Transcript Metabolic Acidosis

Arterial Blood Gas
Interpretation
Dr. Kapila Hettiarachchi
[email protected]
pH is inversely related to [H+]; a pH change of
1.00 represents a 10-fold change in [H+]
pH
7.00
7.10
7.30
7.40
7.52
7.70
8.00
[H+] in nanomoles/L
100
80
50
40
30
20
10
1st Defence
Know the normal values first
Alkalemia:
blood pH > 7.45
Respiratory alkalosis from acute hyperventilation.
Metabolic alkalosis from excessive diuretic therapy
Acidemia:
blood pH < 7.35
Metabolic acidosis from decreased perfusion (lactic acidosis)
Respiratory acidosis from hypoventilation.
RESPIRATORY ALKALOSIS
↓PaCO2 & ↑ pH
Hypoxemia (includes altitude)
Anxiety
Sepsis
Any acute pulmonary insult (e.g., pneumonia, mild asthma attack, early
pulmonary edema, pulmonary embolism)
Respiratory
alkalosis
Expected changes in pH and HCO3- for a 10-mm Hg change in
PaCO2 resulting from respiratory alkalosis:
ACUTE
pH ↑ by 0.08
* Units for HCO3- are mmol/L
HCO3- ↓ by 2
Respiratory Acidosis
Primary Event
Compensatory Event
HCO3Low pH ~ --------High PaCO2
Increased HCO3Low pH ~ ------------------High PaCO2
RESPIRATORY ACIDOSIS
↑PaCO2 & ↓ pH
Central nervous system depression (e.g., drug overdose)
Chest bellows dysfunction
(e.g., Guillain-Barré syndrome, myasthenia gravis)
Disease of lungs and/or upper airway
(e.g., chronic obstructive lung disease, severe asthma attack,
severe pulmonary edema)
Respiratory acidosis
Respiratory acidosis
Expected changes in pH and HCO3- for a 10-mm Hg change in
PaCO2 resulting from respiratory acidosis
ACUTE
pH ↓ by 0.07
* Units for HCO3- are mmol/L
HCO3- ↑ by 1*
Metabolic Alkalosis
Primary Event
High HCO3High pH ~ ---------------
PaCO2
Compensatory Event
High HCO3High pH ~ -------------------Increased PaCO2
Metabolic alkalosis
↑ HCO3- & ↑ pH
Chloride responsive (responds to NaCl or KCl therapy): diuretics,
corticosteroids, gastric suctioning, vomiting
Chloride resistant: any hyperaldosterone state (e.g., Cushing’s
syndrome, Bartter’s syndrome, severe K+ depletion)
Metabolic Alkalosis –
How to detect compensation
1.) PaCO2 = [HCO3] + 15
2.) PaCO2 will ↑ 6 mmHg per 10-mmol/L ↑ in [HCO3]
Metabolic alkalosis
Metabolic alkalosis
Metabolic Acidosis
Primary Event
Low HCO3Low pH ~ -----------PaCO2
Compensatory Event
Low HCO3Low pH ~ -----------Decreased PaCO2
METABOLIC ACIDOSIS
↓HCO3- & ↓ pH
- Increased anion gap
• lactic acidosis; ketoacidosis; drug poisonings (e.g., aspirin,
ethylene glycol, methanol)
- Normal anion gap
- Diarrhea; some kidney problems (e.g., renal tubular acidosis,
interstitial nephritis)
Metabolic Acidosis – How to detect
compensation
1.) PaCO2 = (1.5x HCO3) + 8
or
2.) PaCO2 will ↓ 1.25 mmHg per mmol/L ↓ in [HCO3]
Metabolic acidosis
Metabolic acidosis
Metabolic acidosis
Anion Gap
Metabolic acidosis is conveniently divided into elevated and
normal anion gap (AG) acidosis. AG is calculated as
AG = Na+ - (Cl- + HCO3- )
Note: Normal AG is typically 12 ± 4 mEq/L.
Mixed Acid-base Disorders are Common
In chronically ill respiratory patients, mixed disorders are
probably more common than single disorders,
e.g., RAc + MAlk,
RAc + Mac,
Ralk + MAlk.
