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PATHOPHYSIOLOGY OF ACID-BASE DISORDERSTyler Paradis MDResident AnesthesiologistOHSU APOMSaturday, Nov 21st 2015
1
Objectives
•Understand Pathophysiology of acid base balance
•Understand the various acid base abnormalities
•Understand how to interpret an ABG
2
CO2 TRANSPORT IN THE BLOOD
3
FORMS OF CO2 TRANSPORT IN THE BLOOD
4
1.Combined with Hemoglobin (~10%)
2.Dissolved in Plasma (~5%)
3.Bicarbonate (80-90%)
Carbamino-Hemoglobin
5
HHb–NH2 + CO2 ↔ HHb–NHCOO- + H+
• CO2 binds to terminal amino groups on the globin chains of hemoglobin.
• Deoxygenated Hb binds CO2 more readily than oxygenated Hb – known as the Haldane effect. As a result, CO2 content is higher in venous than arterial blood.
• CO2 also forms small amounts of carbamino compounds with plasma proteins.
Dissolved CO2
6
• Dissolved CO2 content is proportional to its partial pressure and solubility.
• The solubility of CO2 is 20x > O2.
Dissolved CO2
7
• Dissolved CO2 content is usually expressed in mEq/L blood.
• Dissolved CO2 = PCO2 x 0.03 (mm Hg) (mEq CO2/L/mm Hg)
= 40 x 0.03
= 1.2 mEq CO2/L
Bicarbonate Formation and the Chloride Shift
8
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
Carbonic Anhydrase
Cl-
RBC
Plasma
➢The presence of carbonic anhydrase in red blood cells results in rapid formation of bicarbonate within erythrocytes.
➢ There is no carbonic anhydrase in the plasma, so the CO2 hydration equation proceeds very slowly in plasma.
➢ HCO3- diffuses down its concentration gradient from RBCs into the plasma in
exchange for Cl-, a process called the chloride shift.
➢ The chloride shift is mediated by Band 3, a major membrane protein.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
(slow)
ACID-BASE BALANCE AND
DISORDERS
9
CO2 HCO3_ CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3
_
Blood Kidney
Lung
The lungs regulate arterial PCO2 and thus the concentration of dissolved CO2 in blood via changes in alveolar ventilation, while the kidneys regulate [HCO3
_ ] via changes in renal H+ excretion.
Role of the Lungs and Kidneys in Acid-Base Homeostasis
10
Relationship Between The pH And The H+ Concentration In The Physiologic Range
pH H+
nM/L
7.8 16
7.7 20
7.6 26
7.5 32
7.45 36
7.4 40
7.35 45
pH H+
nM/L
7.3 50
7.2 63
7.1 80
7.0 100
6.9 125
6.8 160
11
Henderson - Hasselbalch Equation
pH = pK of carbonic acid + log
= 6.1 + log
[Dissolved CO2]
[HCO3_]
[0.03 PCO2]
[HCO3_]
12
Normal Acid - Base Balance
pH
7.0 7.2 7.4 7.8
H+ nm/L
1.2 24
7.6
40 25 20316379 50
HCO3_Dissolved
CO2
KidneyLung
13
Characteristics Of The Primary Acid - Base Disorders
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
[HCO3_ ]p
[HCO3_ ]p
PCO2
PCO2
PCO2
PCO2
[HCO3_ ]p
[HCO3_ ]p
pH Primary Disturbance
CompensatoryResponse
14
Respiratory Acidosis
An abnormal physiological process in which there is a primary increase in PaCO2 due to a primary decrease in alveolar ventilation (hypoventilation), which results in a decreased ratio of [HCO3
_ ] / [dissolved CO2] and a decrease in the pH of the blood.
