Download Interpreting ABGs

Interpreting ABGs
Suneel Kumar MD
Arterial Blood Gases
• Written in following manner:
–
–
–
–
pH/PaCO2/PaO2/HCO3
pH = arterial blood pH
PaCO2 = arterial pressure of CO2
PaO2 = arterial pressure of O2
HCO3 = serum bicarbonate
concentration
Oxygenation
• Hypoxia: reduced oxygen pressure in
the alveolus (i.e. PAO2)
• Hypoxemia: reduced oxygen
pressure in arterial blood (i.e. PaO2)
Hypoxia with Low PaO2
• Alveolar diffusion impairment
• Decreased alveolar PO2
– Decreased FiO2
– Hypoventilation
– High altitude
• R  L shunt
• V/Q mismatch
Hypoxia with Normal PaO2
• Alterations in hemoglobin
– Anemic hypoxia
– Carbon monoxide poisoning
– Methemoglobinemia
• Histotoxic hypoxia
– Cyanide
• Hypoperfusion hypoxia or stagnant
hypoxia
Alveolar—Arterial
Gradient
• Indirect measurement of V/Q
abnormalities
• Normal A-a gradient is 10 mmHg
• Rises with age
• Rises by 5-7 mmHg for every 0.10
rise in FiO2, from loss of hypoxic
vasoconstriction in the lungs
Alveolar—Arterial
Gradient
A-a gradient = PAO2 – PaO2
• PAO2 = alveolar PO2 (calculated)
• PaO2 = arterial PO2 (measured)
Alveolar—Arterial
Gradient
•
•
•
•
PAO2 = PIO2 – (PaCO2/RQ)
PAO2 = alveolar PO2
PIO2 = PO2 in inspired gas
PaCO2 = arterial PCO2
RQ = respiratory quotient
Alveolar—Arterial
Gradient
PIO2 = FiO2 (PB – PH2O)
• PB = barometric pressure (760 mmHg)
• PH2O = partial pressure of water vapor (47
mmHg)
RQ = VCO2/VO2
• RQ defines the exchange of O2 and CO2
across the alveolar-capillary interface
(0.8)
Alveolar—Arterial
Gradient
PAO2 = FiO2 (PB – PH2O) – (PaCO2/RQ)
Or
PAO2 = FiO2 (713) – (PaCO2/0.8)
Alveolar—Arterial
Gradient
• For room air:
PAO2 = 150 – (PaCO2/0.8)
• And assume a normal PaCO2 (40):
PAO2 = 100
Acid-Base
• Acidosis or alkalosis: any disorder
that causes an alteration in pH
• Acidemia or alkalemia: alteration in
blood pH; may be result of one or
more disorders.
Six Simple Steps
1. Is there acidemia or alkalemia?
2. Is the primary disturbance respiratory
or metabolic?
3. Is the respiratory problem acute or
chronic?
4. For metabolic, what is the anion gap?
5. Are there any other processes in anion
gap acidosis?
6. Is the respiratory compensation
adequate?
Henderson-Hasselbach
Equation
pH = pK + log [HCO3/PaCO2] x K
(K = dissociation constant of CO2)
Or
[H+] = 24 x PaCO2/HCO3
Henderson-Hasselbach
Equation
pH
7.20
7.30
7.40
7.50
7.60
[H+]
60
50
40
30
20
Step 1:
Acidemia or Alkalemia?
• Normal arterial pH is 7.40 ± 0.02
– pH < 7.38  acidemia
– pH > 7.42  alkalemia
Step 2:
Primary Disturbance
• Anything that alters HCO3 is a
metabolic process
• Anything that alters PaCO2 is a
respiratory process
Step 2:
Primary Disturbance
• If 6pH, there is either 5PaCO2 or
6HCO3
• If 5pH, there is either 6PaCO2 or
5HCO3
Step 4:
For Metabolic, Anion Gap?