Mixed Acid-base Disorders are Common
In renal failure (and other conditions) combined MAlk + MAc
is also encountered.
For an increased anion gap metabolic acidosis,
are there other derangements?
• Determining the “corrected bicarbonate concentration”:
Corrected HCO3 = measured HCO3 + (Anion Gap - 12)
• If the corrected HCO3 is less than normal (under 22mEq/L) then there is
an additional metabolic acidosis present.
• Corrected HCO3 values over 26 mEq/L reflect a co-existing metabolic
alkalosis.
• If it remains between 22-26mEq/L only existing Met. Acidosis is present
Correction of severe acidosis
Give NaHCO3 only If PH is < 7.1
but not in respiratory acidosis
Correction of severe acidosis
NaHCO3 in mmol = Base deficit X Body weight
3
Give half of that
Target is keeping PH > 7.2
if ABG not available
NaHCO3 1 mmol per Kg will be enough
Tips to Diagnose Mixed
Acid-base Disorders
TIP 1. Do not interpret any blood gas data for acid-base
diagnosis without closely examining the serum electrolytes:
Na+, K+, Cl-, and CO2.
• A serum CO2 out of the normal range always represents some type of
acid-base disorder
• Note that serum CO2 may be normal in the presence of two or more
acid-base disorders.
Tips to Diagnose Mixed Acid-base Disorders
(cont.)
TIP 2. Single acid-base disorders do not lead to normal blood
pH. Although pH can end up in the normal range (7.35 - 7.45)
with a single mild acid-base disorder, a truly normal pH with
distinctly abnormal HCO3- and PaCO2 invariably suggests two
or more primary disorders.
Tips to Diagnose Mixed Acid-base Disorders
(cont.)
Example:
pH 7.40,
PaCO2 20 mm Hg,
HCO3- 12 mEq/L in a patient with sepsis.
Acid-base Disorders:
Test Your Understanding
A child with severe asthma, arterial blood gas shows
pH of 7.14
PaCO2 of 70 mm Hg
HCO3- of 23 mEq/L
How would you describe the likely acid-base disorder(s)?
Acid-base Disorders:
Test Your Understanding - Answers
Acute elevation of PaCO2 leads to reduced pH, i.e., an acute respiratory
acidosis. However, is the problem only acute respiratory acidosis or is there
some additional process?
For every 10-mm Hg rise in PaCO2 (before any renal compensation), pH falls
about 0.07 units. Because this patient's pH is down 0.26, or 0.05 more than
expected for a 30-mm Hg increase in PaCO2, there must be an additional
metabolic problem.
Also note that with acute CO2 retention of this degree, the HCO3- should be
elevated 3 mEq/L. Thus a low-normal HCO3- with increased PaCO2 is another
way to uncover an additional metabolic disorder. Decreased perfusion leading
to mild lactic acidosis would explain the metabolic component.
Interpreting Oxygenation
• What is ‘normal’ PaO2?
80-100mmHg is normal
Interpreting Oxygenation
•Oxygenation gradually deteriorates during life Several
calculations available for determining ‘normal’ based on
patient age.
PaO2 = 104.2 - (0.27 x age)
•i.e., 30 year old ~ 95 mmHg
60 year old ~ 88 mmHg
Interpreting Oxygenation
P(A-a)O2 difference –Normal is 3-25mmHg
•Increased P(A-a) O2 -
1.Shunt
2.V/Q mismatch
3.Diffusion defect
Interpreting Oxygenation
Oxygenation index- PaO2 /FiO2
• Normal is 500
• 200-300 is moderate shunt/ V/Q abnormality
• <200 severe shunt / V/Q abnormality
PaCO2 Equation: PaCO2 reflects ratio of metabolic CO2
production to alveolar ventilation
PaCO2 =
VCO2 x 0.863
------------------VA
VCO2 = CO2 production
VA = VE – VD
VE = minute (total) ventilation (= resp. rate x
tidal volume)
VD = dead space ventilation (= resp. rate x dead
space volume
0.863 converts VCO2 and VA units to mm Hg
PaCO2
Condition
in blood
State of
alveolar ventilation
> 45 mm Hg
Hypercapnia
Hypoventilation
35 - 45 mm Hg
Eucapnia
Normal ventilation
< 35 mm Hg
Hypocapnia
Hyperventilation
Dangers of Hypercapnia
Besides indicating a serious derangement in the respiratory
system, elevated PaCO2 poses a threat for three reasons:
1) An elevated PaCO2 will lower the PAO2 and as a result will lower the
PaO2.
