This document provides guidance on interpreting arterial blood gases (ABGs). It begins with 11 sample ABG results and asks the reader to identify the primary acid-base disorder in each case. The document then provides detailed explanations for interpreting ABGs and diagnosing mixed acid-base disorders. Key points include calculating the anion gap and excess anion gap to identify concurrent metabolic acidosis or alkalosis complicating the primary disorder suggested by pH, PCO2 and HCO3 levels. The goal is to methodically analyze ABG results to determine the underlying physiologic disturbance(s).

1. Nephrology is the ART of Homeostasis
PREPARED BY
DR MOHAMMED AL
SHAER
NEPHROLOGY SPECILAIST
• KFH-MADINA
ACID BASE DISORDERS

2. CASE 0
 73YEARS OLDWOMEN WITH DM,CHF,CKD .
PRESENTEDWITH NAUSEA,VOMITINGAND
SOP.SHE IS ON INSULIN AND LASIX.WT 6O KG .
 Biochem :Na 140 k 4 Cl 95 HCO3 24 CREAT. 4.1 BUN
54 MG/DL GLU 145 ALB 4.1
 VBG :PH 7.40 PCO2 40 PO2 90 HCO3 24.
 WHAT ISTHE ACID BASE DISORDER:
 A –METAB.ACIDOSIS AND RESP. ACIDOSIS
 B-METAB. ACIDOSIS AND METAB.ALKALOSIS
 C-METAB.ACID.+METAB. ALKALOSIS AND RESP.ACID
 D-RESP.ALKALOSIS AND METAB. ALKALOSIS
 E-NON OFTHE ABOVE…IT IS NORMALABG..

4. WHY 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.
 NB: NOT EVERY DYSPNIC PATIENT NEEDSABG
(e.g. BA,CHF,ESRDWITH MILD SYMPTOMS)

5. Why an ABG instead of Pulse
oximetry ?
 Pulse oximetry does not assess
ventilation (pCO2) or acid base status.
 Pulse oximetry becomes unreliable
when saturations fall below 70-80%.
 Technical sources of error (ambient or
fluorescent light, hypoperfusion, nail
polish, skin pigmentation)
 Pulse oximetry cannot interpret
methemoglobin or carboxyhemoglobin.

11. INFORMATION PROVIDED BY ABG..CONT
• PH MEASURES THE HYDROGEN(H+)IONS IN
THE BLOOD..
• PH OF THE BLOOD IS USUALLY KEPT
BETWEEN 3.35 - 4.45 and RESULTS ABOVE
CONSIDERED ALKALOSIS AND BELOW IS
CONSIDERED ACIDOSIS..
• KEPP IN MIND THAT RESULTS BETWEEN 3.35-
4.45 IS NOT NORMAL AT ALL AND CAN
REFLECT COMPENSATED ACID BASE
DISORDER…..

14. TAKE CARE FOR THE SAMPLE!!

24. INTERPRETITION OF ABG
 WHENYOU INTERPRETE THE ABG
DEPEND ON THE OPTIMAL
FIGURES
 PH 7.40 PCO2 40 HCO3 24
 ANION GAP 10 +/- 2
 AG =Na –OCH + LC( 3) OR
Na – CL –HCO3

25. ABG OR VBG ?!!

26. How to CONFIRM THAT ABG IS TRUE!!
USE HANDERSON-HASELBALCH EQUATION
[H+] = 24(PaCO2 )/ [HCO3 -]

28. PEARLES
 WORK ONLY IFYOU CALCULATETHEANION GAP
(AG) CORRECTLY..
 ABG AND BIOCHEM. DRAWN ATTHE SAMETIME
HAVVALUESWITHIN /+- 2 OF EACH OTHER..
 THERE IS AN SD OF ABOUT 3ON EACH FORMULA
 WEWILL USE 24 FOR HCO3 AND 10-12 FOR AG..
 ALWAYS BE SURE THAT DIAGNOSIS IS
CONSISTANT HISTORYANDTHE PH ..
 YOU CAN HAVE MULTIPLE METABOLIC
DISORDERS BUT ONLY A SINGLE RESPIRATORY
DISORDER..

