Transcript A Practical Approach to Acid

A Practical Approach to
Acid-Base Disorders
Madeleine V. Pahl, M.D., FASN
Professor of Medicine
Division of Nephrology
Course Objectives
What will I learn from this lecture?
I will come away knowing that acid-base
disorders are:
a) boring and irrelevant
b) relevant but obtuse
c) fascinating and clinically important
Introduction
• Daily acid production: 15,000 mmol of CO2 and 50100 meq of non-volatile acid (mostly sulfuric acid
from metabolism of amino acids)
• Balance maintained by renal and pulmonary
excretion
• Renal excretion: combination of H+ with titratable
acids, mainly phosphate and ammonia
• Balance assessed in terms of bicarbonate-carbon
dioxide buffer system, Henderson-Hasselbalch
equation
– pH = 6.10 x log ([HCO3] / [0.03 x pCO2])
Introduction
• Acid-base homeostasis critically affects tissue
and organ performance
• Both acidosis and alkalosis can have severe
and life threatening consequences
• It is the nature of the responsible condition
that determines the prognosis
Definitions
• Acidosis: process that lowers the extracellular fluid
pH (reduction in HCO3 or elevation in pCO2)
– Metabolic acidosis: low pH and low HCO3
– Respiratory acidosis: low pH and high pCO2
• Alkalosis: process that raises extracellular pH
(elevation in HCO3 or fall in pCO2)
– Metabolic alkalosis: high pH and high HCO3
– Respiratory alkalosis: high pH and low pCO2
Compensatory Responses
• Metabolic acidosis: respiratory compensation
begins in the first hour, 1.2 mmHg fall for 1
meq/L HCO3 reduction (Winter’s equation)
– pCO2 = 1.5 x (HCO3) + 8 + 2
• Metabolic alkalosis: rise of 0.6 mmHg pCO2
for 1 meq/L HCO3 elevation
Compensatory Responses
• Respiratory acidosis:
– acute: HCO3 increases 1meq/L for every 10 mmHg rise of
pCO2
– Chronic (renal compensation complete in 3-5 days): HCO3
increases 3.5meq/L for every10 mmHg rise of pCO2
• Respiratory alkalosis:
– acute: HCO3 increases 2 meq/L for every 10 mmHg fall of
pCO2
– Chronic: HCO3 increases 4 meq/L for every 10 mmHg fall of
pCO2
Compensatory Responses
Primary
Compensation
Respiratory:
Acidosis
pCO2
HCO3
Alkalosis
pCO2
HCO3
Metabolic:
Acidosis
Alkalosis
HCO3
HCO3
pCO2
pCO2
Metabolic acidosis: anion gap
• AG (12) = Na+ - (HCO3- + Cl-)
Unmeasured
cations
Unmeasured
anions
HCO3-
• Normal AG:
– HCO3 loss
– RTA
• Elevated AG:
Na+
Cl-
–
–
–
–
ketoacidosis
lactic acidosis
drugs and toxins
uremia
Delta HCO3
• Calculate anion gap
• Obtain the difference from expected anion
gap (12)
• Add the difference to the measured HCO3
• If > 24 then there is a ‘hidden’ metabolic
alkalosis
Problem Solving
• Is an acid-base disorder present?
• What is the primary or dominant
abnormality?
• Is the disorder simple or mixed?
• What is the cause?
Case: 1
67 yo male s/p gastrectomy returns 12 days later with nausea
decreased mental status
PE: T 39o BP 86/50, P 130
Drowsy, minimally responsive
Lungs: clear
Cor: tachy, RR , no rub
Abd: diffusely tender
Ext: no edema
pH
7.24
pCO2
24
132 95
HCO3
10
8.2
pO2
12
BUN = 86
Cr = 5.4
Problem Solving: Case 1
•
•
•
•
•
pH: 7.24 low, acidosis
pCO2: 24 low, respiratory alkalosis
HCO3: 12 low, metabolic acidosis
Anion Gap: 132- (95+12) = 25
Winter’s equation (expected pCO2): (12 x 1.5 = 18)
+ 8 = 26 (observed = 24)
• Delta change HCO3: (25-12= 13)+12(observed) =
25 (a normal HCO3)
• Answer:
– anion gap metabolic acidosis
– compensatory respiratory alkalosis
Case 2:
18 yo male c/o lethargy, SOB for 3-4 days, had ‘stomach flu’ 1
week ago
PE: T 37o BP 100/60, P 129 RR: 30
Lungs: clear
Cor: tachy, RR
Abd: normal
Ext: no edema
Skin: poor turgor, dry mucous membranes
pH
7.1
0
140 100
pCO2
15
6.5
HCO3
5
pO2
110
5
Problem Solving: Case 2
•
•
•
•
•
pH: 7.1 low, acidosis
pCO2: 15 low, respiratory alkalosis
HCO3: 5 low, metabolic acidosis
Anion Gap: 140 - (100+5) = 35
Winter’s equation (expected pCO2): (5 x 1.5 = 7.5) + 8
=15.5 (observed = 15)
• Delta change HCO3: (35-12= 23)+5 (observed) = 28 (an
elevated HCO3)
• Answer:
• anion gap metabolic acidosis
• compensatory respiratory alkalosis
• metabolic alkalosis
Case 3:
40 yo male with history of duodenal ulcer c/o epigastric pain for 2
weeks, severe vomiting for 1 week, unable to keep anything down
PE: T 37o BP 100/70, P 120
Neck veins flat
Lungs: clear
Cor: tachy, RR
Abd: diffusely tender
pH
7.54
pCO2
48
HCO3
40
pO2
80
140 80
2.0
44
Urine:
Na = 2
Cl = 3
K = 21
Problem Solving: Case 3
•
•
•
•
pH: 7.54 high, alkalosis
pCO2: 48 high, respiratory acidosis
HCO3: 44 high, metabolic alkalosis, increased by 20
so expected pCO2
• 0.6 x 20 = 12
• 40 + 12 = 52 Slightly higher than observed
• Answer:
• metabolic alkalosis (Cl- responsive with low
urinary Cl)
• compensatory respiratory acidosis with a slight
respiratory alkalosis ?
