < 7.38 (acidemia)HCO3- < 22 mmol/LInterpretationMetabolic acidosis
15. Replace Values?Presumed arterial blood gasStep 1: Dominant Disorder?pH < 7.38 (acidemia)pCO2 > 5.7 kPaInterpretationRespiratory acidosis
16. Replace Values?Arterial blood gasStep 1: Dominant Disorder?pH > 7.42 (alkalemia)HCO3- > 27 mmol/LInterpretationMetabolic alkalosis
17. Replace Values?Arterial blood gasStep 1: Dominant Disorder?pH > 7.42 (alkalemia)pCO2 < 5.0 kPaInterpretationRespiratory alkalosis
18. Replace Values?Presumed arterial blood gasStep 1: Dominant Disorder?pH < 7.38 (acidemia)pCO2 > 5.7 kPaHCO3- < 22 mmol/LInterpretationThe patient has both of the following:respiratory acidosismetabolic acidosis
19. pH between 7.38 and 7.42If the pH is normal, there are two possibilities:No acid-base disturbance≥ 2 acid-base disturbances
20. Replace Values?Presumably arterial blood gasStep 1: Dominant Disorder?pH normalpCO2 > 5.7 kPaHCO3- > 26 mmol/LInterpretationThe patient has both of the following:respiratory acidosismetabolic alkalosis
21. Replace Values?Arterial blood gasStep 1: Dominant Disorder?pH normalpCO2 < 5.0 kPaLactate > 2 mmol/LInterpretationThe patient has both of the following:respiratory alkalosismetabolic (lactic) acidosis
22. Replace Values?pH 7.40 arterialStep 1: Dominant Disorder?Arterial pH 7.40 (normal)pCO2 6 kPa (slightly elevated)HCO3- 24 mmol/L (normal)InterpretationNo apparent acid-base disturbances
23. Systematic Interpretation If venous values: + 0,03 to pH and - 0.6 from pCO2ADominant Disorder:Acidosis or alkalosis?Metabolic or respiratory?CCompensationID
24. Step 2: Compensation?
25. Respiratory CompensationIn the setting of a metabolic disorder, there should be a respiratory compensationThe respiratory compensation is immediateThe respiratory compensation is proportional to the metabolic disorder
26. Respiratory CompensationYou sprint 100mYou develop a metabolic acidosis (lactate)You hyperventilate (not a disorder, rather a normal physiological response)The degree of hyperventilation is proportional to the degree of acidosisPicture: https://movietvtechgeeks.com/wp-content/uploads/2016/08/usain-bolt-at-rio-olympics.jpg
27. Expected CompensationMetabolic Disturbance∆ pCO2 = SBE x 0.1+/- 1
28. Expected respiratory compensationto a metabolic acidosisSBE-6∆ pCO2Expected pCO2 SBE x 0.1 = -0.65.3 + ∆ pCO2 = 4.7
29. Expected respiratory compensationto a metabolic alkalosisSBE8∆ pCO2Expected pCO2 SBE x 0.1 = 0.85.3 + ∆ pCO2 = 6.1
30. Replace Values?Arterial blood gasStep 1: Dominant Disorder?Metabolic acidosisStep 2: Compensation?Expected ∆ pCO2: SBE x 0.1 (+/- 1)Expected ∆ pCO2: -29 x 0.1 = -2.9Expected pCO2: 5.3 – 2.9 = 2.4 (+/-1)Actual pCO2: 1.9InterpretationExpected respiratory compensationNo apparent respiratory disorder
31. Replace Values?pH: 6.79pCO2: 3.7Step 1: Dominant Disorder?Metabolic acidosisStep 2: Compensation?Expected ∆ pCO2: -27 x 0.1 = -2.7Expected pCO2: 5.3 – 2.7 = 2.6 (+/- 1)Actual pCO2: 3.7InterpretationThe patient also has a respiratory acidosis
32. Replace Values?Arterial blood gasStep 1: Dominant Disorder?Metabolic acidosisStep 2: Compensation?Expected ∆ pCO2: -24 x 0.1 = -2.4 (+/- 1)Expected pCO2: 5.3 – 2.4 = 2.9 (+/-1)Actual pCO2: 1.1InterpretationThe patient also has a respiratory alkalosis
33. Replace Values?Arterial blood gasStep 1: Dominant Disorder?Metabolic alkalosisStep 2: Compensation?Expected ∆ pCO2: 13.5 x 0.1 = 1.3 (+/-1)Expected pCO2: 5.3 + 1.3 = 6.6 (+/-1)Actual pCO2: 6.3InterpretationAppropriate respiratory compensationNo apparent respiratory disorder