15
pH
7.0 7.2 7.4 7.8
H+ nm/L
2.4
24
7.6
40 25 20316379 50
HCO3_Dissolved
CO2
Kidney
Lung
pH
7.0 7.2 7.4 7.8
H+ nm/L
2.440
7.6
40 25 20316379 50
HCO3_
Dissolved CO2
KidneyLung
Respiratory AcidosisAnd Acidemia
Compensated RespiratoryAcidosis
16
Causes of Respiratory Acidosis (Alveolar Hypoventilation)
I. Airway Obstruction A. Chronic Obstructive Lung Disease B. Upper Airway ObstructionII. Chest Wall Restriction A. Kyphoscoliosis B. Pickwickian SyndromeIII. Respiratory Center Depression A. Anesthetics B. Sedatives
C. Opiates D. Brain injury or disease E. Severe hypercapnia, hypoxiaIV. Neuromuscular disorders A. Spinal cord injury B. Phrenic nerve injury C. Poliomyelitis D. Myasthenia Gravis E. Guillian-Barré Syndrome F. Administration of Curare-like Drugs G. Respiratory Muscle Diseases
17
Respiratory Alkalosis
An abnormal physiological process in which there is a primary decrease in PaCO2 due to a primary increase in alveolar ventilation (hyperventilation) resulting in an increase in the ratio of HCO3
- to dissolved CO2, and an increase in arterial pH.
18
pH
7.0 7.2 7.4 7.8
H+ nm/L
0.6
24
7.6
40 25 20316379 50
HCO3_
Dissolved CO2
Kidney
Lung
pH
7.0 7.2 7.4 7.8
H+ nm/L
0.6
14
7.6
40 25 20316379 50
HCO3_
Dissolved CO2
Kidney
Lung
Respiratory AlkalosisAnd Alkalemia
Compensated RespiratoryAlkalosis
19
Causes of Respiratory AlkalosisI. Respiratory Center Stimulation A. CNS 1. Anxiety 2. Hyperventilation Syndrome 3. Inflammation (encephalitis, meningitis) 4. Stroke 5. Tumors B. Drugs or Hormones 1. Salicylates 2. Progesterone 3. Hyperthyroidism C. Reflex 1. Hypoxemia 2. High Altitude 3. Metabolic Acidosis 4. Sepsis, fever 5. Pulmonary Embolism 6. Pulmonary Edema 7. Congestive Heart Failure 8. AsthmaII. Iatrogenic Mechanical OverventilationIII. Liver Failure
20
Metabolic Alkalosis
An abnormal physiological process in which there is a primary increase in [HCO3
_ ] due to an excessive gain of base (HCO3
_ ) or loss of acid (H+) resulting in an increase in the ratio of HCO3
- to dissolved CO2 and a rise in arterial pH.
21
pH
7.0 7.2 7.4 7.8
H+ nm/L
1. 2
34
7.6
40 25 20316379 50
HCO3_
Dissolved CO2
Kidney
Lung
pH
7.0 7.2 7.4 7.8
H+ nm/L
1. 5
34
7.6
40 25 20316379 50
HCO3_
Dissolved CO2
Kidney
Lung
Metabolic AlkalosisAnd Alkalemia
Compensated MetabolicAlkalosis
22
Causes of Metabolic AlkalosisI. Loss of Hydrogen Ions A. Vomiting B. Nasogastric Suction C. Gastric fistulas D. Diuretic therapy E. Severe Magnesium or Potassium Deficiency F. Overproduction of mineralocorticoids (Cushing’s syndrome; Primary hyperaldosteronism; renal artery stenosis) G. Ingestion of mineralocorticoids (Licorice ingestion; chewing tobacco) H. Inherited Disorders (Bartter’s Syndrome; Liddle’s syndrome; Gitelman’s syndrome)
II. Ingestion or administration of excess bicarbonate or other bases A. Intravenous bicarbonate B. Ingestion of bicarbonate or other bases (e.g., antacids)
23
Metabolic Acidosis
An abnormal physiological process in which there is a primary decrease in arterial [HCO3
_ ] due to an excessive loss of base (HCO3
_ ) or gain of acid (H+) resulting in a decrease in the ratio of HCO3
- to dissolved CO2 and thus a decrease in arterial pH. .