Case 4:
24 yo white male s/p gunshot wound to the abdomen required
splenectomy and ileostomy. The pt is intubated, sedated and
paralyzed. He has an NG tube in place, is on multiple antibiotics
and has required post-op pressors.
PE: T 39o BP 100/60, P 113
Looks terrible!
Lots of tubes and drains
pH
7.6
1
140 94
pCO2
30
3.0
HCO3
29
pO2
29
Problem Solving: Case 4
•
•
•
•
•
pH: 7.61 high, alkalosis
pCO2: 30 low, respiratory alkalosis
HCO3: 29 high, metabolic alkalosis
Anion Gap: 140 - ( 94 + 29) = 17
Delta change HCO3: (17-12= 5)+29 (observed) =
34 (‘true’ value without acidosis)
• Answer:
• metabolic alkalosis
• anion gap metabolic acidosis
• respiratory alkalosis
Case 5:
55 yo man collapsed in a bar and was brought to the ER. He was
unresponsive, no BP was obtainable, a sinus tachycardia was
present and he had peritoneal signs.
pH
6.86
pCO2
81
HCO3
14
139 84
3.9
16
He was intubated, started on pressors and treated with HCO3
pH
7.04
148 93
pCO2
34
HCO3
9
4.5
10
Problem Solving: Case 5
Admission
•
•
•
•
•
pH: 6.85 low, acidosis
pCO2: 81 high, respiratory acidosis
HCO3: 16 low, metabolic acidosis
Anion Gap: 139 – (84 + 16) = 39
Winter’s equation (expected pCO2): (16 x 1.5 =
24) + 8 = 32 (lower than observed, 81)
• Delta change HCO3: (39-12 = 27 )+16 (observed)
= 43
• Answer:
– anion gap metabolic acidosis
– respiratory acidosis
– metabolic alkalosis
Problem Solving: Case 5
After Intubation
•
•
•
•
•
pH: 7.04 low, acidosis
pCO2: 34 low, respiratory alkalosis
HCO3:10 low, metabolic acidosis
Anion Gap: 148 – (93 + 10) = 45 (increasing)
Winter’s equation(expected pCO2): (10 x 1.5 = 15)
+ 8 = 23 (lower than observed, 34)
• Delta change HCO3: (45-12 + 33)+10(observed) =
43
• Answer:
– anion gap metabolic acidosis (lactate was 24)
– respiratory alkalosis
– metabolic alkalosis
Case 6:
32 yo hispanic female with a 1 week history of bloody diarrhea
comes to the ER with SOB, weakness and a feeling of doom.
PE: T 38.7o BP 90/40, P 100
Abd: diffusely tender with hyperative bowel sounds and OB+ stools
pH
7.1
1
140 115
pCO2
16
3.7
HCO3
5
5
Problem Solving: Case 6
•
•
•
•
•
pH: 7.11 low, acidosis
pCO2: 16 low, respiratory alkalosis
HCO3: 5 low, metabolic acidosis
Anion Gap: 140 – (115 + 5) = 20
Winter’s equation (expected pCO2): (5 x 1.5 = 7.5) + 8
= 15.5 (same as observed, 16)
• Delta change HCO3: (20-12 = 8)+5 (observed) = 13 (a
low HCO3)
• Answer:
• anion gap metabolic acidosis
• non-anion gap metabolic acidosis
(hyperchloremic)
• compensatory respiratory alkalosis
Case 7:
18 yo female attempts suicide by taking pills found in mother’s
medicine chest. Brought to ER alert but agitated.
PE: 148/60, P 126 T 37o
Cor: tachy
Neuro: no focal findings
pH
7.56
pCO2
15
140 107
HCO3
13
4.5
pO2
13
Problem Solving: Case 7
•
•
•
•
•
pH: 7.56 high, alkalosis
pCO2: 15 low, respiratory alkalosis
HCO3: 13 low, metabolic acidosis
Anion Gap: 140 ( 107 – 13) = 20
Winter’s equation (expected pCO2): (13 x 1.5 =19.5) + 8
= 27.5 (higher than observed, 15)
• Delta change HCO3: (20-12 = 8)+13 (observed) = 21
(only minimally reduced HCO3)
• Answer: salicylate poisoning
• anion gap metabolic acidosis
• primary respiratory alkalosis