34. Metabolic CompensationIn the setting of a respiratory disorder, a metabolic compensation can occurThe metabolic compensationen takes 3-5 days to fully developThe metabolic compensation is proportional to the degree of the chronic respiratory disorder
35. Expected CompensationMetabolic Disturbance∆ pCO2 = SBE x 0.1+/- 1Acute Respiratory DisturbanceSBE = 0+/- 3Chronic Respiratory DisturbanceSBE = ∆ pCO2 x 3+/- 3
36. Chronic vs Acute?How can you distinguish between:chronic respiratory acidosis with metabolic compensationacute respiratory acidosis + metabolic alkalosisYou cannot distinguish between these simply based on the blood gas results.You need to take into consideration the clinical context (Step 4).
37. Expected SBE withchronic respiratory alkalosisExpected SBE = ∆ pCO2 x 3 (+/- 3) mmol/L 5.33.3∆ pCO2 = -2.0-6
38. Expected SBE withchronic respiratory alkalosisExpected SBE = ∆ pCO2 x 3 (+/-) 3 mmol/L 5.37.3∆ pCO2 = 2.06
39. Replace Values?Arterial blood gasStep 1: Dominant Disorder?Respiratory acidosisStep 2: Compensation?∆ pCO2: 12.1 – 5.3 = 6.8Expected SBE if acute: 0Expected SBE if chronic: 6.8 x 3 = 20Actual SBE: 3.4InterpretationRespiratory acidosis most likely acute
40. Replace Values?Arterial blood gasStep 1: Dominant Disorder?Respiratory acidosisStep 2: Compensation?∆ pCO2: 8.0 – 5.3 = 2.7Expected SBE if acute: 0Expected SBE if chronic: 2.7 x 3 = 8Actual SBE: 6.6InterpretationRespiratory acidosis likely chronic
41. Akut respiratorisk alkalosReplace Values?Arterial blood gasStep 1: Dominant Disorder?Respiratory alkalosisStep 2: Compensation?∆ pCO2: 1.7 - 5.3 = -3.6Expected SBE if acute: 0Expected SBE if chronic: -3.6 x 3 = -11Actual SBE: -5InterpretationRespiratory alkalosis acute on chronic
42. Systematic Interpretation If venous values: + 0,03 to pH and - 0.6 from pCO2ADominant Disorder:Acidosis or alkalosis?Metabolic or respiratory?CCompensation:Expected ∆ pCO2 if metabolic disorder: SBE x 0.1 (+/- 1)Expected SBE if acute respiratory disorder: 0Expected SBE if chronic respiratory disorder: ∆ pCO2 x 3 (+/- 3)IIonsD
43. Step 3: IonsCalculate the Anion Gap (AG)Estimate the ∆ AGCalculate ∆ AG + HCO3-
44. Anion GapThe total charge of cations always equals the total charge of anions (electroneutrality principle)The albumin present in blood is negatively chargedNa+, Cl- & HCO3- are the ions with large valuesNa+ has to be greater than (Cl- + HCO3-) to compensate for the negatively charged albumin
45. Anion Gap & ElectroneutralityAG: Na+ - Cl- - HCO3-
46. Normal AG8 (+/- 2) mmol/LDepends on how Na+, Cl- & HCO3- are measured
47. Standard HCO3-The HCO3- that is usually provided is the “standard bicarbonate” “cHCO3- (P,st)”The standard bicarbonate provides a superficial estimate of the metabolic contribution to pH changeIt is calculated according to complex formulas
48. Standard HCO3-
49. Actual HCO3-If we want to do more than a superficial interpretation by calculating the anion gap, we need the actual (real) bicarbonate that is determined by pH and pCO2If pCO2 is +/- 2 kPa from 5.3, then standard bicarbonate is +/-2 mmol/L from actual bicarbonateIf pCO2 is < 3.3 kPa or > 7.3 kPa, replace standard bicarbonate with actual bicarbonate
50. [H+] is determined by pCO2 and [HCO3-][H+] = 180 x pCO2 / [HCO3-]**Henderson-Hasselbalch Equation, pCO2 in kPa
51. http://www-users.med.cornell.edu/~spon/picu/calc/basecalc.htmpCO2 in mm Hg 1 kPa = 7.5 mm Hg
52.
