Dr. Vishram BucheOm Child Trust Hospital NAGPUR
The Goal :
In detail
…A respiratory component …A respiratory acid …Moves opposite to the direction of pH.
…A metabolic component …It is a base (Metabolic) …Moves in the same direction of pH.
…Moves in same direction ...Primary disorder …Moves in opposite direction …Mixed Disorder
COCO22
HCOHCO33
COCO22
HCOHCO33
H 1pH
H+ nmoles /L. pH
20 7.60
30 7.50
40 7.40
50 7.30
60 7.200
14
H+ = 80- last two digits of pH
OH ion
H+ ion
Alkaline
Acidic
CO 2 CHANGES pH in opposite direction
Primary lesion
compensation
pH
HCO3
CO2
METABOLIC ACIDOSIS
HYPER VENTILATION
BICARB CHANGES pH in same direction
LOW HCO3
LOW pH
LOW pCO2
LowAlkali
CO 2 CHANGES pH in opposite direction
Primary lesion
compensation
pH
HCO3
CO2
METABOLIC ALKALOSIS
HYPO VENTILATION
BICARB CHANGES pH in same direction
HIGH HCO3
HIGH pH
HIGH CO2
High Alkali
CO 2 CHANGES pH in opposite direction
Primary lesion
compensation
pH
CO 2
BICARB
Respiratory acidosis
HIGH pCO2
LOW pH
HIGH HCO3
High CO2
CO 2 CHANGES pH in opposite direction
Primary lesion
compensation
pH
CO 2
BICARB
Respiratory alkalosis
LOW pCO2
HIGH pH
LOW HCO3
Low CO2
Body’s physiologic response to Primary disorder in order to bring pH towards NORMAL limit
Full compensationPartial compensationNo compensation…. (uncompensated)
BUT never overshoots, If overcompensation is there, Take it granted it is a MIXED disorder
How to identify the type of compensation…..?
pH HCO3 CO2
7.20 15 40
7.20 15 30
7.37 15 20
Un Compensated
Partially Compensated
Fully Compensated
PaO2
SaO2
OXY (Sat) 98%HAEMOGLOBIN
2 % DissolvedOxygen
O.
D.
C.
PAO2
A.C.I.
CaO2Content of oxygenMl/100 of blood
DeliveryOf
OxygenTo
TissuesDaO2
O2
Cardiac output A.C.I.( Alveolar capillary interface)
FiO2….21%....150 mm of Hg
Now that I have this data,
what does it mean?
----- XXXX Diagnostics ------
Blood Gas Report248 05:36 Jul 22 2000Pt ID 2570 / 00
Measured 37.0o
CpH 7.463pCO2 44.4 mm HgpO2 113.2 mm Hg
Corrected 38.6o
CpH 7.439pCO2 47.6 mm HgpO2 123.5 mm Hg
Calculated DataHCO3 act 31.1 mmol / LHCO3 std 30.5 mmol / LBE 6.6 mmol / LO2 CT 14.7 mL / dlO2 Sat 98.3 %ct CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79
Entered DataTemp 38.6 oCct Hb 10.5 g/dlFiO2 30.0 %
output
-----XXXX Diagnostics-----
Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00
Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg
Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78
Entered DataTemp 38.6 0CFiO2 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
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 37o C are well established as a reliable reflectors of alveolar ventilation
Reliable data on DO2 and oxygen demand are
unavailable at temperatures other than 37o C
Bicarbonate is calculated on the basis of the Henderson equation:
[H+] = 24 pCO2 / [HCO3-]
or for the
Mathematically inclined…
-----XXXX Diagnostics-----
Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00
Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg
Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / LHCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78
Entered DataTemp 38.6 0CFiO2 30.0 %ct Hb 10.5 gm/dl
-----XXXX Diagnostics-----
Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00
Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg
Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78
Entered DataTemp 38.6 0CFiO2 30.0 %ct Hb 10.5 gm/dl
Standard Bicarbonate:Plasma HCO3 after equilibrationto 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
-----XXXX Diagnostics-----
Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00
Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg
Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 ct 15.8 mL / dlO2 Sat 98.4 %O2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78pO2 (a/A) 0.78
