Approach to ABG Analysis

ARTERIAL BLOOD
GASES
Navin Kumar Mishra
Junior Resident-III
IMS-BHU
23-07-2018
BASICS AND INTERPRETATION

OBJECTIVES
• Understand correct sample handling
• Outline an interpretation strategy
• Describe causes for acid-base disturbances
• Delineate a workup strategy for common disorders
@ NKMishra23/07/18 2

ABG
• ABG is a very useful diagnostic tool in our day to day practice.
• The first arterial puncture was performed in 1912 by Hurter, a German
physician.
• Drawn from artery- Radial, Brachial, Femoral, Dorsalis pedis, Posterior
tibial
• It is an invasive procedure.

Acid-Base Physiology
• pH is the negative logarithm to the base 10 of the hydrogen ion
concentration in mmol/L
• pH = - log10[H+]
• An increase in pH indicates a proportionate decrease in the [H+] and a
decrease in the pH indicates a proportionate increase in the [H+].
• H2CO3 generates 12,500 mmol H+ per day.
• Normal metabolism of proteins and nucleotides generates about 100
mmol H+ per day in the form of sulphuric and phosphoric acids.@ NKMishra23/07/18 4

Calculation of pH
• pH is calculated from Henderson-Hasselbalch
equation .
• pH = pK + log acid/bas
• pH = 6.1 + log HCO3-
H2CO3
Kassirer and Bliech modified equation
• H+ = 24 x PCO2/HCO3-

Regulation of pH
pH is maintained in narrow range by
• 1) In seconds: buffer systems
• 2)In minutes: CO2 excretion by the lungs
• 3)In hours to days: renal excretion of H+,
reabsorption of HCO3

Regulation of arterial pH
• 1.BUFFERS –Buffer systems minimize the change in pH resulting from
production of acid .
• Main buffer system in humans is HCO3- in ECF and protein and
phosphate buffers in ICF.
2.ROLE OF THE RESPIRATORY SYSTEM–Elimination of volatile acid CO2.
• a. Respiratory centers in the brain respond to changes in pH of CSF
and blood to affect ventilatory rate.
• b. Ventilation directly controls the elimination of CO2.

ROLE OF KIDNEY
It retains and regenerate HCO3- thereby regenerating the
body buffer with the net effect of eliminating the non-
volatile acid load
a. H+ secretion
1. Free urinary H+ - minimal contribution
2. Ammonia
3. Phosphorus
b. HCO3- reabsorption
1. Proximal tubule – 90%
2. Distal tubule -10%

Applications of ABG
• To document respiratory failure and assess its severity.
• To monitor patients on ventilators and assist in weaning.
• To assess acid base imbalance in critical illness.
• To assess response to therapeutic interventions and
mechanical ventilation.
• To assess pre-op patients.

VENTILATION
PaCO2 = VCO2 x K
VA
Hypercapnea > 45 mm Hg (Hypoventilation)
Respiratory Acidosis
Hypocapnea < 35 mm Hg (Hyperventilation)
Respiratory Alkalosis
VA=Portion of total ventilation participate in gas
exchange with pulmonary blood
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OXYGENATION
• P(A-a)O2
• O2 content
• PaO2 / FiO2 ratio
• arterial-Alveolar O2 tension ratio
@ NKMishra23/07/18 11

P(A-a)O2= PAO2- PaO2
• PAO2- partial pressure of oxygen in alveolar gas
pAO2 = pIO2 – (paCO2 / R)
PAO2= (PB-P h2o) x FiO2- (paCO2/R)
Eg.:
=(760-47) x .21- (40/0.8) =100
PAO2-Pao2= 100-80=20
N= <15
PAO2 = partial pressure of oxygen in alveolar gas, PB = barometric
pressure (760mmHg), Ph2o = water vapor pressure (47 mm Hg),
FiO2 = fraction of inspired oxygen, PCO2 = partial pressure of CO2
in the ABG, R = respiratory quotient (0.8)
@ NKMishra23/07/18 12

P(A-a)O2= PAO2- PaO2
• PaO2- partial pressure of oxygen in blood
PaO2 = FiO2 × 5
PaO2 = 109 - 0.4 (Age)
PaO2 is dependant upon Age, FiO2, Patm
HYPOXEMIA
Mild (60-80) mmHg
Moderate(40-60) mmHg
Severe <40 mmHg
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Arterial Blood pH
7.4
Neutral
Academia
Acidosis?
Alkalemia
Alkalosis?
7.35~7.45
6.8 7.8
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TECHNIQUES of Sampling
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SAMPLE MANAGEMENT
• Sample needs to be iced
• At room temperature sample lasts 15 – 20 min
• PaCO2 increases
• PaO2 decreases
• pH decreases  Red cell glycolysis
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CAUTION
pH PaCO2 PaO2
Heparin ↓ ↓
Air Bubbles ↓ ↑
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Technical Errors
Risk of alteration of results with:
1)size of syringe/needle
2)vol of sample
 Syringes must have > 50% blood
 Use only 3ml or less syringe
25% lower values if 1 ml sample taken in 10 ml syringe (0.25 ml heparin in
needle)
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Parameters Excessive Heparin Air bubbles
pH ↓ or remain the same ↑
PCO2 ↓ ↓
PO2 May altered May altered
HbO2% sat May altered May altered
HbCO2% sat Will not altered Will not altered
Hb content ↓ Is not altered
HCO3 ↓ ↓
Base Excess ↓ ↓
Oxygen content May be altered Maybe altered
@ NKMishra23/07/18 21