53. Standard HCO3: 21Actual HCO3: 37Anion gap usingstandard HCO3: 27Anion gap usingactual HCO3: 11
54. Standard HCO3: 21Actual HCO3: 37Anion gap usingstandard HCO3: 27Anion gap usingactual HCO3: 11
55. Why bother with AG?Narrows the differential diagnosis of the metabolic acidosis
56. Two Types of Metabolic AcidosesIncreased AGMetabolic AcidosisHyperchloremicMetabolic AcidosisNormal AGNo metabolic acidosis
57. DDx Metabolic Acidosis Increased AGMethanol, MetforminUremiaDiabetic ketoacidosPropylene glycol, Pyroglutamic acidIron, IsoniazidLactate (L or D)Ethylene glycol, Ethanol ketoacidosSalicylates, solvents, starvation ketoacidosisHCMALoss of HCO3- fromBowel (e.g. diarrhea)Kidney (e.g. RTA)Administration of lots of NaCl
58. ∆ AG∆ AG = Actual AG - Normal AGIn the presence of a ∆ AG, extra anions are present, i.e. the patient has an increased AG metabolic acidosisIt is the ∆ AG that needs to be explained, not the whole AG
59. Why bother with ∆ AG?Reveals whether a metabolic acidosis is being concealed by a concurrent metabolic alkalosisReveals whether the lactate accounts for all or part of the increased AGA low, even negative anion gap can reveal another disorder
60. Normal AGNormal HCO3Normal pHNo disturbancesIncreased AGDecreased HCO3Decreased pHMetabolic acidosisNormal AGIncreased HCO3Increased pHMetabolic alkalosisIncreased AGNormal HCO3Normal pHMetabolic acidosis + Metabolic alkalosisSodium Chloride HCO3 Anion Gap
61. ∆ AG + HCO3-1 mmol HCO3- is consumed by 1 mmol of added anionIf the patient has a ∆ AG of 10 mmol/L, the HCO3- has dropped by roughly 10 mmol/LActual HCO3 + ∆ AG can be thought of as HCO3- ’prior’ to acquisition of extra anions
62. ∆ AG + HCO3-∆ AG + HCO3- ≈ 24 mmol/L: no sign of an additional metabolic disturbance∆ AG + HCO3- > 26 mmol/L: background metabolic alkalosis or compensation for a chronic respiratory acidosis∆ AG + HCO3- < 22 mmol/L: background hyperchloremic metabolic acidosis (HCMA)
63. Replace Values?pH: 7.40pCO2: 4.6Step 1: Dominant Disorder?Respiratory alkalosis (mild)Metabolic acidosis (lactate; mild)Step 2: Compensation?Two disturbances balancing out each otherNo compensationStep 3: Ions?AG: 141 – 92 – 21 = 28Assuming AG 8, ∆ AG is 20∆ AG + HCO3- = 41Interpretation Metabolic acidosis: DKA (glucose 28)?Metabolic alkalosis (vomiting? dehydration?)