24
pH
7.0 7.2 7.4 7.8
H+ nm/L
1. 2
12
7.6
40 25 20316379 50
HCO3_Dissolved
CO2
Kidney
Lung
pH
7.0 7.2 7.4 7.8
H+ nm/L
0.612
7.6
40 25 20316379 50
HCO3_
Dissolved CO2
KidneyLung
Metabolic AcidosisAnd Acidemia
Compensated MetabolicAcidosis
25
Serum Electrolyte Concentrations
Na+ 136 - 145 mEq/L
K+ 3.5 - 5.0 mEq/L
Cl_ 100 - 106 mEq/L
CO2 Content (HCO3-) 24 - 28 mEq/L
Ca++ 4.3 - 5.3 mEq/L
Mg++ 1.5 - 2.5 mEq/L
HPO4_ _ , H2PO4
_ 1.5 - 3.0 mEq/L
H+ 40 x 10 _ 6 mEq/L
Anion Gap
[Na+ ] - ([Cl_ ] + [HCO3_ ]) Normal = 12 ± 4 mEq/L
140 - (103 + 25) = 12
26
Normal Metabolic Acidosis
Hyperchloremic
(Normal Gap)
Normochloremic
(Increased Gap)
A_
24
A_
12A_
12
HCO3-
25
HCO3-
13 HCO3-
13
Cl_
103
Cl_
115 Cl_
103
Na+
140
Na+
140Na+
140
Classification of metabolic acidosis arrived at by using anion gap. Bar A shows the normal relationship of the unmeasured anions (A_, anion gap) to plasma electrolytes; B and C illustrate the respective anion compositionof plasma in hyperchloremic and normochloremic metabolic acidosis.
A B C
27
Differential Diagnosis of Metabolic Acidosis
Normal Anion Gap (Hyperchloremia)
I. Gastrointestinal loss of HCO3_
A. Diarrhea B. Small bowel or pancreatic drainage or fistula C. Ureterosigmoidostomy, jejunal loop, ileal loop conduit D. DrugsII. Renal Loss of HCO3
_ A. Carbonic anhydrase inhibitors B. Renal tubular acidosis (RTA) III. Miscellaneous A. Dilutional acidosis B. Hyperalimentation
I. Lactic AcidosisII. Ketoacidosis A. Diabetic B. Starvation C. AlcoholicIII. Ingestion of Toxic Substances A. Salicylate overdose B. Paraldehyde poisoning C. Methyl alcohol ingestion D. Ethylene glycol ingestionIV. Failure of Acid Excretion A. Acute renal failure B. Chronic renal failure
Increased Anion Gap (Normochloremic)
28
Excess H+
H+ + HCO3_ ↔ H2CO3
_
H2CO3_ ↔ CO2 + H20
Diffusion Into Cells
Renal H+ Excretion
( Immediate )
ExtracellularBuffering
(Minute to Hours)
Respiratory Compensation
(2 - 4 hours )
IntracellularBuffering
(Hours to Days)Renal
Compensation
Lungs
29
Magnitude of Compensatory Responses To Acid - Base Disorders
Respiratory Compensation:
Metabolic Acidosis:1.2 mmHg ↓ in PCO2 per mEq/L ↓ in [HCO3
_ ]
Metabolic Alkalosis:0.7 mmHg ↑ in PaCO2 for every mEq/L ↑ in [HCO3
_ ]
Renal Compensation:
0.4 mEq/L Δ [HCO3_ ] for every mmHg Δ PaCO2
30
Simple vs. Mixed Acid-Base Disturbances
Simple Acid-Base Disturbance:
A single primary disturbance in acid-base balance that may or may not be associated with a secondary compensatory process modifying the change in pH.
Mixed Acid-Base Disturbance:
Combination of 2 or more primary disturbances of acid-base balance occurring at the same time,each of which would independently alter the pH.
1. Combination of primary acid-base disturbances that may augment their effect in pH. A. Combined metabolic and respiratory acidosis. B. Combined metabolic and respiratory alkalosis.
2: Combination of primary acid-base disturbances that tend to cancel out their effects on pH. A. Metabolic acidosis and respiratory alkalosis. B. Respiratory acidosis and metabolic alkalosis.
31
Interpretation of Arterial Blood Gas Results
A. Examine pH to determine if acidosis ( < 7.40 ) or alkalosis ( > 7.40 ).
B. Examine PaCO2 to see if there is a respiratory component to the acid-base disorder.
C. Examine [HCO3_ ] to see if there is a metabolic component to the
acid-base disorder.
D. Is there compensation? Is it appropriate ? Simple vs. Mixed disorder?
E. For mixed disturbance can calculate ΔAG (AG – normal AG)
F. Examine PaO2 to assess oxygenation.
32
Characteristics Of The Primary Acid - Base Disorders
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
[HCO3_ ]p
[HCO3_ ]p
PCO2
PCO2
PCO2
PCO2
[HCO3_ ]p
[HCO3_ ]p
pH Primary Disturbance
CompensatoryResponse
33
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
34
The Bigger Picture - The Systematic Approach to A Patient With An Acid-Base Disorder.