Entered DataTemp 38.6 0CFiO2 30.0 %ct Hb 10.5 gm/dl
Alveolar-arterial O2 Difference
* When FiO2 = 21 % :PiO2 = (760-45) x .21= 150 mmHg
OO22
COCO22
PAO2 = 150 – 1.2 (PCO2)
= 150 – 1.2 40
= 150 – 50 = 100 mm Hg
PaO2 = 90 mmHg
………..PAO2 – PaO2 = ? PAO2 = PiO2* -(PCO2/0.8)
PAO2 – PaO2 = 10 mmHg PaO2
PAO2
Alveolar-arterial Difference
OO22
COCO22
Alveolar – arterial G.100 - 45 = 55 ……………….Wide A-a
Oxygenation Failure Wide Gap
PCO2 = 40PaO2 = 45PAO2 = 150 – 1.2 (40) = 150 - 50 = 100
Ventilation FailureNormal Gap
PCO2 = 80PaO2 = 45PAO2 = 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
Always mention and see… FiO2 ct Hb
-----XXXX Diagnostics----Blood Gas Report
Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg
Calculated Data
HCO3 act 31.2 mmol / LO2 Sat 98.4 %O2 ct 15.8pO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78
Entered Data
FiO2 %Ct Hb gm/dl
Technical Errors
Glass vs. plastic syringe: Changes in pO2 are not clinically importantNo effect on pH or pCO2
Heparin (1000 u / ml):Need <0.1 ml / ml of bloodpH of heparin is 7.0; pCO2 trends downAvoided by heparin flushing & drawing 2-4 cc blood
Delay in measurement:Rate of changes in pH, pCO2 and pO2 can be reduced to 1/10 by cooling in ice slush(4o C) No major drifts up to 1 hour
1.
2. Look at pH?
3. Who is the culprit ?...Metabolic / Respiratory
4. If respiratory…… acute and /or chronic
5. If metabolic acidosis,
Anion gap ed and/or normal or both?
6. Is more than one disorder present?
7. Correlate clinically
Consider the clinical settings! Anticipate the disorder
7 s
teps
to a
naly
ze
AB
G
Step 2
Look at the pH
Is the patient acidemic pH < 7.35or alkalemic pH > 7.45
If pH = 7.4 …… Normal Mixed or Fully compensated
Step 3 ……. CULPRIT?
HCO3…… METABOLIC
> 26 ….. Met. Alkalosis
< 22 ……Met. Acidosis
PCO2 ……RESPIRATORY
> 45 …… Resp. Acidosis
< 35 …… Resp. Alkalosis
HCO3 = BaseNormal…22-26
CO2 = ACIDNormal…35-45
Step 4
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
pH
Step 5If it is a primary Metabolic disturbance,whether respiratory compensation appropriate?For metabolic acidosis:Expected PCO2 = (1.5 x [HCO3]) + 8 + 2
(Winter’s equation)
For metabolic alkalosis:Expected PCO2 = 6 mm… for 10 mEq. rise in Bicarb.………UNCERTAIN COMPENSATION
CO2 is equal to Last two digits
of pH
Remember If : Suspect .............
actual PaCO2 is more than expected : additional ...respiratory acidosis
actual PaCO2 is less than expected : additional...respiratory alkalosis
Step 5 cont.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 ethanolU remiaD iabetic KetoacidosisP araldehydeI nfection (lactic acid)E thylene GlycolS alicylate
Common pediatric causes
Lactic acidosisMetabolic disordersRenal failure
Step 6…
-- Clinical history
-- pH normal, abnormal PCO2 n HCO3
-- PCO2 n HCO3 moving opposite directions
-- Degree of compensation for primary
disorder is inappropriate
-- Rise of anion gap and Fall of HCO3….. ……..R/F equation
Validity of ABG report… a lab error
H= 24 xPCO2
HCO3
e.g. pH = 7.30, PCO2 = 38.1, 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
Ready Chart………
Limitations…..
SIMPLE DISORDERS LOOKS LIKE MIXED1. Not enough time lapsed for compensation 2. 5% out of confidence Bandse.g. pH = 7.20, HCO3 = 18, PCO2 = 33
MIXED DISORDERS LOOKS LIKE SIMPLEe.g. pH =7.24, PCO2 = 65, HCO3 = 26Chronic Resp acidosis + Metabolic AcidosisChronic case …. History helps
Partially compensated Metabolic Acidosis
pH = 7.4PaCO2 = 40 HCO3 = 24
1
Uncompensated Metabolic Acidosis
pH = 7.4PaCO2 = 40 HCO3 = 24
2
Partially compensated Metabolic Alkalosis
pH = 7.4PaCO2 = 40 HCO3 = 24
3
Fully compensated Respiratory Alkalosis
pH = 7.4PaCO2 = 40 HCO3 = 24
4
Partially compensated Respiratory Acidosis
pH = 7.4PaCO2 = 40 HCO3 = 24
5
Uncompensated Uncompensated Metabolic AlkalosisMetabolic Alkalosis
pH = 7.4PaCO2 = 40 HCO3 = 24
6
Normal A.B.G.