Technical Errors
WBC Counts
0.01 ml O2 consumed/dL/min
Marked increase in high TLC/plt counts : dec.pO2
Chilling / immediate analysis
ABG Syringe must be transported earliest via COLD CHAIN
Change/10 min Uniced 370C Iced 40C
pH 0.01 0.001
pCO2 1 mm Hg 0.1 mm Hg
pO2 0.1% 0.01%
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Site Selection
• Radial Artery – 45. insertion angle
• Brachial Artery – 60-90 .insertion angle
• Femoral Artery – 90 insertion angle
• Dorsalis Pedis Artery
• Posterior Tibial artery
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Contraindication
• No absolute contraindications
• Dialysis shunt – choose another site
• Patient on anticoagulant/aspirin therapy – may have to hold
pressure on puncture site longer than normal
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Site specific contraindication
Radial : Buergers disease
Raynauds
Absent Ulnar collateral circulation
AV dialysis shunt
Femoral: Local infection
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@ NKMishra
Only the person who collect the sample can tell if he has
drawn a pulsating blood
Partly mixed sample- Difficult to recognize
Arterial Venous
PH 7.35 - 7.45 7.36- 7.39
Pco2 35- 45 44 - 48
Po2 80 - 100 38 - 42
Hco3 24- 26 20 - 24
Sao2 95- 100 % 75%
23/07/18 33

A Stepwise
Approach to
Solving
Acid-Base
Disorders

Step 1: Assess the internal consistency of the values using
the Henderseon-Hasselbach equation:
• [H+] = 24(PaCO2)
[HCO3-]
@ NKMishra23/07/18 35

pH Approximate [H+]
(mmol/L)
7.00 100
7.05 89
7.10 79
7.15 71
7.20 63
7.25 56
7.30 50
7.35 45
7.40 40
7.45 35
7.50 32
7.55 28
7.60 25
7.65 22
If the pH and the [H+] are inconsistent, the ABG is probably not valid.
@ NKMishra23/07/18 36

Step 2: Is there alkalemia or acidemia present?
• pH < 7.35 acidemia
pH > 7.45 alkalemia
• This is usually the primary disorder
• Remember: an acidosis or alkalosis may be present even if the pH is in
the normal range (7.35 – 7.45)
• You will need to check the PaCO2, HCO3- and anion gap
@ NKMishra23/07/18 37

• Step 3: Is the disturbance respiratory or metabolic?
• What is the relationship between the direction of change in the pH and the
direction of change in the PaCO2?
• In primary respiratory disorders, the pH and PaCO2 change in opposite directions;
in metabolic disorders the pH and PaCO2 change in the same direction.
Acidosis Respiratory pH ↓ PaCO2 ↑ ROME
Acidosis Metabolic& pH ↓ PaCO2 ↓ ROME
Alkalosis Respiratory pH ↑ PaCO2 ↓ ROME
Alkalosis Metabolic pH ↑ PaCO2 ↑ ROME
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• Step 4: Is there appropriate compensation for the primary disturbance?
• Usually, compensation does not return the pH to normal (7.35 – 7.45) except….?
Disorder Expected compensation Correction factor
Metabolic acidosis PaCO2 = (1.5 x [HCO3-]) +8 ± 2
Acute respiratory acidosis Increase in [HCO3-]= ∆ PaCO2/10 ± 3
Chronic respiratory acidosis (3-5 days) Increase in [HCO3-]= 3.5(∆ PaCO2/10)
Metabolic alkalosis Increase in PaCO2 = 40 + 0.6(∆HCO3-)
Acute respiratory alkalosis Decrease in [HCO3-]= 2(∆ PaCO2/10)
Chronic respiratory alkalosis Decrease in [HCO3-] = 5(∆ PaCO2/10) to 7(∆
PaCO2/10)
If the observed compensation is not the expected compensation, it is likely that more than one acid-base disorder is
present. @ NKMishra23/07/18 39

• Step 5: Calculate the anion gap (if a metabolic acidosis exists):
• AG= [Na+]-( [Cl-] + [HCO3-] )=12 ± 2
• A normal anion gap is approximately 12 meq/L.
• In patients with hypoalbuminemia, the normal anion gap is lower than 12 meq/L; the
“normal” anion gap in patients with hypoalbuminemia is about 2.5 meq/L lower for
each 1 gm/dL decrease in the plasma albumin concentration (for example, a patient
with a plasma albumin of 2.0 gm/dL would be approximately 7 meq/L.)
• If the anion gap is elevated, consider calculating the osmolal gap in compatible clinical
situations.
• Elevation in AG is not explained by an obvious case (DKA, lactic acidosis, renal failure
• Toxic ingestion is suspected
• OSM gap = measured OSM – (2[Na+] - glucose/18 – BUN/2.8
• The OSM gap should be < 10
@ NKMishra23/07/18 40

• Step 6: If an increased anion gap is present, assess the relationship between
the increase in the anion gap and the decrease in [HCO3-].
• Assess the ratio of the change in the anion gap (∆AG ) to the change in [HCO3-
] (∆[HCO3-]): ∆AG/∆[HCO3-]
• This ratio should be between 1.0 and 2.0 if an uncomplicated anion gap
metabolic acidosis is present.
• If this ratio falls outside of this range, then another metabolic disorder is
present:
• If ∆AG/∆[HCO3-] < 1.0, then a concurrent non-anion gap metabolic acidosis is
likely to be present.
• If ∆AG/∆[HCO3-] > 2.0, then a concurrent metabolic alkalosis is likely to be
present.
• It is important to remember what the expected “normal” anion gap for your
patient should be, by adjusting for hypoalbuminemia (see Step 5, above.)
@ NKMishra23/07/18 41