29. PEARLS…CONTINUED
 INTERPRETE USINGTHE OPTIMAL FIGURES: PH 7.40
PCO2 40 HCO3 24 so results above or below
should be considered abnormal until finishing
evaluation.
 PCO2 AND HCO3 ARE CLOSE FRIENDS! Move
up or down togather but within limits.
 Compensation for metabolic disorder is single
phase while for respiratory two phases(acute
and chronic)..
 LUNG STARTTO COMPENSATE WITHIN 1-3
MINUTES WHILE KIDNEYSTAKES HOURSTO
DAYS..

31.  PH < 3.35 indicates ACIDEMIA..
 PH > 4.45 indicates ALKALOSIS..
 PH BELOW 6.8 AND ABOVE 7.8 IS
INCOMPATABLEWITH LIFE..MEANS DEATH
 DON’T FORGET THAT PH BETWEEN 3.35 -
4.45 IS THE NORMAL RANGE BUT
DOESN’T RULE OUT ABNORMAL ACID
BASE DISORDER ..!!!!YOU SHOULD LOOK
TO ALL PARAMETERS ANDTHE ANION
GAP..

37. • IN GENERAL WE USE THIS EQUATION
AG = NA –CL-HCO3

40. NORMAL OR NON ANION GAP

45. BEST WAY TO CALCULATE
COMPENSATION

52. TYPES OF ACID BASE DISORDERS

62. CAUSES OF METABOLIC ACIDOSIS
 KULT
 K:KETOSIS(Diabetic,Starvation,AL
COHOLIC)
 U: UREMIA
 L: LCATIC ACIDOSIS
 T:Toxins(salicylate,propafol,parace
tamol..)

72. SUMMERY OF ABG APPROACH 1
 Step 1: is it reliable ? HANDERSON-HASELBALCH
 STEP2 : Comprehensive history and physical
examination.
 STEP 3 : Acidosis or alkalosis..? See the pH (7.40)
 STEP 4 : Identify the primary disorder See the change
in PCo2 & HCO3
 STEP 5 : Calculate the compensatory response Is IT
adequately compensated???
 STEP 6 : Calculate Anion Gap ESPECIALLY IN
Metabolic Acidosis
 STEP 7 : Calculate the Delta Gap= DELTA-DELTA =
DELTA AG (unmask hidden mixed disorders) ..

73. SUMMERY OF ABG APPROACH 2
 STEP -8 : Calculate the osmolar gap (for
HighAG acidosis)
 STEP -9 : Calculate the urinary anion gap (IN
Non AG MetabolicAcidosis)
 STEP -10 : Formulate Differential Diagnosis

74. CASE 1
 73YEARS OLDWOMEN WITH
DM,CHF,CKD,PRESENTEDWITH NAUSEA,VOMITING
AND SOP.SHE IS ON INSULIN AND LASIX.WT 6O KG .
 Biochem :Na 140 k 4 Cl 95 HCO3 24 CREAT. 4.1
BUN 54 MG/DL GLU 145 ALB 4.1
 VBG :PH 7.40 PCO2 40 PO2 90 HCO3 24.
 WHAT ISTHE ACID BASE DISORDER:
 1 –METAB.ACIDOSIS AND RESP. ACIDOSIS
 2-METAB. ACIDOSIS AND METAB.ALKALOSIS
 3-METAB.ACID.+METAB. ALKALOSIS AND RESP.ACID
 4-RESP.ALKALOSIS AND METAB. ALKALOSIS
 5-NON OFTHE ABOVE…

75. CASE 1 ANSWER B
 AG = 140 – 95 – 24 =21 HIGH AG MEANS
SHE HAS HAGMA (DUETO RENAL
FAILURE)
 SHE has vomiting and receiving
furosemide which causes METABOLIC
ALKALOSISWHICH NORMALISES THE
PH AND HCO3..
 DELAT AG = 21-10 = 11
 11 +24( HCO3) = 35.. (IF > 30 there is
Metabolic Alkalosis and if < 24 NAGMA)

76. CASE 2
ABG:
 PaCO2 27 mm Hg HCO3 13
 PaO2 105 mm Hg
 H+ 70 mmhg
 Na+ 134 mmol/L
BIOCHEM:
K+ 3.7 mmol/L Cl- 109 mmol/L
HCO3 20 mmol/L Albumin 4.0 g/dL
• AG 12

