Abg may 2021

DR J P SONI
Professor and Head of the Department Paediatrics
Division of Paediatric Cardiology
DR S N Medical College
Jodhpur
Doc_jpsoni@yahoo.com

❑ It is the reflection of cardio respiratory status
❑ Integral part of NICU – Interpretation is
important for diagnosis, treatment & prognosis
❑ Neonate is vulnerable to acid base disorders due to
➢ Lesser blood buffer quantity
➢ Immature kidney
➢ Fragile cardiovascular status
❑ Among Preterm babies- above deficiencies are exaggerated
ABG -Introduction

❑ The patients with Severe respiratory or metabolic
disorders.
❑ Clinical features of hypoxia or hypercarbia
❑ Shock
❑ Sepsis
❑ Decreased cardiac output
❑ Renal failure
❑ Ideally any baby on oxygen therapy
Indications for ABG

Precautions Before blood Sampling
❑ Wait for steady state
❑ Avoid air bubbles in syrige.
❑Transport sample in slush of ice, process within 30 min,
1-2hrs in ice.
❑ Always discard few drops of blood before testing.
❑ If Sample is processed late - blood cells will utilize
oxygen & this will lower pO2 & raise pCO2 levels.

Changes in ABG every 10 min in vitro
If Delay in processing - Blood sample should
be stored at 4 ° c

precaution while Collection
❑ Air bubble in sample – will shift pO2 values towards 150
and pCO2 falls precipitously
❑ Use least heparin ( <0.1 ml / ml of blood ).
❑ Use low strength heparin ( 1000 u / ml).
❑ Carefully mix the blood after sampling.
❑ Excess heparin in sample - results in falsely low values of
pCO2 and base excess & lowers pH.

Type of blood sample
❑ Arterial
❑Capillary – arterialized
❑ Venous

Site of Collection
Arterial Puncture
❑Umbilical
❑Radial, Posterior Tibial, Ulnar
❑Avoid - Brachial, Femoral, Supf. Temporal

Technique of Arterial Blood sampling
ALLEN’S TEST
ALLEN’S TEST
FOR TESTING
ADEQUACY OF
COLLATERAL
BETWEEN ULNAR
AND RADIAL
PALMER
CIRCULATION

ALLEN’S TEST:-
• Step 1: tight fist x 20 sec
• Step 2: Occlude radial and ulnar arteries
• Step 3: Open hand and look for blanching
• Step 4: release ulnar artery and look for capillary refill (5-7 sec)

Technique of Arterial puncture
ALLEN’S TEST
• Position wrist
• Prep skin
• Insert needle ~45 degrees,
bevel up
• Apply pressure x 5min
post procedure

Collection of capillary blood
ALLEN’S TEST
Capillary - arterialized
❑Pre-warmed heel, avoid squeezing.
❑Rotate in palm & seal, ensure no
air bubble.
❑Good for pH, pCO2 when peripheral perfusion
good.
❑pO2 - 10-20 mmHg less than actual
❑Problems - ? Poor perfusion

Collection of Venous blood
ALLEN’S TEST
Venous blood gases
❑ Drawn against flow of blood to heart.
❑ No tourniquet to be applied.
❑ Good for HCO3 estimation.
❑ Bad for pH, pCO2, pO2

Comparison of Blood Gas Analysis at Different Sites
ALLEN’S TEST

pH
HCO3
PCO2
“ALWAYS TREAT THE PATIENT NOT THE ABG”

pH………..7.40 (7.35-7.45)
PCO2 …..40 (35-45) mm of Hg
HCO3 (act) …..24 (22-26) mEq/L
PO2 ……. 80-100 mm of Hg
O2 Sat…. >95-99, BE/BD +/- 1
It is incomplete without FiO2
HCO3
PCO2
PO2
pH
Na
K
Cl
Ca
It Is Incomplete without…… Hb
ct