Disorder pH Primary problem Compensation
Metabolic acidosis ↓ ↓ in HCO3- ↓ in PaCO2
Metabolic alkalosis ↑ ↑ in HCO3- ↑ in PaCO2
Respiratory acidosis ↓ ↑ in PaCO2 ↑ in [HCO3-]
Respiratory alkalosis ↑ ↓ in PaCO2 ↓ in [HCO3-]
Table 1: Characteristics of acid-base disturbances
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Table 2: Selected etiologies of respiratory acidosis
oAirway obstruction
- Upper
- Lower
o COPD
o asthma
o other obstructive lung disease
oCNS depression
oSleep disordered breathing (OSA or OHS)
oNeuromuscular impairment
oVentilatory restriction
oIncreased CO2 production: shivering, rigors, seizures, malignant hyperthermia, hypermetabolism,
increased intake of carbohydrates
oIncorrect mechanical ventilation settings
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Table 3: Selected etiologies of respiratory alkalosis
oCNS stimulation: fever, pain, fear, anxiety, CVA, cerebral edema,
brain trauma, brain tumor, CNS infection
oHypoxemia or hypoxia: lung disease, profound anemia, low FiO2
oStimulation of chest receptors: pulmonary edema, pleural effusion,
pneumonia, pneumothorax, pulmonary embolus
oDrugs, hormones: salicylates, catecholamines,
medroxyprogesterone, progestins
oPregnancy, liver disease, sepsis, hyperthyroidism
oIncorrect mechanical ventilation settings
@ NKMishra23/07/18 44

Table 4: Selected causes of metabolic alkalosis
oHypovolemia with Cl- depletion
o GI loss of H+
o Vomiting, gastric suction, villous adenoma,
diarrhea with chloride-rich fluid
o Renal loss H+
o Loop and thiazide diuretics, post-hypercapnia
(especially after institution of mechanical
ventilation)
oHypervolemia, Cl- expansion
o Renal loss of H+: edematous states (heart failure,
cirrhosis, nephrotic syndrome),
hyperaldosteronism, hypercortisolism, excess
ACTH, exogenous steroids, hyperreninemia,
severe hypokalemia, renal artery stenosis,
bicarbonate administration
@ NKMishra23/07/18 45

Table 5: Selected etiologies of metabolic acidosis
oElevated anion gap:
o Methanol intoxication
o Uremia
o Diabetic ketoacidosisa, alcoholic ketoacidosis,
starvation ketoacidosis
o Paraldehyde toxicity
o Isoniazid
o Lactic acidosisa
o Type A: tissue ischemia
o Type B: Altered cellular metabolism
o Ethanolb or ethylene glycolb intoxication
o Salicylate intoxication
a Most common causes of metabolic acidosis with an
elevated anion gap
b Frequently associated with an osmolal gap
oNormal anion gap: will have increase in [Cl-]
o GI loss of HCO3-
o Diarrhea, ileostomy, proximal colostomy,
ureteral diversion
o Renal loss of HCO3-
o proximal RTA
o carbonic anhydrase inhibitor (acetazolamide)
o Renal tubular disease
o ATN
o Chronic renal disease
o Distal RTA
o Aldosterone inhibitors or absence
o NaCl infusion, TPN, NH4+ administration
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Disorder Characteristics Selected situations
Respiratory acidosis with
metabolic acidosis
↓in pH
↓ in HCO3
↑ in PaCO2
•Cardiac arrest
•Intoxications
•Multi-organ failure
Respiratory alkalosis with
metabolic alkalosis
↑in pH
↑ in HCO3-
↓ in PaCO2
•Cirrhosis with diuretics
•Pregnancy with vomiting
•Over ventilation of COPD
Respiratory acidosis with
metabolic alkalosis
pH in normal range
↑ in PaCO2,
↑ in HCO3-
•COPD with diuretics, vomiting, NG suction
•Severe hypokalemia
Respiratory alkalosis with
metabolic acidosis
pH in normal range
↓ in PaCO2
↓ in HCO3
•Sepsis
•Salicylate toxicity
•Renal failure with CHF or pneumonia
•Advanced liver disease
Metabolic acidosis with metabolic
alkalosis
pH in normal range
HCO3- normal
•Uremia or ketoacidosis with vomiting, NG suction,
diuretics, etc.
Table 6: Selected mixed and complex acid-base disturbances
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ARTERIAL BLOOD GAS
MEASURES
• PaCO2
• PaO2
• pH
CALCULATES
• Bicarbonate
• SaO2
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INTERPRETATION
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Normal Values
• pH - 7.35 - 7.45
• PaCO2 - 35-45 mmHg
• PaO2 - 80-100 mmHg
• HCO3 - 22-26
• O2sat - 95-100%
• Base Excess - +/-2 m Eq/L
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Base excess and deficit
• The BE (or base deficit) is defined as the amount of acid (or base) required
to be added to whole blood to achieve a pH of 7.4 at 37˚C and paCO2 of
40mmHg.
If the base is in excess
• may be due to decrease in metabolic acids
• may be due to increase in buffers (e.g. HCO3-)
If the base is in deficit
• may be due to excess metabolic acids
@ NKMishra23/07/18 51

 Step 1: is it reliable??
 STEP -2 : Comprehensive history and physical
examination.
 STEP -3 : Acidosis or alkalosis..???
See the pH (<7.35 or >7.45)
 STEP -4 : Identify the primary disorder
See the change in PCo2 & HCO3
 STEP -5 : Calculate the compensatory response
Is adequately compensated???
@ NKMishra23/07/18 52

 STEP -6 : Calculate anion gap
STEP -7 : Calculate the delta gap (unmask hidden
mixed disorders)
STEP -8 : Calculate the osmolar gap (for high AG
acidosis)
 STEP -9 : Calculate the urinary anion gap (Non AG
metabolic acidosis)
 STEP -10 : Formulate differential diagnosis
@ NKMishra23/07/18 53