In the clinical setting, the acid-base laboratory data (pH, PaCO2, HCO3_ ) should never be evaluated
independently of :
A. Patient History - Evaluate for potential processes related to acid-base disturbances.
B. Physical Examination - Are there clues relevant to acid-base, cyanosis, fever, nasogastric tube breathing pattern, tetany, hypotension, COPD, etc.
C. Plasma Electrolyte Concentrations
1. Na+, K+, Cl-, CO2 content
2. Anion gap (unmeasured anions) = Na+ - [HCO3_ + Cl- ]
a. Normal value = 12 mEq/L
b. Useful in clinical diagnosis of different causes of metabolic acidosis 1) High Anion Gap acidosis 2) Normal Anion Gap Acidosis
D. Other laboratory Data - Blood sugar, BUN, blood ammonia, blood cultures, pulmonary function data, x-rays, urine data, etc.
35
Acid-Base InterpretationpH PaCO2 [HCO3
-]
1. 7.63 20 22
Acid-Base InterpretationpH PaCO2 [HCO3
-]
1. 7.63 ↑ 20 22
alkalosis
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
Acid-Base InterpretationpH PaCO2 [HCO3
-]
1. 7.63 20 ↓ 22
alkalosis
respiratory
Is there any compensation?
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
[HCO3_ ]p
[HCO3_ ]p
PCO2
PCO2
PCO2
PCO2
[HCO3_ ]p
[HCO3_ ]p
pH Primary Disturbance
CompensatoryResponse
Acid-Base InterpretationpH PaCO2 [HCO3
-]
1. 7.63 20 22 -Normal range
Uncompensated respiratory alkalosis
Acid-Base InterpretationpH PaCO2 [HCO3
-]
2. 7.47 20 16
Acid-Base InterpretationpH PaCO2 [HCO3
-]
2. 7.47 ↑ 20 16
alkalosis
Acid-Base InterpretationpH PaCO2 [HCO3
-]
2. 7.47 20 ↓ 16
alkalosis
respiratory
IS THERE COMPENSATION FOR THE RESPIRATORY ALKALOSIS?
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
[HCO3_ ]p
[HCO3_ ]p
PCO2
PCO2
PCO2
PCO2
[HCO3_ ]p
[HCO3_ ]p
pH Primary Disturbance
CompensatoryResponse
Acid-Base InterpretationpH PaCO2 [HCO3
-]
2. 7.47 20 16 ↓
Yes, there is renal compensation for the respiratory alkalosis.
Is compensation appropriate?
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
Acid-Base InterpretationpH PaCO2 [HCO3
-]
7.47 20 16
Therefore, Compensated respiratory alkalosis
PCO2 is decreased from 40 to 20= 20
So there can be 2 x 4 or 8 mEq/L ↓ in [HCO3
-]24-8 =
16
Compensation for respiratory alkalosis:4 mEq/L ↓ [ HCO3
_ ] per 10 mm Hg ↓ PCO2
Comparing # 1 and # 2, pH PaCO2 [HCO3
-]
1. 7.63 20 26
2. 7.47 20 16
Renal compensatory excretion of HCO3
- lowered pH down towards normal
level.
Acid-Base InterpretationpH PaCO2 [HCO3
-]
3. 7.22 60 24
50
Acid-Base InterpretationpH PaCO2 [HCO3
-]
3. 7.22 ↓ 60 24
acidosis
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
52
Acid-Base InterpretationpH PaCO2 [HCO3
-]
3. 7.22 ↑ 60 24
acidosisrespiratory
Is there any compensation for the Respiratory Acidosis?
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
[HCO3_ ]p
[HCO3_ ]p
PCO2
PCO2
PCO2
PCO2
[HCO3_ ]p
[HCO3_ ]p
pH Primary Disturbance
CompensatoryResponse
Acid-Base InterpretationpH PaCO2 [HCO3
-]
3. 7.22 60 24 - Normal
uncompensated (acute) respiratory acidosis
Acid-Base InterpretationpH PaCO2 [HCO3
-]
4. 7.33 60 30
56
Acid-Base InterpretationpH PaCO2 [HCO3
-]
4. 7.33 ↓ 60 30
acidosis
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
58
Acid-Base InterpretationpH PaCO2 [HCO3
-]
4. 7.33 ↑ 60 30
acidosis
respiratory
Is there any compensation for the Respiratory Acidosis?