The physiological range of pH
7. 45
7.35
Above 7.45 = alkalosis
7.45 to 7.35 – normal range
Below 7.35 = acidosis

Component of ABG
Normal pH: 7.35 – 7.45 (Narrow Range)

Component of ABG
ALLEN’S TEST
PaCO2
Partial pressure of CO2
Normal value: 35 to 45 mmHg
TCO2
Total Carbon dioxide
Sum of HCO3 & amount of dissolved CO2
For each mmHg pCO2, 0.03 ml of CO2 is dissolved
per 100 ml of plasma
Normal value: 23 to 27 mmol/L

Component of ABG
ALLEN’S TEST
➢HCO-3 : actual bicarbonate (Normal :22 to 26 mmol/L)
➢St HCO–3 : standard HCO3 (at pCO2of 40mmHg & at
370C) (as it changes with CO2 levels).
➢Total buffer base (BB): 48 to 49 mmol/L (constituted by Hb –
25%, bicarbonate - 50% and other s- 25% )
➢Base excess (BE)/ Base deficit :
Actual base (above/below) from total buffer base(BB)
Normal value: - 2 to + 2

Component of ABG
ALLEN’S TEST
PaO2: Partial pressure of O2
➢ Normal value :80 to 100 mmHg
➢ SaO2: Percentage Oxygen saturation of Hb
➢ O2CT: Sum of oxygen bound to Hb & oxygen dissolved in
plasma.
1. For each mmHg of Po2, 0.003 ml of oxygen is dissolved per
100 ml of plasma
2. Each gram of Hb binds to 1.34ml of oxygen

pH
ALLEN’S TEST
Normal pH: 7.35 – 7.45 (Narrow Range)
The precise control of pH necessary for:
❑Cellular functions
❑Enzymatic reactions
❑Protein conformation
❑CNS function

The Regulation of the pH of the Body
ALLEN’S TEST
For optimal functioning of organism, pH should be
maintained near 7.4; by buffers.
❑Buffer = Mixture of weak acids + their salts
or weak bases + their salts
❑Buffers : Chemical buffers
Respiratory buffers
Kidneys

The Regulation of the pH of the Body
ALLEN’S TEST
Chemical buffers
➢Bicarbonate – Carbonic acid buffer
➢Phosphate buffer
➢Protein buffer
➢Hemoglobin buffer
Respiratory buffers: Regulated by sensors located in
CNS, Carotid body & aortic arch.
Kidneys:
➢Reabsorption of Bicarbonate
➢Acidification of buffer salt (Phosphate)
➢Excretion of Ammonia

Acid base physiology & Components of ABG
ALLEN’S TEST
pH
Normal pH: 7.35-7.45
CO2 + H2O <--> H2CO3 <--> HCO3
- + H+
ph = 6.10 + log ([HCO3] / [0.03 x PCO2])
Negative logarithm of H+ ion concentration
pH Inversely related to H+ ion concentration
Change of pH by 1 = 10 fold change in H+ ion concentration

Ratio of Base : Acid at 7.4 pH is 20:1

The Terms
• ACIDS
• Increase in H+ ion :
Acidic / Low pH
– Acidemia / Acidosis
• Respiratory acidosis
CO2
• Metabolic acidosis
HCO3
• BASES
• Decrease in H+ ion :
Alkaline/ High pH
– Alkalemia / Alkalosis
• Respiratory Alkalosis
CO2
• Metabolic Alkalosis
HCO3

The Relationship Between Plasma pH, [HCO3-] and PCO2
ALLEN’S TEST

Respiratory Disturbance
ALLEN’S TEST

CO 2 CHANGES
pH in opposite direction
Primary lesion
compensation
pH
CO 2
BICARB
Respiratory acidosis
HIGH pCO2
LOW pH
HIGH HCO3 (compensated)
High
CO2
 Ventilation