INTERPRETATION STEPS
1. pH
2. PaCO2
3. Anion Gap
- Delta ratio
4. Compensation
pH PaCO2 Disorder
↓ ↓ Metabolic Acidosis
↑ ↑ Metabolic Alkalosis
↓ ↑ Respiratory Acidosis
↑ ↓ Respiratory Alkalosis
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ANION GAP (AG)
• Normal value  8 – 12
• Primarily determined by negatively charged plasma proteins
• Albumin ↓ 1 g/dL (below 4/4.5)  AG ↓ 2.5
• AG needs correction for hypoalbuminemia
@ NKMishra23/07/18 55

UNADJUSTED AG ADJUSTED AG
Decreased 26.7 %
Decreased 4.7 %
Normal 22.0 %
Normal 62.6 %
Normal 36.4 %
Increased 26.3 %
Increased 10.7 % Increased 10.7 %
J Lab Clin Med 2005;146:317–20 @ NKMishra23/07/18 56

Some quick clues:
• 1. if CO2 and HCO3 in opposite direction , it indicates mixed d/o
• 2. if pH and Co2 same direction, it indicates Metabolic d/o=ROME
• 3. if pH and Co2 same direction, it indicates Respiratory d/o= ROME
• If the difference between digits after decimal in PH and CO2 is
• <15= metabolic
• > 15= Respiratory
@ NKMishra23/07/18 57

Case 0
• pH 7.28
• PaCO2 27 mm Hg
• PaO2 105 mm Hg
• H+ 70 mmhg
• Na+ 134 mmol/L
• K+ 3.7 mmol/L
• Cl- 109 mmol/L
• HCO3
- 13 mmol/L
• Albumin 4.0 g/dL
• AG 12
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• H+ = 24 X 27/ 13
• = 49.8
• Not Reliable test.
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CASE 1
• pH 7.28
• PaCO2 27 mm Hg
• PaO2 105 mm Hg
• H+ 50 mmhg
• Na+ 134 mmol/L
• K+ 3.7 mmol/L
• Cl- 109 mmol/L
• HCO3
- 13 mmol/L
ANION GAP = 12
@ NKMishra23/07/18 62

1. pH: ↓
2. PCO2: ↓
3. Anion Gap:
- Delta ratio
4. Compensation
CASE 1
METABOLIC ACIDOSIS
NON-ANION GAP
pH PaCO2 Disorder
Normal
@ NKMishra23/07/18 63

COMPENSATION
PaCO2 = 1.5 x HCO3
- + 8 ± 2
PaCO2 = 1.5 x 13 + 8 ± 2
PaCO2 = 19.5 + 8 ± 2
PaCO2 = 27.5 ± 2
PaCO2 = 25.5 - 29.5
PaCO2 = 27
@ NKMishra23/07/18 64

1. pH: ↓
2. PCO2: ↓
3. Anion Gap: Normal
- Delta ratio
4. Compensation
CASE 1
METABOLIC ACIDOSIS
NON-ANION GAP
ADEQUATE
COMPENSATION
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NON-ANION GAP ACIDOSIS
• Hyperchloremic: Net
Bicarbonate loss
• Two main different routes
• GI: diarrhea, fistulas
• Renal: RTAs, drugs
• What is used to differentiate?
Urine AG = Na+ + K+ - Cl-
UAG = Negative normally bcz CL- content is high in
urine.
• NAG Acidosis:
Diarrhea
NAG M.Acidosis
UAG= Neg
K+= dec
RTA
NAG M.Acidosis
UAG= Positive
K+= Inc
@ NKMishra23/07/18 66

NEJM 1988;318:594-99
CONTROL
CONTROL
CONTROL
RTA
DIARRHEA
DIARRHEA
DIARRHEA
RTA
RTA
@ NKMishra23/07/18 67

URINE ANION GAP
[Na+] + [K+] + [NH4
+] + [UC] ≈ [Cl-] + [HCO3-] + [UA]
[Na+] + [K+] - [Cl-] ≈ [UA] - [UC]
UAG ≈ [Na+] + [K+] - [Cl-]
?
@ NKMishra23/07/18 68

HOW TO USE IT
NORMAL RESPONSE TO ACIDOSIS
UAG ≈ [Na+] + [K+] - [Cl-]
UAG < 0
IF KIDNEYS UNABLE TO EXCRETE H+
UAG ≈ [Na+] + [K+] - [Cl-]
UAG > 0
NH4
+ Cl- NH4
+ Cl
-
@ NKMishra23/07/18 69

NON ANION GAP ACIDOSIS
GASTROINTESTINAL
- Diarrhea
- External pancreatic or bowel
drainage
- Ureterosigmoidostomy
- Drugs
* Calcium Chloride
* Magnesium Sulfate
* Cholestyramine
RENAL
- Hypokalemic
* Proximal RTA (type 2)
* Distal RTA (type 1)
* Drugs: Acetazolamide, Amphotericin B
- Hyperkalemic
* Type 4 RTA
* Mineralocorticoid deficiency
* Tubulointerstitial disease
* Ammonium excretion defect
* Drugs: K+ sparing diuretics, ACEi
ARBs, Trimethoprim, Pentamidine,
NSAIDs, Cyclosprine, tacrolimus, etc
@ NKMishra23/07/18 70