pH = 7.4PaCO2 = 40 HCO3 = 24
7
Uncompensated Respiratory Acidosis
pH = 7.4PaCO2 = 40 HCO3 = 24
8
Uncompensated Respiratory Alkalosis
pH = 7.4PaCO2 = 40 HCO3 = 24
9
Fully compensated Respiratory Acidosis
pH = 7.4PaCO2 = 40 HCO3 = 24
10
Combined Alkalosis
pH = 7.4PaCO2 = 40 HCO3 = 24
11
Combined Acidosis
pH = 7.4PaCO2 = 40 HCO3 = 24
12
-----XXXX Diagnostics----Blood Gas Report
Measured 37.0 0CpH 7.301 pCO2 75.1 mm HgpO2 45.3 mm Hg
Calculated Data
HCO3 act 35.2 mmol / LO2 Sat 78.4 %O2 ct 15.8pO2 (A -a) 9.5 mm Hg pO2 (a/A) 0.83
Entered Data
FiO2 21 %Ct Hb 12 gm/dl
CO2 =75-40=35Expected pH ( Acute ) = 7.11Expected pH ( Chronic ) = 7.30
Chronic resp. acidosis
pH <7.30 …Acidosis
Respiratory Acidosis
Normal A-a gradientHypoxia due toDue to hypoventilation
Hypoxia….???
Case 1 6 year old male with progressive
respiratory distress due to
Muscular dystrophy .
Case 28-year-old male asthmatic;3 days of cough, dyspneaand orthopnea notresponding to usualbronchodilators.
O/E: Respiratory distress;suprasternal and intercostal retraction;tired looking; on 4 L NC.
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o C
pH 7. 24pCO2 49.1 mm HgpO2 66.3 mm Hg
Calculated DataHCO3 act 18.0 mmol / L
O2 Sat 92 %pO2 (A - a) mm Hg pO2 (a / A)
Entered DataFiO2 30 %
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o C
pH 7. 24pCO2 49.1 mm HgpO2 66.3 mm Hg
Calculated DataHCO3 act 18.0 mmol / L
O2 Sat 92 %pO2 (A - a) mm Hg pO2 (a / A)
Entered DataFiO2 30 %
153-66= 87
pH <7.35 , acidosis
pCO2 >45; respiratory acidosis
Wide A / a gradient
Hypoxia
WITH INCREASE IN CO2 BICARB MUST RISE ? Metabolic acidosis + respiratory acidosis
CO2 = 49 - 40 = 9Expected pH ( Acute ) = 9/10 x 0.08 = 0.072Expected pH ( Acute ) = 7.40 - 0.072 = 7.328Acute resp. acidosis
30 × 5 = 150
Case 3
8 year old diabetic
with respi. distress
fatigue and loss of
appetite.
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o C
pH 7.23pCO2 23 mm HgpO2 110.5 mm Hg
Calculated DataHCO3 act 14 mmol / L
O2 Sat %pO2 (A - a) mm Hg pO2 (a / A)
Entered DataFiO2 21.0 %
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o C
pH 7.23pCO2 23 mm HgpO2 110.5 mm Hg
Calculated DataHCO3 act 14 mmol / L
O2 Sat %pO2 (A - a) mm Hg pO2 (a / A)
Entered DataFiO2 21.0 %
pH <7.35 , Acidosis
HCO3 <22; metabolic acidemia
Last two digits of pHCorrespond with co2
If Na = 130, Cl = 90Anion Gap = 130 - (90 + 14)
= 130 – 104 = 26
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o
CpH 7.34pCO2 38.1 mm HgpO2 90.3 mm Hg
Calculated DataHCO3 act 30 mmol / L
O2 Sat 98.3 %pO2 (A - a) 10 mm Hg pO2 (a / A) 0.93
Entered DataFiO2 21.0 %
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o
CpH 7.34pCO2 38.1 mm HgpO2 90.3 mm Hg
Calculated DataHCO3 act 30 mmol / L
O2 Sat 98.3 %pO2 (A - a) 10 mm Hg pO2 (a / A) 0.93
Entered DataFiO2 21.0 %
Case 4
16 year old female withsudden onset of dyspnea.