RESPIRATORY ACIDOSIS
ALLEN’S TEST
A retention of CO2 generally
caused by respiratory problems,
hypoventilation
Pulmonary Causes
-CNS Depression
-Respiratory muscle
-Tube block / dislodgement
-Opening of PDA
-Pulm interstitial edema
-Pulmonary air leak
-Collapse / consolidation

COMPENSATED RESPIRATORY ACIDOSIS
ALLEN’S TEST

Primary lesion
compensation
pH
CO 2
BICARB
Respiratory alkalosis
Low
CO2
CO 2 CHANGES
pH in opposite direction
LOW pCO2
HIGH pH
LOW HCO3 (compensated)
Ventilation
CO2  HCO3
Cl to balance charges
Hyperchloremia

RESPIRATORY ALKALOSIS
ALLEN’S TEST

COMPENSATED RESPIRATORY ALKALOSIS 7
ALLEN’S TEST

Respiratory Acidosis
Acute Respiratory Acidosis
– Acute - little kidney involvement.
Buffering via titration via Hb for example
• pH by 0.08 for 10mmHg  in CO2
Chronic Respiratory Acidosis
– Chronic - Renal compensation via synthesis and
retention of HCO3
– Cl to balance charges hypochloremia
• pH by 0.03 for 10mmHg in CO2

Metabolic Disturbance
ALLEN’S TEST

Primary lesion
compensation
pH
HCO3
CO2
METABOLIC ACIDOSIS
HYPER VENTILATION
HCO3 changes
pH in same direction
Low
Alkali
LOW HCO3
LOW pH
LOW pCO2 (compensated)
The degree of compensation is
assessed via the
Winter’s Formula
PCO2 = 1.5(HCO3) +8  2

METABOLIC ACIDOSIS
ALLEN’S TEST

COMPENSATED METABOLIC ACIDOSIS
ALLEN’S TEST

Primary lesion
compensation
pH
HCO3
CO2
METABOLIC ALKALOSIS
HYPO VENTILATION
BICARB CHANGES
pH in same direction
HIGH HCO3
HIGH pH
HIGH pCO2 (compensated)
High
Alkali

METABOLIC ALKALOSIS
ALLEN’S TEST

COMPENSATED METABOLIC ALKALOSIS
ALLEN’S TEST

• Patients may have two or more acid-base disorders
at one time
• Delta Gap
Delta HCO3 = HCO3 + Change in anion gap
>24 = metabolic alkalosis
Mixed Acid-Base Disorders

Urinary classification of metabolic alkalosis
• Why is this useful?
– If urinary chloride is low,
• The alkalosis is likely due to volume depletion and/or gastric
losses
• will respond to saline infusion
- If urinary chloride is high,
- Likely the alkalosis is due to hypokalemia or aldosterone
excess
- Will not respond to saline infusion
'Acid-base pHysiology' by Kerry Brandis

Compensation
• Respiratory compensation is quick
• Metabolic compensation is slow
• Compensation is not usually complete
• Patients never over compensate

Body’s physiologic response to Primary disorder
in order to bring pH towards NORMAL limit
✓Full compensation
✓Partial compensation
✓Uncompensated
BUT compensation never overshoots,
If a overshoot pH is there,
Take it granted it is a MIXED disorder
pH HCO3 CO2
7.37 15 20
7.25 15 30
7.20 15 40
How Compensation Occurs ?

ALLEN’S TEST
Metabolic Disorders
➢Respiratory Compensation Starts within minutes
➢Complete within 12 - 24 hrs
➢50% in 6 hrs
➢100% in 16 - 24 hrs

ALLEN’S TEST
Respiratory Disorders
➢ Metabolic Compensation is through Kidneys
➢ Much Slower : Starts within Hours
➢ Complete within 2 - 5 days