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
[HCO3_ ]p
[HCO3_ ]p
PCO2
PCO2
PCO2
PCO2
[HCO3_ ]p
[HCO3_ ]p
pH Primary Disturbance
CompensatoryResponse
Acid-Base InterpretationpH PaCO2 [HCO3
-]
4. 7.33 60 30 ↑
Yes, there is renal compensation.
Is it appropriate?
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
Acid-Base Interpretation
pH PaCO2 [HCO3-]
7.33 60 30
Partially compensated respiratory acidosis
(possibly a mixed disorder)
[HCO3-] is increased 6 mEq/L above normal of
24.
2 x 4 = 8 mEq/L
PaCO2 is increased 20 mmHg above normal.
In respiratory acidosis, compensatory limit is 4 mEq/L ↑ [ HCO3
_ ] per 10 mm Hg ↑PCO2
Comparing # 3 and # 4, pH PaCO2 [HCO3
-]
3. 7.22 60 24
4. 7.33 60 30
Renal compensatory excretion of H+ raised pH up
towards normal level.
Acid-Base InterpretationpH PaCO2 [HCO3
-]
5. 7.25 30 16
Acid-Base InterpretationpH PaCO2 [HCO3
-]
7.25↓ 30 16
acidosis
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
Acid-Base InterpretationpH PaCO2 [HCO3
-]
7.25 30 ↓ 16
acidosis
respiratory
Not
Skip PCO2 for now, look at bicarb-->Metabolic Acidosis?
Metabolic Acidosis?pH PaCO2 [HCO3
-]
7.25 30 16 ↓
YES
Is there compensation?
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis
[HCO3_ ]p
[HCO3_ ]p
PCO2
PCO2
PCO2
PCO2
[HCO3_ ]p
[HCO3_ ]p
pH Primary Disturbance
CompensatoryResponse
Acid-Base InterpretationpH PaCO2 [HCO3
-]
7.25 30 ↓ 16
There is compensatory hyperventilation.
Is compensation appropriate?
Arterial Blood Sample
Acidosis Alkalosis
Metabolic acidosis
Respiratory acidosis
MetabolicAlkalosis
Respiratoryalkalosis
Respiratory compensation
Renal compensation
Respiratory compensation
Renal compensation
pH < 7.4
pH > 7.4
PCO2 < 35 mmHg[ HCO3_ ] > 26 mEq/LPCO2 > 45 mmHg[ HCO3
_ ] < 22 mEq/L
PCO2 < 35 mm Hg [ HCO3_ ] > 26 mEq/L PCO2 > 45 mm Hg [ HCO3
_ ] < 22 mEq/L
* 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ]
*4 mEq/L ↑ [ HCO3_ ]
Per 10 mm Hg ↑PCO2 * 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ]
* 4 mEq/L ↓ [ HCO3_ ]
per 10 mm Hg ↓ PCO2
* If the compensatory response is not appropriate, a mixed acid-base disorder should be suspected
Acid-Base Interpretation
pH PaCO2 [HCO3-]
7.25 30 16
Compensated metabolic acidosis
[HCO3-] is decreased by 8 mEq/L (24-
16)
8 x 1.2 = 9.6 mm Hg40-10= 30 mmHg
∴
In metabolic acidosis, compensatory limit is 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ].
In a man undergoing surgery, it was necessary to aspirate the contents of the upper gastro-intestinal tract. After surgery, the following values were obtained from an arterial blood sample: pH 7.55, PCO2 52 mm Hg and HCO3- 40 mmol/l. What is the underlying disorder
74
pH PaCO2 [HCO3-]
6. 7.55 52 40
In a man undergoing surgery, it was necessary to aspirate the contents of the upper gastro-intestinal tract. After surgery, the following values were obtained from an arterial blood sample: pH 7.55, PCO2 52 mm Hg and HCO3- 40 mmol/l. What is the underlying disorder
75
pH PaCO2 [HCO3-]
6. 7.55 52 40
Alkalosis
In a man undergoing surgery, it was necessary to aspirate the contents of the upper gastro-intestinal tract. After surgery, the following values were obtained from an arterial blood sample: pH 7.55, PCO2 52 mm Hg and HCO3- 40 mmol/l. What is the underlying disorder
76
pH PaCO2 [HCO3-]
6. 7.55 52 40 Metabolic alkalosis
In a man undergoing surgery, it was necessary to aspirate the contents of the upper gastro-intestinal tract. After surgery, the following values were obtained from an arterial blood sample: pH 7.55, PCO2 52 mm Hg and HCO3- 40 mmol/l. What is the underlying disorder
77
pH PaCO2 [HCO3-]
6. 7.55 52 40
Metabolic acidosis with respiratory Compensation
Acid-Base Interpretation
Compensated metabolic alkalosis
[HCO3-] is increased by 16 mEq/L (40-
24)
16 x 0.7 = 11.2 mm Hg
40+11= 51 mmHg
∴
In metabolic alkalosis, compensatory limit is 0.7 mm Hg ↑ PCO2
per mEq/L ↑ [ HCO3_ ].