2) An elevated PaCO2 will lower the pH
3) The higher the baseline PaCO2, the greater it will rise for a given fall in
alveolar ventilation,
PCO2 vs. Alveolar Ventilation
?
Case 3 –
Patient with
Severe Abdominal Pain
Case 3 – Patient with Severe Abdominal Pain
An obese 70 year old man has diabetes of 25 years
duration complicated by coronary artery disease
(CABG x 4 vessels 10 years ago), cerebrovascular
disease (carotid artery endarterectomy 3 years ago)
and peripheral vascular disease (Aorto-bifem 2 years
ago). [“VASCULOPATH”]
Case 3 – Patient with Ischemic Bowel
Case 3 – Patient with Ischemic Bowel
Case 3 – Patient with Ischemic Bowel
ABGs obtained
in the ICU
pH
7.18
PCO2
20 mmHg
HCO3
7 mEq/L
Case 3 – Patient with Ischemic Bowel
ABGs obtained in the ICU
pH
PCO2
HCO3
7.18
20 mmHg
7 mEq/L
What is the primary disorder?
What is the physiologic response to this disorder?
Case 3 – Patient with Ischemic Bowel
Step 1: Acidemic, alkalemic, or normal?
ACIDEMIC
Case 3 – Patient with Ischemic Bowel
Step 2: Is the primary disturbance respiratory or metabolic?
METABOLIC
Case 3 – Patient with Ischemic Bowel
Step 3: For a primary respiratory disturbance, is it acute or
chronic?
NOT APPLICABLE
Case 3 – Patient with Ischemic Bowel
Step 4: For a metabolic disturbance, is the respiratory system
compensating OK?
"Winter's formula":
Expected PCO in metabolic acidosis
2
= 1.5 x HCO3 + 8 (range: +/- 2)
= 1.5 x 7 + 8 = 18.5
pH
7.18
PCO2 20 mm Hg
HCO3
7 mEq / L
Case 3 – Patient with Ischemic Bowel
Step 5: For a metabolic acidosis, is there an increased
anion gap?
FOR THIS STEP ONE MUST OBTAIN SERUM
ELECTROLYTE DATA
Case 3 – Patient with Ischemic Bowel
SERUM ELECTROLYTE DATA
Serum Na
Serum HCO3
Serum Cl
135
mEq/L
7
mEq/L
98
mEq/L
Anion Gap = Na – (Cl – HCO3)
Anion Gap =
= 135 – (98 -7) mEq/L
= 30 mEq/L
(ELEVATED)
SERUM ELECTROLYTE DATA
Serum Na
135
mEq/L
Serum HCO3
7
mEq/L
Serum Cl
98
mEq/L
Case 3 – Patient with Ischemic Bowel
Step 5: For a metabolic acidosis, is there an
increased anion gap?
ANSWER: YES
Case 3 – Patient with Ischemic Bowel
Step 6: For an increased anion gap metabolic acidosis, are there
other derangements?
To determine if there are other metabolic derangements present
we start by determining the “corrected bicarbonate
concentration”:
Corrected HCO3 = measured HCO3 + (Anion Gap - 12)
If the corrected HCO3 is less than normal (under 22mEq/L) then
there is an additional metabolic acidosis present. Corrected HCO3
values over 26 mEq/L reflect a co-existing metabolic alkalosis.
Case 3 – Patient with Ischemic Bowel
Corrected HCO3 = measured HCO3 + (Anion Gap - 12).