No Cough or Chest Pain
Vitals normal but RR 56,anxious.
Acidosis
Low CO2..???
High HCO3…???LAB ERROR!
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
3
2
24HCO
PaCOH
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o C
pH 7.46pCO2 28.1 mm HgpO2 55.3 mm Hg
Calculated DataHCO3 act 19.2 mmol / L
O2 Sat %pO2 (A - a) mm Hg pO2 (a / A)
Entered DataFiO2 24.0 %
----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0o C
pH 7.46pCO2 28.1 mm HgpO2 55.3 mm Hg
Calculated DataHCO3 act 19.2 mmol / L
O2 Sat %pO2 (A - a) mm Hg pO2 (a / A)
Entered DataFiO2 24.0 %
Case 5 :
10 year old child with encephalitis
pH almost within normal rangeMild alkalosis
Co2 is low , respiratoryCo2 low by around 10 ( Acute ) by .08 (Chronic ) by .03
Bicarb looks low ?Is it expected ?
Case 6……
6 yrs old girl having type 1 Diabetic with H/O persistant vomiting
Lab: pH 7.37, pCO2 35 mm Hg, HCO3 22 Na 140, Cl 90, Blood sugar : 300
Mild Metabolic acidosis ? Should we send her Home?
NO !!Anion gap = (140 - 112) = 28Correlate Rise of Anion Gap with Fall of HCO3
Anion Gap ↑ed by 18, HCO3 should ↓ed by 18, but ↓ed by 2 onlyHCO3 retention (production) due to vomiting
Metabolic acidosis, metabolic alkalosis
1 month baby having malrotation, (having intermitent vomiting) posted for surgery .
His pre-op ABG shows on Room air pH ………. 7.39pCO2 ……..l5paO2 ……...90 HCO3 ……..8
b) Metabolic acidosis with compensatory Hypocapnia c) Acute respiratory alkalosis fully compensated. d) Chronic respiratory alkalosis fully compensated.
Case 3……….
a) Primary metabolic acidosis with respiratory alkalosis
What is the probable cause for the above findings ? Are they OK ?As far as oxygenation is concerned ?
Patient was hypo- volumic , received Normal Saline bolus... Corrected acidosisHe was operated …but post-op… drowsy His ABG……..FiO2….30%
pH ……..7.39PaCO2 …38PaO2 ……60
1) Why hypoxemia ?2) Were the lungs bad to begin with ? ( Pre OP PaO2 … 90 mmHg )
3) Micro atelectesis during surgery ? Anesthetist goofed up the case 4) Pure and simple hypoventilation …..Sedation ?
Why hypoxemia ?Lungs were bad to begin with ?Micro atelectesis during surgeryPure and simple hypoventilation ? sedation
PRE OP ….ABG on room airpH 7.39 PaCO2 l5mmHg PaO2 90 mmHgHCO3 8mmol/L
Pre OP .....A/a gradient PAO2 = PiO2 – 1.2 (PaCO2 )
= 150 – 1.2 x 90= 150 – 18 = 132 mm Hg
132 – 90= 42 WIDE A / a gradient
Oxygenation status good …..?
One click
Apparently the lungs looked good with PaO2 of 90…But have a good look at the ABG again With wash out of CO 2 ……….The expected PaO2 should have been more than 90 .
This coupled with correction of acidosis( normalizing PaCO2 )Lowered the PaO2 …post operatively.
Conclusion …….. Lungs were not normal at the beginning
Correlate PaO2 with FiO2
But please also correlate with PaCO2
Learning point
No click
1. I shall use only minimal amount of heparin to rinse the syringe.
(Excess heparin causes pCO2 & shift pO2 to near 150. pH remains unchanged.)
2. I shall do ALLEN’S test for collateral circulation and, ALSO confirm that the sample sent is arterial and not venous
3. I shall ensure there are no air bubbles in the blood .
4. I shall send the sample in ice and analyze it quickly, … and keep the TLC in mind, esp. when there is a delay.
5. I shall always take FiO2 into consideration when interpreting pO2 values. I shall also look at the pCO2 values carefully.
6. I shall take the history into consideration before instituting therapy for Chronic respiratory failure.7. I shall always remember the acronym “ DOPE “ in situations of sudden deterioration of ABG values
D- DisplacementO- ObstructionP- PneumothoraxE- Equipment failure
8. I shall practice gentle mechanical ventilation and not try to bring ABG to perfect normal.
9. I shall treat the patient not the ABG report
10. I shall always correlate ABG report clinically.
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