URINE ANION
GAP
NEGATIVE
Gastrointestinal
Diarrhea
Small bowel / pancreatic
drainage
IATROGENIC
Parenteral nutrition
Saline
Anion exchange resins
POSITIVE
URINE pH > 5.5
Type I RTA
URINE pH < 5.5
Low K+
Type II RTA
High K+
Aldosterone deficiency
Type IV RTA
@ NKMishra23/07/18 71

CASE 2
• pH 7.30
• PaCO2 34 mm Hg
• PaO2 235 mm Hg
• H+ 48 mmhg
• Na+ 144 mmol/L
• K+ 4.5 mmol/L
• Cl- 105 mmol/L
• HCO3
- 17 mmol/L
ANION GAP = 22
@ NKMishra23/07/18 73

1. pH: ↓
2. PCO2: ↓
3. Anion Gap: ↑
- Delta Ratio
4. Compensation
CASE 2
METABOLIC ACIDOSIS
ANION GAP
pH PaCO2 Disorder
@ NKMishra23/07/18 74

DELTA RATIO
• Delta ratio = Δ anion gap / Δ bicarbonate
Actual AG – 12
Delta Ratio =
24 – HCO3
-
22 – 12 12
Delta Ratio = = = 1.7
24 – 17 7
@ NKMishra23/07/18 75

NON-ANION GAP ACIDOSIS
(↑↑ AG – 12)
Delta Ratio = -----------------------
(24-↓↓↓HCO3)
• Delta Ratio < 0.8
METABOLIC ALKALOSIS
(↑↑ AG – 12)
Delta Ratio = -----------------------
(24 - ≈↑HCO3)
• Delta Ratio > 1.5
Δ AG (↑AG-12)
Delta Ratio = ------------- = -----------------
Δ HCO3- (24-↓HCO3)
(0.8-1.5)1
@ NKMishra23/07/18 76

DELTA RATIO
Δ RATIO
< 0.8 0.8 - 1.5 > 1.5
Anion Gap metabolic acidosis
Anion Gap metabolic
acidosis
Non anion gap acidosis Metabolic alkalosis
@ NKMishra23/07/18 77

1. pH: ↓
2. PCO2: ↓
3. Anion Gap: ↑
- Delta Ratio = 1.7
4. Compensation
CASE 2
METABOLIC ACIDOSIS
ANION GAP
METABOLIC
ALKALOSIS
@ NKMishra23/07/18 78

COMPENSATION
PaCO2 = 1.5 x HCO3
- + 8 ± 2
PaCO2 = 1.5 x 17 + 8 ± 2
PaCO2 = 25.5 + 8 ± 2
PaCO2 = 33.5 ± 2
PaCO2 = 31.5 - 35.5
PaCO2 = 34
@ NKMishra23/07/18 79

1. pH: ↓
2. PCO2: ↓
3. Anion Gap: ↑
- Delta Ratio = 1.7
4. Compensation
CASE 2
METABOLIC ACIDOSIS
ANION GAP
METABOLIC
ALKALOSIS
ADEQUATE
COMPENSATION
@ NKMishra23/07/18 80

HIGH ANION GAP
M ethanol
U remia
D iabetic Ketoacidosis: alcohol, starvation
P araldehyde, paracetamol
I ron, Isoniazid, inborn errors of metabolism
L actic Acidosis
E thylene glycol, Ethanol
S alicylates
@ NKMishra23/07/18 81

CASE 3
• pH 7.24
• PaCO2 60 mm Hg
• PaO2 158 mm Hg
• Na+ 140 mmol/L
• K+ 4.4 mmol/L
• Cl- 103 mmol/L
• HCO3
- 25 mmol/L
ANION GAP = 12
@ NKMishra23/07/18 83

1. pH: ↓
2. PCO2: ↑
CASE 3
RESPIRATORY
ACIDOSIS
pH PaCO2 Disorder
ACUTE OR CHRONIC?
@ NKMishra23/07/18 84

RESPIRATORY DISORDERS
Δ10 PaCO2 ACUTE CHRONIC
Δ pH 0.08 0.03
@ NKMishra23/07/18 85

ACUTE OR CHRONIC?
• PaCO2 = 60
• pH = 7.24
Δ pH 0.08 0.03
Δ 2x10 = 20
Δ 2x0.08 =0.16
@ NKMishra23/07/18 86

1. pH: ↓
2. PCO2: ↑
- Delta Ratio
4. Compensation
CASE 3
RESPIRATORY
ACIDOSIS
ACUTE
@ NKMishra23/07/18 87

HCO3
- ???
1 2
4 5
Δ 10 PaCO2
ACUTE
CHRONIC
ACIDOSIS ALKALOSIS
@ NKMishra23/07/18 88

CASE 3
• pH 7.24
• PaCO2 60 mm Hg
• HCO3
- 25 mmol/L
1 2
4 5
Δ 10 PaCO2
ACUTE
CHRONIC
ACIDOSIS ALKALOSIS
Δ 2x10=20
Δ 1
@ NKMishra23/07/18 89

1. pH: ↓
2. PCO2: ↑
- Delta Ratio
4. Compensation
CASE 3
RESPIRATORY
ACIDOSIS
ACUTE
ADEQUATE
COMPENSATION
@ NKMishra23/07/18 90

CASE 4
• pH 7.33
• PaCO2 60 mm Hg
• PaO2 158 mm Hg
• Na+ 146 mmol/L
• K+ 4.4 mmol/L
• Cl- 102 mmol/L
• HCO3
- 32 mmol/L
ANION GAP = 12
@ NKMishra23/07/18 92

1. pH: ↓
2. PCO2: ↑
CASE 4
RESPIRATORY
ACIDOSIS
pH PaCO2 Disorder
ACUTE OR CHRONIC?
@ NKMishra23/07/18 93