Corrected HCO3 = 7 + (30 - 12) = 25
REMEMBER
If the corrected HCO3 is less than normal (under 22mEq/L) then
there is an additional metabolic acidosis present. Corrected HCO3
values over 26 mEq/L reflect a co-existing metabolic alkalosis.
Case 3 – Patient with Ischemic Bowel
Step 6: For an increased anion gap metabolic acidosis, are there
other derangements?
ANSWER: NO OTHER DERANGEMENTS NOTED
Case 3 – Patient with Ischemic Bowel
ANSWER FROM
“Primary metabolic acidosis, with increased anion gap,
with full respiratory compensation”
Case 3 – Patient with Ischemic Bowel
BUT … What is the cause of the
elevated anion-gap metabolic acidosis?
Case 3 – Patient with Ischemic
Bowel
The most common etiologies of a metabolic acidosis
with an increased anion gap are shown below:
 Lactic acidosis
 Ingestion of:
(from poor perfusion)
 Ethylene glycol
 Starvation
 Methanol
 Renal failure
 Salicylate
 Ketoacidosis (as in diabetic ketoacidosis)
Notes on Lactic Acidosis
“Lactic acidosis is a disease characterized by a pH less
than 7.25 and a plasma lactate greater than 5 mmol/L.”
Case 3 – Patient with Ischemic Bowel
Case 3 – Patient with Ischemic Bowel
By the time the patient is admitted to the ICU he looks
absolutely terrible. He is moaning in agony, having
received no pain medications at all.
Vital signs in ICU
BP
HR
RR
Temp
O2 sat
Pain Score
82/50
112
35
35.5 Celsius
84%
10/10
Case 3 – Patient with Ischemic Bowel
Because of the extreme pain, the patient is given
morphine 8 mg IV push, a somewhat generous dose.
When reexamined 15 minutes later the patient appears
to be more comfortable.
New vital signs are obtained.
BP
HR
RR
Temp
O2 sat
Pain Score
75/45
102
22
35.5 Celsius
82%
7/10
Case 3 – Patient with Ischemic Bowel
BP
HR
RR
Temp
O2 sat
Pain Score
75/45
102
22
35.5 Celsius
82%
7/10
What is the next thing we should do for
this patient?
Case 3 – Patient with Ischemic Bowel
Case 3 – Patient with Ischemic Bowel
ABGs obtained in the ICU after morphine has
been given
pH
7.00 (was 7.18)
PCO2
HCO3
25 mmHg (was 20)
7 mEq/L
REMEMBER THAT MORPHINE IS A
RESPIRATORY DEPRESSANT AND
WILL ELEVATE PCO2
Case 3 – Patient with Ischemic Bowel
pH
7.00
PCO2
25 mmHg
HCO3
7 mEq/L
Here is what says
“Primary metabolic acidosis, with increased anion
gap, with superimposed respiratory acidosis”
Case 3 – Patient with Ischemic Bowel
“Primary metabolic acidosis, with increased anion
gap, with superimposed respiratory acidosis”
BUT …
How could there be a respiratory acidosis when
the PCO2 is very much below 40 mm Hg?
Normal Values (arterial blood)
pH
= 7.35 to 7.45
PCO2 = 35 to 45 mm Hg
HCO3 = 22 to 26 mEq/L
Case 3 – Patient with Ischemic Bowel
How could there be a respiratory acidosis when
the PCO2 is very much below 40 mm Hg?
ANSWER
The expected degree of respiratory compensation
is not present.
Case 3 – Patient with Ischemic Bowel
The expected degree of respiratory compensation is not
present.
Expected PCO2 in metabolic acidosis
= 1.5 x HCO3 + 8 (range: +/- 2)
= 1.5 x 7 + 8 = 18.5
BUT … we got a PCO2 of 25 mm Hg (as a result of
respiratory depression from morphine administration) so the
expected degree of respiratory compensation is not present.
Case 3 – Patient with Ischemic Bowel
THERAPY FOR THIS PATIENT
1.Oxygen
2.Metabolic tuning (blood sugar etc.)
3.Mechanical ventilation
4.Fluid resuscitation
5.Hemodynamic monitoring
6.Surgical, anesthesia, ICU consultation
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