64. Replace Values?Arterial blood gasStep 1: Dominant Disorder?Respiratory alkalosis (mild)Metabolic acidosis (lactate)Step 2: Compensation?Two disturbances balancing out each otherNo compensationStep 3: Ions?AG: 128 – 79 – 23 = 26Assuming AG 8, ∆ AG is 18∆ AG + HCO3- = 41Interpretation Metabolic acidosisMetabolic alkalosis
65. Replace Values?pH: 7.21pCO2: 4.8Step 1: Dominant Disorder?Metabolic acidosisStep 2: Compensation?Expected ∆ pCO2: -12 x 0.1 = -1.2 (+/- 1)Expected pCO2: 5.3 – 1.2 = 4.1 (+/-1)Actual pCO2: 4.8No apparent respiratory disorderStep 3: Ions?AG: 135 – 101 – 13 = 21Assuming AG 8, ∆ AG is 13∆ AG + HCO3- = 26Interpretation Lactate accounts for most of ∆ AGMetabolic acidosis due to lactic acidosis
66. Replace Values?Assuming venous blood gaspH 7,36pCO2 3,7Step 1: Dominant Disorder?Metabolic acidosisStep 2: Compensation?Expected ∆ pCO2: -7 x 0.1 = -0.7 (+/-1)Expected pCO2: 5.3 – 0.7 = 4.6 (+/-1)Actual pCO2: 3,7Borderline respiratory alkalosisStep 3: Ions?AG: 145 – 117 – 19 = 9Assuming AG 8, ∆ AG is 1Interpretation Hyperchlormic metabolic acidosisBordernline respiratory alkalosis
67. DDx Low/Negative Anion GapMnemonic LIMBS:Low albumin, Li2+IodideMyelomaBromideSalicylates
68. 54-Year-OldBipolar DiseaseFound UnconsciousReplace Values?Arterial blood gasStep 1: Dominant Disorder?Metabolic acidosisStep 2: Compensation?Expected ∆ pCO2: -8 x 0.1 = -0.8Expected pCO2: 5.3 – 0.8 = 4.5 (+/-1)Actual pCO2: 5.3Borderline respiratory acidosisStep 3: Ions?AG: 123 – 107 – 18 = -2Interpretation Negative AG in a bipolar patientLithium toxicity?
69. Systematic Interpretation If venous values: + 0.03 to pH and – 0.6 from pCO2ADominant Disorder:Acidosis or alkalosis?Metabolic or respiratory?CCompensation:Expected ∆ pCO2 if metabolic disorder: SBE x 0.1 (+/- 1)Expected SBE if acute respiratory disorder: 0Expected SBE if chronic respiratory disorder: ∆ pCO2 x 3 (+/- 3)IIons:If pCO2 < 3.3 or > 7.3: use actual HCO3-Calculate anion gap (AG): Na – Cl – HCO3-Calculate ∆ AG: AG - 8Calculate ∆ AG + HCO3-DDiagnosis?Take all available clinical data into account
70. Step 4: DiagnosisTake into consideration all available information:Background: past medical history, meds . . . HistoryPhysical findingsOther bedside test results
71. DDxL-lacticAcidosis
72. DDx Metabolic Alkalosis
73. DDx Respiratory Acidosis
74. DDx Respiratorisk Alkalosis
75. ACIDReplace ValuesVenous BG: add 0.03 to pH and remove 0.6 from pCO21-Acidosis/alkalosis?Dominant disorderMetabolic acidosis? Respiratory acidosis?Metabolic alkalosis? Respiratory alkalosis?2-Compensation?If dominant disorder is metabolic, is expected respiratory compensation present?If dominant disorder is respiratory, does the disorder appear to be acute or chronic?3-Ions?Use actual HCO3- if pCO2 < 3.3 or > 7.3Calculate AG: Na– Cl – HCO3- Estimate ∆ AG: actual AG – expected AG (8 mmol/L)Calculated ∆ AG + HCO3-4-Diagnosis?Account for acid-base disorders using all available information