RESPIRATORY DISORDERS
Δ pH 0.08 0.03
@ NKMishra23/07/18 94

ACUTE OR CHRONIC?
• PaCO2 = 60
• pH = 7.33
Δ pH 0.08 0.03
Δ 2x10=20
Δ2x0.03= 0.06
@ NKMishra23/07/18 95

1. pH: ↓
2. PCO2: ↑
- Delta Ratio
4. Compensation
CASE 4
RESPIRATORY
ACIDOSIS
CHRONIC
@ NKMishra23/07/18 96

CASE 4
• pH 7.33
• PaCO2 60 mm Hg
• HCO3
- 32 mmol/L
1 2
4 5
Δ 10 PaCO2
ACUTE
CHRONIC
ACIDOSIS ALKALOSIS
↑2 x 10 = 20
↑ 2 x 4 = 8
@ NKMishra23/07/18 97

1. pH: ↓
2. PCO2: ↑
- Delta Ratio
4. Compensation
CASE 4
RESPIRATORY
ACIDOSIS
CHRONIC
ADEQUATE
COMPENSATION
@ NKMishra23/07/18 98

CASE 5
• pH 7.47
• PaCO2 45 mm Hg
• PaO2 146 mm Hg
• Na+ 143 mmol/L
• K+ 4.2 mmol/L
• Cl- 101 mmol/L
• HCO3
- 33 mmol/L
ANION GAP = 9
@ NKMishra23/07/18 100

1. pH: ↑
2. PCO2: ↑
- Delta Ratio
4. Compensation
CASE 5
METABOLIC
ALKALOSIS
pH PaCO2 Disorder
@ NKMishra23/07/18 101

COMPENSATION
PaCO2 = 0.7 x HCO3
- + 21 ± 2
PaCO2 = 0.7 x 33 + 21 ± 2
PaCO2 = 23.1 + 21 ± 2
PaCO2 = 44.1 ± 2
PaCO2 = 42.1 - 46.1
PaCO2 = 45
@ NKMishra23/07/18 102

1. pH: ↑
2. PCO2: ↑
- Delta Ratio
4. Compensation
CASE 5
METABOLIC
ALKALOSIS
ADEQUATE
COMPENSATION
@ NKMishra23/07/18 103

METABOLIC ALKALOSIS
• Decreased H+ concentration
• Chloride responsive
• Chloride resistant
• Measure Urine Cl-
• IF Urine CL <20MEQ = Cl responsive M.Alk
• IF Urine CL >20MEQ = Cl resistant M.Alk
@ NKMishra23/07/18 104

URINE CHLORIDE
CHLORIDE
RESPONSIVE
Gastric fluid loss
Diuretics
Post hypercapnia
Villous adenoma
Congenital chloridorrhea
CHLORIDE RESISTANT
Laxative abuse
Severe K+ depletion
Diuretic abuse
Bartter or Gitelman Syndrome
Primary aldosteronism
Adrenal Hyperplasia
Cushing syndrome
< 20 mEq/L > 20 mEq/L
@ NKMishra23/07/18 105

METABOLIC ALKALOSIS
RENAL HYDROGEN LOSS
- Primary mineralocorticoid excess
- Mineralocorticoid excess-like
states
* Licorice ingestion
* Liddle syndrome
- Loop or thiazide diuretics
- Bartter or Gitelman syndrome
- Post-hypercapnic alkalosis
- Hypercalcemia and the milk-alkali
syndrome
INTRACELLULAR shift of H+
- Severe hypokalemia
* Villous Adenoma
* Laxative abuse
GASTROINTESTINAL H+ LOSS
- Vomiting or nasogastric suction
- Congenital chloride diarrhea
ALKALI ADMINISTRATION WITH
REDUCED RENAL FUNCTION
CONTRACTION ALKALOSIS
@ NKMishra23/07/18 106

CASE 6
• pH 7.40
• PaCO2 38 mm Hg
• PaO2 106 mm Hg
• Na+ 141 mmol/L
• K+ 4.8 mmol/L
• Cl- 97 mmol/L
• HCO3
- 23 mmol/L
ANION GAP = 21
@ NKMishra23/07/18 108

1. pH: Normal
2. PCO2: Normal
3. Anion Gap: ↑
- Delta Ratio
4. Compensation
CASE 6
METABOLIC ACIDOSIS
ANION GAP
pH PaCO2 Disorder
@ NKMishra23/07/18 109

DELTA RATIO
• Delta ratio = Δ anion gap / Δ bicarbonate
Actual AG – 12
Delta Ratio =
24 – HCO3
-
21 – 12 9
Delta Ratio = = = 9
24 – 23 1
@ NKMishra23/07/18 110

DELTA RATIO
Δ RATIO
< 0.8 0.8 - 1.5 > 1.5
Anion Gap metabolic
acidosis
@ NKMishra23/07/18 111

1. pH: Normal
2. PCO2: Normal
3. Anion Gap: ↑
- Delta Ratio = 9
4. Compensation
CASE 6
METABOLIC ACIDOSIS
ANION GAP
METABOLIC
ALKALOSIS
@ NKMishra23/07/18 112

COMPENSATION
PaCO2 = 1.5 x HCO3
- + 8 ± 2
PaCO2 = 1.5 x 23 + 8 ± 2
PaCO2 = 34.5 + 8 ± 2
PaCO2 = 42.5 ± 2
PaCO2 = 40.5 - 44.5
PaCO2 = 38
@ NKMishra23/07/18 113

1. pH: Normal
2. PCO2: Normal
3. Anion Gap: ↑
- Delta Ratio = 9
4. Compensation
CASE 6
METABOLIC ACIDOSIS
ANION GAP
METABOLIC
ALKALOSIS
RESPIRATORY
ALKALOSIS
@ NKMishra23/07/18 114