Oxyhaemoglobin Dissociation Curve
ALLEN’S TEST

Assessment of Oxygenation AaDO2
ALLEN’S TEST
AaDO2 = PAO2 – PaO2
PAO2 = PB - PH20 x FiO2 - PaCO2 / 0.8
PAO2 = 760 - 47 x 0.21 – 40/0.8 = 90
If paO2 = 80,
pAO2 - paO2 = 10 Normal : 10 – 25
>250 Ventilation for respiratory failure
>600 on 100 % FiO2 for 12 hrs ECMO

ALLEN’S TEST
Respiratory index
RI = AaDO2 / PaO2
>1 Need for O2
> 1.8 Ventilation
> 2 Weaning contraindicated
> 5 Refractory hypoxemia

ALLEN’S TEST
a / A Ratio
= PaO2 / PAO2
Normal – 0.7-0.9
< 0.6 Need for O2 therapy
< 0.22 Need for surfactant
< 0.15 Severe hypoxemia

ALLEN’S TEST
Oxygenation Index
OI = MAP x FiO2 X 100 /post ductal PaO2
OI > 15 – severe respiratory compromise
> 30-35 – failure for present mode of ventilation
> 40 – need for ECMO , 80 % mortality

* When FiO2 = 21 % :
PiO2 = (760-45) x .21= 150 mmHg
O2
CO2
 (calculated)PAO2 = 150 – 1.2 (PCO2)
= 150 – 1.2  40
= 150 – 50 = 100 mm Hg
(measured) PaO2 = 90 mmHg
………..PAO2 – PaO2 = ?
PAO2 = PiO2* -(PCO2/0.8)
PAO2 – PaO2 = 10 mmHg
PaO2
PAO2
1.Classify Respiratory Failure
2.Ventilation–perfusion mismatch
……FiO2 dependant derivation

✓…To calculate A-a gradient….
Is the baby hypoxic?
Type and severity of Hypoxia.
✓ …Relationship of PaO2 and FiO2?
FiO2 X 5 = Expected PaO2
✓…Whether PaO2 is appropriate for the given FiO2?
✓…Is the O2 content (CaO2) enough to prevent
hypoxia?

O2
CO2
Alveolar – arterial G.
100 - 45 = 55
……………….Wide A-a
Oxygenation
Failure
Wide Gap
PCO2 = 40
PaO2 = 45
PAO2 = 150 – 1.2 (40)
= 150 - 50
= 100
Ventilation
Failure
Normal Gap
PCO2 = 80
PaO2 = 45
PAO2 = 150-1.2(80)
= 150-100
= 50
Alveolar arterial G.
50 – 45 = 5
…………….Normal A-a

20 × 5 = 100
Expected PaO2 =
FiO2 × 5 = PaO2
Normal

It is essential to have
ELECTROLYTES
for
crucial interpretation of ABG.
esp. Na, Cl, K
We always correlate PaO2 with
FiO2
BUT………………………….
One should never forget to
correlate with PaCO2

----- XXXX Diagnostics ------
Blood Gas Report
248 05:36 Jul 22 2000
Pt ID 2570 / 00
Measured 37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6
o
C
pH 7.439
pCO2 47.6 mm Hg
pO2 123.5 mm Hg
Calculated Data
TPCO2 49
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
ct CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %

-----XXXX Diagnostics-----
Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
pO2 (A -a) 30.2 mm Hg 
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl
Measured values…
most important
Temperature Correction :
Is there any value to it ?
Calculated Data :
Which are useful one?
Entered Data :
Important
Now that I have
this data, what
does it mean?