76. KOL patientReplace Values?Arterial blood gasStep 1: Dominant Disorder?Respiratory acidosisStep 2: Compensation?Expected SBE if acute: 0Expected SBE if chronic: 13 x 3 = 39So likely acute respiratory acidosisStep 3: Ions?Actual HCO3-: 34AG: 143 – 101 – 34 = 8Interpretation The patient’s actual HCO3 suggests either compensation for a chronic respiratory acidosis or a metabolic alkalosis. The clinical picture suggested acute on chronic respiratory acidosis75-Year-OldSevere COPDBarely conscious
77. Four Fundamental ConceptsConceptEquation/Law[H+] is determined by pCO2 and [HCO3-]Henderson-Hasselbalch Equation[H+] is “produced” or “consumed” by changes in pCO2 and [HCO3-]Law of Mass ActionHomeostasis: the lungs/kidneys strive to normalize [H+] if acid-base disorder presentCompensation equations[HCO3-] is determined by the concentrations of ions in the bloodLaw of Electrical Neutrality
78. 1-Hendersson-Hasselbalch Equation[H+] + [HCO3-] ↔ [H2CO3-] ↔ CO2 + H2O[H+] = 180 x pCO2 / [HCO3-]*pH = -log [H+]*This is the H-H equation in plasma; pCO2 units in kPapH < 7.38AcidemiapH > 7.42Alkalemia
79. 2-Law of Mass ActionChangeShiftResult↑ [HCO3-][H+] + [HCO3-] → [H2CO3] → CO2 + H2O↓ [H+]↓ [HCO3-][H+] + [HCO3-] ← [H2CO3] ← CO2 + H2O↑ [H+]↑ CO2[H+] + [HCO3-] ← [H2CO3] ← CO2 + H2O↑ [H+]↓ CO2[H+] + [HCO3-] → [H2CO3] → CO2 + H2O↓ [H+]pH < 7.38[HCO3-] < 22Metabolic acidosispH < 7,38pCO2 > 5.7Respiratory acidosispH > 7.42[HCO3-] > 27Metabolic alkalosispH > 7,42pCO2 < 5.0Respiratory alkalosis
80. 3-HomeostasisDisorderAbnormalityCompensationMetabolic acidosis↓ [HCO3-]↓ CO2Metabolic alkalosis↑ [HCO3-]↑ CO2Respiratory acidosis↑ CO2↑ [HCO3-]Respiratory alkalosis↓ CO2↓ [HCO3-]
81. 4-Law of Electrical NeutralityAnion Gap is defined as Na+ – (Cl- + HCO3-)
82. From a Clinical PerspectiveMachine measures pH and pCO2Actual [HCO3-] can be derived from H-H equationDominant disorder derived from pH, pCO2 and [HCO3-]No respiratory compensation for a metabolic disorder means that a respiratory disorder is presentNa+ – (Cl- + HCO3-) reveals presence of excess anions and coexisting metabolic acidosis + metabolic alkalosisThe clinical context is key to interpreting causality
83. Base Excess and Standard HCO3-cBase(Ecf)c and cHCO3- (P,st)c are calculated valuesCorrespond to the summative metabolic contribution to the change in pHCannot reveal simultaneous metabolic acidosis and metabolic alkalosisCalculating the AG reveals metabolic acidosis hidden by a metabolic alkalosisThe AG is calculated using Actual HCO3-HCO3- (act) ≈ cHCO3- (P,st)c if pCO2 3.3 – 7.3 kPa
84. Base Excess and Standard HCO3-SBE = HCO3- - 24.8 + 16.2 x (pH – 7,40)