Case 7
• pH: 7.4
• PCO2: 40MM/HG
• HCO3: 25MMOL/L
• AG: 30
• M.AC with M.Alk with R.Alk
@ NKMishra23/07/18 115

• Case 8. A 54y/o male, Alcoholic presented to ER with c/o vomiting and
increased respiratory rate.
• PH: 7.4
• PCO2: 30
• HCO3:25
• AG: 30
• M. AC(Alcoholic Ketoacidosis)
• with M.Alk(Vomiting)
• with R. Alk( Hyperventilation due to hepatic dysfxn or alcohol withdrawal)
@ NKMishra23/07/18 116

SUMMARY
@ NKMishra23/07/18 117

SUMMARY
1. pH
2. PaCO2
3. Anion Gap
- Delta ratio
4. Compensation
DISORDER EXPECTED PCO2
Metabolic Acidosis PaCO2 = 1.5 x HCO3 + 8 ± 2
Metabolic Alkalosis PaCO2 = 0.7 x HCO3 + 21 ± 2
Δ RATIO
< 0.8 0.8 - 1.5 > 1.5
Anion Gap metabolic
acidosis Anion Gap metabolic
acidosis
Anion Gap metabolic
acidosis
pH PaCO2 Disorder
@ NKMishra23/07/18 119

SUMMARY
1. pH
2. PaCO2
3. Anion Gap
- Delta ratio
4. Compensation
Δ pH 0.08 0.03
1 2
4 5
Δ 10 PaCO2
ACUTE
CHRONIC
ACIDOSIS ALKALOSIS
@ NKMishra23/07/18 120

Treat the patient not the ABG.
Thank you!!
Have a Nice Day!!
@ NKMishra23/07/18 121

Case-1
• 60 years old M, presents to the ED with rapid
breathing and less responsive than usual. No other
history available.
ABG results
pH 7.31
PCO₂ 10
HCO₃ 5
Na 123
K 5
Cl 99
@ NKMishra23/07/18 122

Stepwise interpretation
1. At pH 7.3 H+ conc. Should be ≈50nmol/L
• Calculated H+ = 24 × 10/5 = 24 × 2 = 48
• Both values corroborate, hence result is valid.
2. pH is 7.3, i.e Acidosis
3. HCO₃ value has gone down, primary process is
metabolic
4. Respiratory compensation:
• Calculated PCO₂ = (1.5 × 5)+8 ± 2 = 13.5 to 17.5
• Partially compensated M.Acidosis a/w respiratory
alkalosis :- Mixed disorder
@ NKMishra23/07/18 123

5. Anion gap: (123+5) – (99 + 5) = 24
• High anion gap metabolic acidosis
Finally:- Mixed acid base disorder, with presence of high
AG metabolic acidosis and respiratory alkalosis.
@ NKMishra23/07/18 124

Case-2
• A k/c/o COPD with cor pulmonale on treatment
presented with progressive breathlessness.
ABG results
pH 7.42
PCO₂ 67
HCO₃ 42
Na 140
K 3.5
Cl 88
@ NKMishra23/07/18 125

• pH is normal; but PCO₂ & HCO₃ both are increased.
• Change in PCO₂ is 67-40 = 27
• Expected rise in HCO₃ should be 27 × 0.4 = 10.8
• Expected HCO₃ = 24+10.8 ≈ 35
• Actual HCO₃ = 42
• AG = 12 (N)
• Mixed disorder, both respiratory acidosis & metabolic
alkalosis.
@ NKMishra23/07/18 126

Case -3
• A known case of chronic kidney disease, discontinued
dialysis & presented to the emergency in an altered
state of sensorium. Attendants gave history of
repeated episodes of vomiting at home.
ABG results
pH 7.42
PCO₂ 40
HCO₃ 25
Na 140
K 3.0
Cl 95@ NKMishra23/07/18 127

• pH, PCO₂, HCO₃ all WNL
• AG = 23 (↑)
• Delta gap = 13 – 1 = 12 (↑)
• AG >> HCO3
–
• Mixed disorder with presence of both high AG
metabolic acidosis and metabolic alkalosis.
@ NKMishra23/07/18 128

Case-4
• 65 yrs old M, past h/o Acute MI on medication,
presented with high grade fever with, cough & yellowish
expectoration for 5 days. Acute increase in shortness of
breath.
ABG results
pH 7.3
PCO₂ 38
HCO₃ 16
Na 136
K 4
Cl 102
@ NKMishra23/07/18 129

• pH 7.3 = Acidosis
• HCO₃ is low ; primary disorder is metabolic acidosis
• Expected PCO₂ = (1.5 × 16) + 8 = 32
• Calculated PCO₂ < estimated PCO₂
• AG = 22
• Delta gap = (10-8) = 2
• Mixed disorder with metabolic acidosis & respiratory
acidosis
@ NKMishra23/07/18 130

Case-6
• ABHG parameters are following:-
PH = 7.39
PaCO2 = 24
HCO3 = 14
Na = 140
K = 4
Cl = 106
AG = 20
@ NKMishra23/07/18 134

• Result:- High anion gap Metabolic Acidosis with Respiratory Alkalosis
• Examples- Lactic acidosis,Sepsis
@ NKMishra23/07/18 135

Case-7
PH = 7.42
PaCO2 = 67
HCO3 = 42
Na = 140
K = 3.5
Cl = 88
AG = 10
@ NKMishra23/07/18 136

• Result:- Metabolic Alkalosis with Respiratory Acidosis
• Examples- COPD Patients
@ NKMishra23/07/18 137