➢Uncorrected pH & pCO2 are reliable reflections of
in-vivo acid base status
➢Temperature correction of pH & pCO2 do
not affect calculated bicarbonate
“ There is no scientific basis ... for applying temperature corrections to blood gas
measurements…” Shapiro BA, OTCC, 1999.
➢pCO2 reference points at 37
o
C are well established
as a reliable reflectors of alveolar ventilation
➢Reliable data on DO2 and oxygen demand are
unavailable at temperatures other than 37
o
C
Measured values should be considered
And
Corrected values should be discarded

Bicarbonate is calculated on the basis of the
Henderson equation:
[H
+
] = 24 pCO2 / [HCO3
-
]
or
for the
Mathematically
inclined…
Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
pO2 (A -a) 30.2 mm Hg 
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl

Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
pO2 (A -a) 30.2 mm Hg 
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl
Standard Bicarbonate:
Plasma HCO3 after equilibration
to a PCO2 of 40 mm Hg
: reflects non-respiratory acid base change
: does not quantify the extent of the buffer
base abnormality
: does not consider actual buffering capacity of
blood
Base Excess:
D base to normalise HCO3 (to 24) with PCO2 at 40
mm Hg
(Sigaard-Andersen)
: reflects metabolic part of acid base D
: no info. over that derived from pH, pCO2
and HCO3
: Misinterpreted in chronic or mixed disorders

Oxygenation
Parameters:
/limitations
O2 Content of blood:
(Hb x1.34x O2 Sat + 0.003x Dissolved O2 )
Remember Hemoglobin
Oxygen Saturation:
( remember this is calculated …error prone)
Alveolar / arterial gradient:
( classify respiratory failure)
Arterial / alveolar ratio:
Proposed to be less variable
Same limitations as A-a gradient
Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
pO2 (A -a) 30.2 mm Hg 
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl

pH………..7.40 (7.35-7.45)
PaCO2 …..40 (35-45) mm of Hg
HCO3 (act) …..24 (22-26) mEq/L
PaO2 ……. 80-100 mm of Hg
O2 Sat…. >95-99, BE/BD +/- 1
It is incomplete without FiO2
HCO3
PaCO2
PaO2
pH
Na
K
Cl
Ca It Is Incomplete without…… Hb ct

1. ABG Right or Wrong
2. Consider the clinical setting
3. Look at pH
4. Who is the culprit ?...Metabolic / Respiratory
5. Match pCO2 or HCO3 with pH change is acute or chronic
6. Metabolic acidosis – find out associated respiratory compensation
7. Ainon Gap – increased or decreased, Delta anion gap
7
steps
to
analyze
ABG

• First pearl is stick to “RULE ”

H= 24 x
PaCO2
HCO3
e.g. pH = 7.30, PCO2 = 38, HCO3 = 30
By Henderson-Hasselbach
H+ = 24 x pCO2/HCO3
= 24 x (38/30) = 30
80 - last two digit pH = H+
80 - H+ = last two digit pH (after 7)
pH should be 7.50

? Is the patient
clinical features of Acidosis
or
Alkalosis
or
Fully compensated

If the patient pH is < 7.35 Acidemia
or pH is > 7.45 Alkalemia
If the patient pH is = 7.4 …… Normal
Mixed Disorder
(metabolic acidosis & Resp. alkalosis)
or Fully compensated

pH is < 7.35 acidemia
pH is > 7.45 alkalemia
pCO2
pCO2
HCO3
HCO3
HCO3
HCO3
pCO2
pCO2
METABOLIC
METABOLIC
RESPIRATORY
RESPIRATORY
Compensation

• First pearl is Spontaneously breathing child (Nature)
will never over compensate
for
Metabolic or respiratory changes.
If the compensation it appropriate – Simple acid-base
disorder
If the compensation it inappropriate – Mixed acid-base
disorder

If there is a primary Respiratory disturbance,
is it acute ?
.08 change in pH ( Acute )
.03 change in pH (Chronic)
10 mm
Change
PaCO2
=
Remember………… relation of CO2 and pH

7.60
20
7.50
30
7.40
40
7.30
50
7.20
60
7.10
70
pH
PaCO2
pH
Last two digits
80 – PaCO2

* If measured pH is less than predicted pH than
it is respiratory acidosis
* If measured pH is more than predicted pH than
it is respiratory alkalosis