77. CASE 2 ANSWER
 H+ = 24 X 27/ 13 = 49.8
 Not Reliable test as H +ON
ABG IS 70 WHILE USING
HANDERSON EQUATION IT S 50,AT
THE SAMETIME NOTETHAT HCO3
ON ABG IS 13WHILE ON BIOCHEM
20

78. CASE 3
ABG
• pH 7.28 • PaCO2 27 mm Hg
• PaO2 105 mm Hg • H+ 50 mmhg •
BIOCHEM:
Na+ 134 mmol/L • K + 3.7 mmol/L • Cl-
109 mmol/L • HCO3 - 14 mmol/L •
Albumin 4.0 g/Dl
 AG 12

79. CASE 3 ANSWER
 1. pH: ↓(acidosis)
 2-THE CAUSEWOULD BE EITHER LOW
HCO3 OR HIGH PCO2
 Anion Gap: Normal . Compensation
COMPENSATED(expected pco2 (27-
31) using winter formula
 NON-ANION GAP METABOLIC ACIDOSISWITH
ADEQUATE COMPENSATION

80. CASE 4
pH 7.50
pCO2 29
HCO3 22
Variable Primary
Disorder
Normal
Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Acute Respiratory
Alkalosis

81. CASE 5
pH 7.25
pCO2 60
HCO3 26 Variable Primary
Disorder
Normal
Range, arterial
Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Acute Respiratory Acidosis

82. CASE 6
pH 7.34
pCO2 60
HCO3 31 Variable Primary
Disorder
Normal
Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Chronic Respiratory Acidosis
with Metabolic
Compensation

83. CASE 7
pH 7.50
pCO2 48
HCO3 34 Variabl
e
Primary
Disorder
Normal
Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Metabolic Alkalosis

84. CASE 8
pH 7.20
pCO2 21
HCO3 8 Variabl
e
Primary
Disorder
Normal Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Metabolic Acidosis with
Respiratory Compensation

85. CASE 9
pH 7.40
pCO2 40
HCO3 24
Na 145
Cl 100 Variable Primary
Disorder
Normal
Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-
7.45
Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Anion Gap Metabolic Acidosis
and Metabolic Alkalosis
1. Look at the pH to determine the
primary process.
2. Calculate the anion gap: Na – (Cl +
HCO3)
3. Calculate the excess anion gap (total
anion gap minus the normal anion gap)
and add this to the measured HCO3
concentration, if >30, there is
underlying metabolic alkalosis; if <24,
there is underlying non-gap metabolic
acidosis

86. What’s the Diagnosis?
Chronic renal failure in a
patient with vomiting as his
uremia worsened.

87. Example #9
pH 7.50
pCO2 20
HCO3 15
Na 145
Cl 100 Variable Primary
Disorder
Normal
Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Respiratory alkalosis, Anion
Gap Metabolic Acidosis and
Metabolic Alkalosis
1. Look at the pH to determine the
primary process.
2. Calculate the anion gap: Na – (Cl +
HCO3)
3. Calculate the excess anion gap (total
anion gap minus the normal anion gap)
and add this to the measured HCO3
concentration, if >30, there is
underlying metabolic alkalosis; if <24,
there is underlying non-gap metabolic
acidosis

88. What’s the Diagnosis?
History of vomiting
(metabolic alkalosis),
alcoholic ketoacidosis
(metabolic acidosis), and
bacterial pneumonia
(respiratory alkalosis)

89. Example #10
pH 7.10
pCO2 50
HCO3 15
Na 145
Cl 100 Variable Primary
Disorder
Normal
Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Respiratory Acidosis, Anion
gap Metabolic Acidosis,
Metabolic Alkalosis
1. Look at the pH to determine the
primary process.
2. Calculate the anion gap: Na – (Cl +
HCO3)
3. Calculate the excess anion gap (total
anion gap minus the normal anion gap)
and add this to the measured HCO3
concentration, if >30, there is
underlying metabolic alkalosis; if <24,
there is underlying non-gap metabolic
acidosis