Anion gap = Na+ - (Cl- + HCO3-)
– Normal is < 12
Increased Anion Gap
• Ingestion of drugs or toxins
–
–
–
–
–
–
–
Ethanol
Methanol
Ethylene glycol
Paraldehyde
Toluene
Ammonium chloride
Salicylates
Increased Anion Gap
• Ketoacidosis
– DKA
– Alcoholic
– Starvation
• Lactic acidosis
• Renal failure
Step 4:
For Metabolic, Anion Gap?
• If + AG, calculate Osm gap:
Calc Osm = (2 x Na+) + (glucose/18) +
(BUN/2.8) + (EtOH/4.6)
Osm gap = measured Osm – calc Osm
Normal < 10 mOsm/kg
Nongap Metabolic
Acidosis
• Administration of acid or acidproducing substances
– Hyperalimentation
– Nonbicarbonate-containing IVF
Nongap Metabolic
Acidosis
• GI loss of HCO3
– Diarrhea
– Pancreatic fistulas
• Renal loss of HCO3
– Distal (type I) RTA
– Distal (type IV) RTA
– Proximal (type II) RTA
Nongap Metabolic
Acidosis
• Calculate urine anion gap:
Urine AG = (Na+ + K+) – Cl– Positive gap indicates renal impaired
NH4+ excretion
– Negative gap indicates normal NH4+
excretion and nonrenal cause
Nongap Metabolic
Acidosis
• Urine Cl- < 10 mEq/l is chloride
responsive and accompanied by
“contraction alkalosis” and is “saline
responsive”
• Urine Cl- > 20 mEq/l is chloride
resistant, and treatment is aimed at
underlying disorder
Step 5: Any other process
with elevated AG?
• Calculate rgap, or “gap-gap”:
rGap = Measured AG – Normal AG (12)
Step 5: Any other process
with elevated AG?
• Add rgap to measured HCO3
– If normal (22-26), no other metabolic
problems
– If < 22, then concomitant metabolic
acidosis
– If > 26, then concomitant metabolic
alkalosis
Step 6: Adequate
respiratory compensation?
Winter’s Formula
Expected PaCO2 = (1.5 x HCO3) + 8 ± 2
– If measured PaCO2 is higher, then
concomitant respiratory acidosis
– If measured PaCO2 is lower, then
concomitant respiratory alkalosis
Step 6: Adequate
respiratory compensation?
• In metabolic alkalosis, Winter’s
formula does not predict the
respiratory response
– PaCO2 will rise > 40 mmHg, but not
exceed 50-55 mmHg
– For respiratory compensation, pH will
remain > 7.42
Clues to a Mixed
Disorder
• Normal pH with abnormal PaCO2 or
HCO3
• PaCO2 and HCO3 move in opposite
directions
• pH changes in opposite direction for
a known primary disorder
Case 1
• A 24 year old student on the 6 year
undergraduate plan is brought to the
ER cyanotic and profoundly weak. His
roommate has just returned from a
semester in Africa. The patient had
been observed admiring his
roommate's authentic African
blowgun and had scraped his finger
on the tip of one of the poison darts
(curare).
Case 1
138
100
26
7.08/80/37
Case 1
• What is the anion gap?
AG = 138 – (100 + 26)
AG = 12
Case 1
• Acute respiratory acidosis
Case 2
• A 42 year old diabetic female who
has been on insulin since the age of
13 presents with a 4 day history of
dysuria which has progressed to
severe right flank pain. She has a
temperature of 38.8ºC, a WBC of
14,000, and is disoriented.
Case 2
135
99
4.8
12
7.23/25/113
Case 2
• What is the A-a gradient?
A-a = [150 – 25/0.8] – 113 = 6
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
AG = 135 – (99 + 12) = 24
Case 2
• What is the rgap?
rGap = 24 – 12 = 12
rGap + HCO3 = 12 + 12 = 24
– No other metabolic abnormalities
• Is the respiratory compensation
appropriate?