RESPIRATORY disorders…
Calculate Expected HCO3 for a Change in CO2 ......... 1 2 3 4
Acidosis…. (expected) HCO3 = 0.1 x ∆ CO2
Alkalosis…. (expected) HCO3 = 0.2 x ∆ CO2
Acidosis…. (expected) HCO3 = 0.35 x ∆ CO2
Alkaosis…. (expected) HCO3 = 0.4 x ∆ CO2
Acute respiratory
Chronic respiratory

• Third pearl is Look Respiratory compensation for
metabolic acidosis is adequate or not
By expected PaCO2 = last two digits of pH
Winter formula or
pH 7.xy;
xy is expected PaCO2

For metabolic acidosis:
Expected PCO2 = (1.5 x [HCO3]) + 8 + 2
(Winter’s equation)
Expected PaCO2 is equal to
Last two digits
of pH
pH 7.xy; . xy is Expected PaCO2

If it is a primary Metabolic disturbance,
whether respiratory compensation appropriate or not?
Remember If :
Suspect .............
actual PaCO2 is more than expected
additional...respiratory acidosis
actual PaCO2 is less than expected
additional...respiratory alkalosis

• PaCO2 up to 10
Metabolic Acidosis
• PaCO2 up to 60
Metabolic Alkalosis
Metabolic disorder
body will try to compensate by respiration
CO2 ex-halation(Acidosis) or retention (Alkalosis)

• Bicarb up to 40
Respiratory
Acidosis
• Bicarb up to 10
Respiratory
Alkalosis
Respiratory disorder
body will try to compensate by renal
HCO3 retention(Acidosis) or excretion (Alkalosis)

Compensation for metabolic alkalosis
• The normal response is hypoventilation
• The key is to compensate by increasing pCO2
• How much pCO2 is enough?
Expected pCO2 = 0.7 [HCO3] + 20 mmHg (range: +/- 5)

• Patients can have two or more acid-base
disorders at one time

• second pearl is Look for Serum Chloride.
• Hypochloremic Anion acidosis – Look for other anion
• Hperchloremic Anion acidosis – Normal anion gap
acidosis

Find out anion gap
What is anion gap? ...
Na - (Cl
-
+ HCO3
-
) = Anion Gap
usually <12
If >12, Anion Gap Acidosis :
If Metabolic Disorder
Na+
Cl-
HCO3
-
Alb-

Metabolic Acidosis: The “Anion Gap”
Na+
Cl-
HCO3
-
Alb-
 [Na+] - ([Cl-] + [HCO3
-])
~ 10-12 mM/L
Na+
Cl-
HCO3
-
Alb-
Na+
Cl-
HCO3
-
Alb-
Find chloride
Wide anion gap
metabolic acidosis
High chloride
Low chloride
Low anion hperchloremic
Metabolic acidosis
wide anion gap
metabolic acidosis
Lactate
Creatinine
Glucose
Urine ketone

If metabolic acidosis is there
How is anion gap? Is it wide ...
Na - (Cl
-
+ HCO3
-
) = Anion Gap usually <12
If >12, Anion Gap Acidosis : M ethanol
U remia
D iabetic Ketoacidosis
P araldehyde
I nfection (lactic acid)
E thylene Glycol
S alicylate
Common pediatric causes
✓ Lactic acidosis
✓ Metabolic disorders
✓ Renal failure

Equivalent rise of AG and Fall of HCO3……
….Pure Anion Gap Metabolic Acidosis
Discrepancy…….. in rise & fall
+ Non AG M acidosis, + M Alkalosis

• third pearl is Look for other metabolic Disturbance
along with metabolic acidosis

-- pH normal, abnormal PCO2 & HCO3
-- PCO2 & HCO3 moving opposite direction
-- Degree of compensation for primary
disorder is inappropriate
Find Delta Gap

What is Delta gap ?
• Delta Gap Δ AG
Corrected HCO3
Delta HCO3 = HCO3 + Anion gap - 12