pH PaCO2 [HCO3-]
7.55 52 40
A person was admitted to hospital in a coma. Analysis of the arterial blood gave the following values: PCO2 16 mm Hg, HCO3- 5 mmol/l and pH 7.1. What is the underlying acid-base disorder?
79
pH PaCO2 [HCO3-]
7. 7.1 16 5
A person was admitted to hospital in a coma. Analysis of the arterial blood gave the following values: PCO2 16 mm Hg, HCO3- 5 mmol/l and pH 7.1. What is the underlying acid-base disorder?
80
pH PaCO2 [HCO3-]
7. 7.1 16 5
Acidosis
A person was admitted to hospital in a coma. Analysis of the arterial blood gave the following values: PCO2 16 mm Hg, HCO3- 5 mmol/l and pH 7.1. What is the underlying acid-base disorder?
81
pH PaCO2 [HCO3-]
7. 7.1 16 5
Metabolic Acidosis
A person was admitted to hospital in a coma. Analysis of the arterial blood gave the following values: PCO2 16 mm Hg, HCO3- 5 mmol/l and pH 7.1. What is the underlying acid-base disorder?
82
pH PaCO2 [HCO3-]
7. 7.1 16 5
Metabolic acidosis with Respiratory compensation
Acid-Base Interpretation
pH PaCO2 [HCO3-]
7. 7.1 16 5
Compensated metabolic acidosis
[HCO3-] is decreased by 19 mEq/L (24-
5)
19 x 1.2 = 22.8 mm Hg
40-23= 17 mmHg
∴
In metabolic acidosis, compensatory limit is 1.2 mm Hg ↓ PCO2
per mEq/L ↓ [ HCO3_ ].
2 yo receiving deep sedation by the adult ED attending who gives him 4 mg morphine, respiratory rate is 6
84
pH PaCO2 [HCO3-]
8. 7.16 70 24
Acidosis
2 yo receiving deep sedation by the adult ED attending who gives him 4 mg morphine, respiratory rate is 6
85
pH PaCO2 [HCO3-]
8. 7.16 70 24
Respiratory Acidosis
2 yo receiving deep sedation by the adult ED attending who gives him 4 mg morphine, respiratory rate is 6
86
pH PaCO2 [HCO3-]
8. 7.16 70 24
Compensation? --> No, HCO3 is normal
Uncompensated (acute) respiratory acidosis
35 yo Cystic Fibrosis patient on the Peds floor with end-stage lung disease
87
pH PaCO2 [HCO3-]
9. 7.30 89 38
Acidosis
35 yo Cystic Fibrosis patient on the Peds floor with end-stage lung disease
88
pH PaCO2 [HCO3-]
9. 7.30 89 38
Respiratory acidosis
35 yo Cystic Fibrosis patient on the Peds floor with end-stage lung disease
89
pH PaCO2 [HCO3-]
9. 7.30 89 38
Compensation? --> yes, HCO3 is increased
Acid-Base Interpretation
pH PaCO2 [HCO3-]
7.3 89 38
Partially compensated respiratory acidosis
(possibly a mixed disorder)
[HCO3-] is increased 14 mEq/L above normal of
24.
5 x 4 = 20 mEq/L
PaCO2 is increased 49 (~50) mmHg above norm.
In respiratory acidosis, compensatory limit is 4 mEq/L ↑ [ HCO3
_ ] per 10 mm Hg ↑PCO2
References
•Adapted from PATHOPHYSIOLOGY OF ACID-BASE DISORDERS, Patricia J. Metting, Ph.D. Professor Departments of Physiology & Pharmacology and Medicine, University of
Toledo COM, 2011
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