Case-8
PH = 7.3
PaCO2 = 38
HCO3 = 18
Na = 140
K = 4
Cl = 102
AG = 20
@ NKMishra23/07/18 138

• Result:- Metabolic Acidosis with Respiratory Acidosis
• Examples- Pneumonia, Pulmonary Edema
@ NKMishra23/07/18 139

Case-9
PH = 7.42
PaCO2 = 40
HCO3 = 25
Na = 140
K = 3
Cl = 95
AG = 23
@ NKMishra23/07/18 140

• Result:- Mixed Disorder-Metabolic Acidosis with Metabolic Alkalosis
• Examples- Uremia with Vomiting
@ NKMishra23/07/18 141

Case-10
• ABG parameters are following:-
PH = 7.2
PaCO2 = 25
HCO3 = 10
Na = 132
K = 3
Cl = 110
AG = 15
@ NKMishra23/07/18 142

• Result:- Normal AG Hyperchloremic Metabolic Acidosis
• Examples- DKA
@ NKMishra23/07/18 143

METABOLIC ACIDOSIS
@ NKMishra23/07/18 144

Causes of High AG Met Acidosis
1. Ketoacidosis:
Diabetic
Alcoholic
Starvation
2. Lactic Acidosis:
Type A (Inadequate O2 Delivery to Cells)
Type B (Inability of Cells to utilise O2)
Type D (Abnormal bowel anatomy)
3. Toxicity:
Salicylates Paraldehyde
Methanol Toluene
Ethylene Glycol
4. Renal Failure
5. Rhabdomyolsis
@ NKMishra23/07/18 145

CAUSES OF NORMAL ANION GAP METABOLIC
ACIDOSIS
1.HCO3 loss:
GIT Diarrhoea
Pancreatic or biliary drainage
Urinary diversions
(ureterosigmoidostomy)
Renal Proximal (type 2) RTA
Ketoacidosis (during therapy)
Post-chronic hypocapnia
@ NKMishra23/07/18 146

2. Impaired renal acid excretion:
Distal (type 1) RTA
Hyperkalemia (type 4) RTA
Hypoaldosteronism
3. Misc:
Acid Administration (NH4Cl)
Hyperalimentation
Cholestyramine Cl
HCl therapy (Rx of severe met alkalosis)
@ NKMishra23/07/18 147

METABOLIC ALKALOSIS
@ NKMishra23/07/18 148

CAUSES OF METABOLIC ALKALOSIS
1. EXOGENOUS HCO3- LOADS
Acute alkali administration
Milk – alkali syndrome
2. Effective ECFV contraction, normotention, K+ deficiency and
secondary Hyperreninemic hyperaldosteronism:
GI LOSS: Vomiting
Gastric Aspiration
Villous adenoma
@ NKMishra23/07/18 149

RENAL LOSS : Diuretics
Post hypercapnic state
Hypercalcaemia
Recovery from LA/KA
Mg2+ deficiency
Bartters/Gitelmans syndr
Nonreabs anions – penicill
3. ECFV expansion, hypertension,K+ deficiency, and
mineralocorticoid excess:
HIGH RENIN : RAS
Accelerated hypertension
Renin sec tumor
@ NKMishra23/07/18 150

LOW RENIN :
PRIMARY ALDOSTERONISM –
Adenoma, hyperplasia , carcinoma
ADRENAL ENZYME DEFECTS –
11 b Hydroxylase
CUSHINGS SYNDROME OR DIS.
4. Gain of function mutation of renal sodium channel
with ECF expansion , hypertension , K+ deficiency
and hyporeninemic hypoaldosteronism : called as
LIDDLES SYNDROME
@ NKMishra23/07/18 151

RESPIRATORY ACIDOSIS
@ NKMishra23/07/18 152

Causes of Respiratory Acidosis
1. CENTRAL :
Drugs( anesthetics, morphine , sedatives)
Stroke
Infection
2. AIRWAY :
Obstruction
Asthma
3. PARENCHYMA :
Emphysema
Pneumoconiosis
Bronchitis
ARDS
Barotrauma
@ NKMishra23/07/18 153

• 4. NEUROMUSCULAR :
• Poliomyelitis
• Kyphoscoliosis
• Myasthenia
• Muscular dystrophies
5. MISCELLANEOUS
• Obesity
• Hypoventilation
• Permissive Hypercapnia
@ NKMishra23/07/18 154

Respiratory Alkalosis
@ NKMishra23/07/18 155

Causes of Respiratory Alkalosis
1.CENTRAL NERVOUS SYSTEM STIMULATION
Structural Causes Non Structural Causes
Head trauma Pain
Brain tumor Anxiety
CVA Fever
Meningitis, encephalitis Psychosis
2. HYPOXEMIA OR TISSUE HYPOXIA
Pneumonia, pulm oedema
Aspiration
High Altitude
@ NKMishra23/07/18 156

3. STIMULATION OF CHEST RECEPTORS :
• Hemothorax
• Flail chest
• Cardiac failure
• Pulmonary embolism
4. MIXED/UNKNOWN MECHANISMS:
Drugs – Salicylates Nicotine
Progesterone Thyroid hormone
Catecholamines
Xanthines (Aminophylline & related compounds)
Cirrhosis
Gram –ve Sepsis
Pregnancy
Heat exposure
Mechanical Ventilation @ NKMishra23/07/18 157

• ABG is a very useful diagnostic tool for our day to day
practice.
• Approach to interpret should be step wise & in a
systematic manner.
• Any abnormal result should be analyzed cautiously in
light of clinical context.
• Appropriate use of this tool using clinical judgment is of
paramount importance
@ NKMishra23/07/18 158

Approach to ABG Analysis

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