✓Delta gap = HCO3 + AG-12 (∆ AG)
✓Delta Gap = 24….Pure AG acidosis
✓ < 24 = AG M Acidosis + non AG acidosis
✓ > 24 = AG M Acidosis + metabolic alkalosis

N-HCO3 = 24, N-Anion Gap = 12
Delta Gap = HCO3 + ∆AG
e.g. if HCO3= 12, AG = 24, ∆ AG = 12
Delta gap = 12 + 12 = 24
….Pure AG Metabolic Acidosis
Delta Gap = 24 ……AG met Acidosis
< 24 ….. + Non AG Mac
> 24 ….. + Meta. Alkalosis
Delta Gap = HCO3 + ∆ AG
e.g. if HCO3 = 12, AG = 20, ∆ AG = 8
Delta Gap = 12 + 8 = 20,
< 24 …AG + Non AG metabolic Acidosis
Delta Gap = HCO3 + ∆ AG
e.g. if HCO3 = 12, AG = 30, ∆ AG = 18
Delta Gap = 12 + 18 = 30
> 24 ….AG + metabolic Alkalosis

Fourth pearl
“TREAT THE PATIENT NOT THE ABG”
Be systematic with “ABG” analysis

ALLEN’S TEST
AaDO2 = PAO2 – PaO2
pAO2 = PB - PH20 x FiO2 - PaCO2 / 0.8
pAO2 = 760 - 47 x 0.21 – 40/0.8 = 90
If paO2 = 80,
pAO2 - paO2 = 10 Normal : 10 – 25
>250 Ventilation for respiratory failure
>600 on 100 % FiO2 for 12 hrs ECMO

ALLEN’S TEST
a / A Ratio
=PaO2 / PAO2
Normal – 0.7-0.9
< 0.6 Need for O2 therapy
< 0.22 Need for surfactant
< 0.15 Severe hypoxemia

PaO2/Fio2
ALLEN’S TEST
PaO2 / FiO2 Ratio
=PaO2 / FiO2
<300 = Acute lung injury
<200 = Acute respiratory distress
syndrome

PaO2/Fio2
ALLEN’S TEST
oxygen index
(MAP x FiO÷PaO2 ) x 100
Ventilation Index
PIP x Vent. Rate/min x PaCO2
÷1ooo

It’s not magic
understanding
ABG’ s,
it just takes a
little practice!

Experience is a wonderful
thing.
It enables you to recognize a mistake
when you make it (again).

Partially compensated
Metabolic Acidosis
pH = 7.4
PaCO2 = 40
HCO3 = 24
9 months old male with Acute Enteritis…..
Metabolic Acidosis

Metabolic Alkalosis
pH = 7.4
PaCO2 = 40
HCO3 = 24

Fully compensated
Respiratory Alkalosis
pH = 7.4
PaCO2 = 40
HCO3 = 24

pH = 7.4
PaCO2 = 40
HCO3 = 24

Uncompensated
Metabolic Alkalosis
pH = 7.4
PaCO2 = 40
HCO3 = 24

Normal A.B.G.
pH = 7.4
PaCO2 = 40
HCO3 = 24

Uncompensated
pH = 7.4
PaCO2 = 40
HCO3 = 24

Uncompensated
Respiratory Alkalosis
pH = 7.4
PaCO2 = 40
HCO3 = 24

Fully compensated
pH = 7.4
PaCO2 = 40
HCO3 = 24

Combined Alkalosis
pH = 7.4
PaCO2 = 40
HCO3 = 24

Combined Acidosis
pH = 7.4
PaCO2 = 40
HCO3 = 24

▲Respiratory
Alkalosis
What is the
Diagnosis ?
pH ………7.563
PCO2 ….19.8
HCO3 ….18.7
For a 10 mm change of PCO2
pH changes by 0.08 ……Acute
by 0.03 ……Chronic
Is it acute / Chronic?
Acute Respiratory Alkalosis

Abg may 2021

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