90. What’s the Diagnosis?
Patient presented in an obtunded state
(respiratory acidosis), history of vomiting
(metabolic alkalosis), DKA (anion gap metabolic
acidosis)
Or
Chronic respiratory acidosis and metabolic
compensation in whom an acute anion gap
metabolic acidosis developed

91. Example #11
pH 7.15
pCO2 15
HCO3 5
Na 140
Cl 110 Variable Primary
Disorder
Normal
Range,
arterial Gas
Primary
Disorder
pH Acidemia 7.35-7.45 Alkalemia
pCO2 Respiratory
alkalosis
35 - 45 Respiratory
acidosis
HCO3 Metabolic
acidosis
22 – 26 Metabolic
Alkalosis
Anion Gap and Non-Anion
Gap Metabolic Acidoses
1. Look at the pH to determine the
primary process.
2. Calculate the anion gap: Na – (Cl +
HCO3)
3. Calculate the excess anion gap (total
anion gap minus the normal anion gap)
and add this to the measured HCO3
concentration, if >30, there is
underlying metabolic alkalosis; if <24,
there is underlying non-gap metabolic
acidosis

92. What’s the Diagnosis?
DKA with non-gap acidosis
during recovery phase of
DKA due to failure to
regenerate HCO3 from
keto-acids lost in the urine

93. Case 12
40YEARSWOMEN,HISTORY OF SHORT
BOWEL SYNDROME,LIKES ICE
CREAM,PRESENTEDWITH SLURRED
SPEECH AND CONFUSION
PH 7.27 PCO2 24 HCO3 16 AG 20
OG, CREATININE,LACT. : NORMAL.NO KETONS.
WHAT ISTHE MOST LIKELY DIAGNOSIS ?
A –L-LACTATE B-PYROGLUTAMIC ACID
C-D-LACTATE D-METHANOL
E-TOPIRAMATE

94. ANSWER 12
D-LACTATE
 ALL EXCEPT TOPIRAMATE CAN CAUSE HAGMA.
 NO OSMOLAL GAP SO NO METANOL!
 NO HISTORY OF ACETAMINOPHEN USE OR
ANTIBIOTIC USE SO NOT PYROGLUTAMIC.
 NORMAL LACTATE RULE OUT L-LACTICACIDOSI.
 D-LACTATEOCCURS IN SHORT BOWEL
SYNDROMEWITH HIGH CARBOHYDRATE
INTAKE.ITSASSOCIATEDWITH NEUROLOGICAL
MANIFISTATIONS..

95. CASE 13
 60 years old man known COPD LASTVISIT 1
MO.AGO HIS ABG :PH 7.31 PCO2 70 HCO3 36
 HE PRESENTEDWITH AKI DUETO SEVERE GE.
 BIOCHEN; CREAT 400 MCOL/L N a 145 cl 95
 ABG ; PH 7.20 PCO2 70 HCO3 24
 WHAT ISTHE ACID BASE DISORDER HE HAS ?
A-RESP.ACIDOSISAND HAGMA.
B –RESP.ACID. + HAGMA+METAB.ALKALOSIS
C-RESP.ACID.+HAGMA+RESP.ALKALOSIS.
D-RESPIRATORYACIDOSISAND MET.ALKALOSIS.
E-RESPIRATORY ACIDOSISONLY.

96. CASE 13 ANSWER
 THE ANSWER IS B
PH SHOWES ACIDOSIS.HE HAS COPDWITH
PCO2 70,SO HIS EXPECTED HCO3 IS 36..
NOW HIS HCO3 IS 24 MEANS 10 BELOW
EXPECTED SO HE HAS METAB. ACIDOSIS.
AG IS HIGH 145-95-24=26
DELTA AG =26-10=16
ADDTHE 16TOTHE MEASURED HCO3
16+24=40
As we said if >30 there is METABOLIC ALK.

97. Conclusions
 Acid-base disturbances are easy to analyze if
approached systematically
 Determine primary abnormalities based on pH
 Calculate the anion gap
 Calculate the delta gap and add to the
measured HCO3
 Calculate an anion gap on EVERY chemistry
you see
 If there is an elevated anion gap, remember to
get an ABG!!