Expected PCO2 = (1.5 x 12) + 8 ± 2 = 24 ± 2
– It is appropriate
Case 2
• Acute anion gap metabolic acidosis
(DKA)
Case 3
• A 71 year old male, retired machinist,
is admitted to the ICU with a history
of increasing dyspnea, cough, and
sputum production. He has a 120
pack-year smoking history, and quit 5
years previously. On exam he is
moving minimal air despite using his
accessory muscles of respiration. He
has acral cyanosis.
Case 3
135
93
30
7.21/75/41
Case 3
• What is the A-a gradient?
A-a = [150 – 75/.8] – 41 = 15
• Acidemic or alkalemic?
• Primary respiratory or metabolic?
• Acute or chronic?
– Acute 5PCO2 by 35 would 6pH by 0.28
– Chronic 5PCO2 by 35 would 6pH by 0.105
• Somewhere in between
Case 3
• What is the anion gap?
AG = 135 – (93 + 30) = 12
Case 3
• Acute on chronic respiratory acidosis
(COPD)
Case 3b
• This same patient is intubated and
mechanically ventilated. During the
intubation he vomits and aspirates.
He is ventilated with an FiO2 of 50%,
tidal volumes of 850cc, PEEP of 5,
rate of 10. One hour later his ABG is
7.48/37/215.
Case 3b
• What is the A-a gradient?
A-a = [FiO2 (713) – 37/.8] – 215
A-a = 310 – 215 = 95
• Why is he alkalotic with a normal
PCO2?
– Chronic compensatory metabolic
alkalosis and acute respiratory alkalosis
Case 4
• A 23 year old female presents to the
Emergency Room complaining of
chest tightness and lightheadedness. Other symptoms include
tingling and numbness in her
fingertips and around her mouth. Her
medications include Xanax and birth
control pills, but she recently ran out
of both.
Case 4
135
109
22
7.54/22/115
Case 4
• What is the A-a gradient?
A-a = [150 – 22/.8] – 115 = 8
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• Acute or chronic?
– Acute 6CO2 by 18 would 5pH by 0.144
• What is the anion gap?
AG = 135 – (109 + 22) = 4
Case 4
• Acute respiratory alkalosis (panic
attack)
Case 5
• 72 year old woman admitted from a
nursing home with one week history
of diarrhea and fever.
133
118
5
7.11/16/94
Case 5
• What is the A-a gradient?
A-a = [150 – 16/.8] – 94 = 36
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
AG = 133 – (118 + 5) = 10
• Is respiratory compensation
adequate?
PCO2 = (1.5 x 5) + 8 ± 2 = 16 ± 2
Case 5
• Non anion gap metabolic acidosis
(diarrhea)
• Compensatory respiratory alkalosis
Case 6
• A 27 year old pregnant alcoholic with
IDDM is admitted one week after
stopping insulin and beginning a
drinking binge. She has experienced
severe nausea and vomiting for
several days.
Case 6
136
70
19
7.58/21/104
Case 6
• What is the A-a gradient?
A-a = [150 – 21/.8] – 104 = 20
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
AG = 136 – (70 + 19) = 47
• What is the rgap?
rGap = 47-12 = 35
rGap + HCO3 = 54
Case 6
• Primary respiratory alkalosis
(pregnancy)
• Anion gap metabolic acidosos
(ketoacidosis)
• Metabolic alkalosis (vomiting)
Case 7
• 35 year old male presents to the ER
unconscious.
145
70
23
7.61/24/78
Creat 6.1
Case 7
• What is the A-a gradient?
A-a = [150 – 24/.8] – 78 = 42
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
AG = 145 – (70 + 23) = 52
Case 7
• What is the rgap?
rGap = 52 - 12 = 40
rGap + HCO3 = 63
– Metabolic alkalosis
Case 7
• Respiratory alkalosis
• Anion gap metabolic acidosis (renal
failure)
• Metabolic alkalosis
Bonus Case #1
• 51 year old man with polysubstance
abuse, presented to ER with 3-4 day
h/o N/V and diffuse abdominal pain.
Reports no EtOH or cocaine in 2
weeks. He has been taking “a lot” of
aspirin for pain. Denies dyspnea, but
has been tachypneic since arrival.
Bonus Case #1
• Afebrile, P 89, R 20, BP 142/57.
Lethargic but arrousable, easily
aggitated. Lungs clear, and abdomen
is soft with mild tenderness in LUQ
and LLQ.
Bonus Case #1
126
93
58
3.4
11
1.8
218
UA 1+ ketones
Acetone negative
Lactate 6.9
EtOH 0
Osm 272
7.46/15/107
Bonus Case #1
• What is the A-a gradient?
A-a = [150 – 15/.8] – 107 = 25
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
AG = 126 – (93 + 11) = 22
Anion gap metabolic acidosis
Bonus Case #1
• What is the rgap?
rGap = 22 - 12 = 10
rGap + HCO3 = 21
Non gap metabolic acidosis
• What is the osmolar gap?
Calc Osm = 2x126 + 218/18 + 58/2.8
Calc Osm = 265
Osm gap = 272 – 265 = 7
Bonus Case #1
• Respiratory alkalosis (aspirin)
• Anion gap metabolic acidosis (aspirin)
• Non gap metabolic acidosis
Bonus Case # 2
• 20 year old college student brought
to the ER by his fraternity brothers
because they cannot wake him up. He
had been in excellent health until the
prior night.
Bonus Case #2
• Afebrile, P 118, R 32, BP 120/70.
Anicteric sclerae, pupils 8mm and
poorly responsive to light.
Fundoscopic exam with slight blurring
of discs bilaterally and increased
retinal sheen. Remainder of exam
unremarkable.
Bonus Case #2
142
98
4.3
10
14
108
UA negative
EtOH 45
Osm 348
7.22/24/108
Bonus Case #2
• What is the A-a gradient?
A-a = [150 – 24/.8] – 108 = 12
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
AG = 142 – (98 + 10) = 34
Anion gap metabolic acidosis
Bonus Case #2
• What is the rgap?
rGap = 34 - 12 = 22
rGap + HCO3 = 32
Metabolic alkalosis
Bonus Case #2
• What is the osmolar gap?
Calc Osm = 2x142 + 108/18 + 14/2.8 + 45/4.6
Calc Osm = 305
Osm gap = 348 - 305 = 43
• Is the respiratory compensation
adequate?
PCO2 = (1.5 x 10) + 8 ± 2 = 23 ± 2
Bonus Case #2
• Anion gap metabolic acidosis with
elevated osmolar gap (methanol)
• Metabolic alkalosis
• Compensatory respiratory alkalosis
Bonus Case #3
• A 23 year old man presents with
confusion. He has had diabetes since
age 12, and has been suffering from
an intestinal flu for the last 24
hours. He has not been eating much,
has vague stomach pain, stopped
taking his insulin, and has been
vomiting. His glucose is high.
Bonus Case #3
130
80
10
7.20/25/68
Bonus Case #3
• What is the A-a gradient?
A-a = [150 – 25/.8] – 68 = 51
• Acidemia or alkalemia?
• Primary respiratory or metabolic?
• What is the anion gap?
AG = 130 – (80 + 10) = 40
Anion gap metabolic acidosis
Bonus Case #3
• What is the rgap?
rGap = 40 - 12 = 28
rGap + HCO3 = 38
Metabolic alkalosis
• Is the respiratory compensation
adequate?
PCO2 = (1.5 x 10) + 8 ± 2 = 23 ± 2
Bonus Case #3
• Anion gap metabolic acidosis (DKA)
• Metabolic metabolic alkalosis
(emesis)
• Compensatory respiratory alkalosis