ABG Lecture Dr Lenora Fernandez

ARTERIAL BLOOD GAS INTERPRETATIONARTERIAL BLOOD GAS INTERPRETATION

Lenora C. Fernandez, MD FPCCP

Philippine General Hospital

OBJECTIVES

To review the components of an ABG examination

To discuss a systematic way of interpreting the arterial blood gas

To recognize existing acid base disorders

To become familiar with the concept of anion gap


COMPONENTS OF AN ABG

pH Measurement of acidity or alkalinity,

based on the hydrogen (H+) ions present.

Negative log of the free H+ ion concentration

The normal range is 7.35 to 7.45



PaO2 The partial pressure of oxygen that is

dissolved in arterial blood. The normal range is 80 to 100 mm Hg.

SaO2 The arterial oxygen saturation. The normal range is 95% to 100%.



PaCO2 The amount of carbon dioxide dissolved

in arterial blood. The normal range is 35 to 45 mm Hg.



HCO3 The calculated value of the amount of bicarbonate in

the bloodstream. The normal range is 22 to 26 mEq/liter (24 + 2)B.E. The base excess indicates the amount of excess or

insufficient level of bicarbonate in the system. The normal range is –2 to +2 mEq/liter (0 + 2). (A negative base excess indicates a base deficit in

the blood.)


Steps in ABG CollectionSteps in ABG Collection

1.1. Prepare the materials needed.Prepare the materials needed.

2.2. Prepare the syringe with needle.Prepare the syringe with needle.

3.3. Select the puncture site.Select the puncture site.

4.4. Perform the modified Allen test.Perform the modified Allen test.

5.5. Collect the sample.Collect the sample.

6.6. Apply pressure on puncture site.Apply pressure on puncture site.

7.7. Prepare the specimen for transport.Prepare the specimen for transport.


Which ABG collection error/s will Which ABG collection error/s will falsely elevate the pH?falsely elevate the pH?

A.A. Failure to cool bloodFailure to cool blood

B.B. Dilution with heparinDilution with heparin

C.C. Venous admixtureVenous admixture

D.D. None of the aboveNone of the above


Which ABG collection error/s will Which ABG collection error/s will NOT affect the paONOT affect the paO22??

A. Failure to cool bloodA. Failure to cool blood

B. Dilution with heparinB. Dilution with heparin

C. Venous admixtureC. Venous admixture

D. Air contaminationD. Air contamination


Effects of ABG collection errors on pH, Effects of ABG collection errors on pH, paCOpaCO22 and paO and paO22

ABG COLLECTION ERRORABG COLLECTION ERROR pH pH paCO paCO22 paOpaO22

1. Dilution with heparin 1. Dilution with heparin INC INC DEC DEC NCNC

2. Air contamination 2. Air contamination INC INC DEC DEC INCINC

3. Venous admixture3. Venous admixture DEC DEC INC INC DECDEC

4.4. Failure to cool bloodFailure to cool blood DEC DEC INC INC

DECDEC

Legend: INC=increase, DEC=decrease, NC=no changeLegend: INC=increase, DEC=decrease, NC=no change

ANALYSIS OF RESULTS


The arterial blood gas is used to evaluate both acid-base balance and oxygenation, each representing separate conditions. Acid-base evaluation requires a focus on three of the reported components: pH, PaCO2 and HCO3.

pH ~ [HCO3] ~ kidney PaCO2 lungs


STEP ONE: Acidosis vs. Alkalosis

Assess the pH to determine if the blood is within normal range, alkalotic or acidotic.

Normal: 7.35 to 7.45


STEP ONE: Acidosis vs. Alkalosis

pH Degree of impairment

< 7.20 Severe acidemia

7.20-7.29 Moderate

7.30-7.34 Mild acidemia

7.35-7.45 Normal pH

7.46-7.50 Mild alkalemia

7.51-7.55 Moderate

> 7.55 Severe alkalemia


STEP TWO: Respiratory vs. Metabolic

pHpH

< 7.4< 7.4 >7.4 >7.4 acidemiaacidemia alkalemiaalkalemia

HCOHCO33 < 24 pCO < 24 pCO22 > 40 > 40 HCO HCO33 > 24 > 24 pCO pCO22 < 40 < 40

metabolicmetabolic respiratory respiratory metabolic metabolic respiratory respiratory

acidosisacidosis alkalosis alkalosis

Determine the primary disorder.Determine the primary disorder.



To check for the primary disorder determine the degree of deviation of the values of pCO2 and HCO3 from the normal

RespiratoryChange in PCO2/ 40 > change in HCO3/24

Metabolic change in HCO3/24 > Change in PCO2/ 40


SAMPLE

pH 7.22

PaCO2 55

HCO3 25

Step 1: Acidosis

Step 2:

Change in PCO2 = 37.5%

Change in HCO3 = 4.2%

Therefore, respiratory acidosis



HINT: If pH and PaCO2 are moving in

opposite directions, then the problem is primarily respiratory in nature

If they are moving in the same direction, then the problem is primarily metabolic in nature.


Classification of Lab Metabolic Acid-base Component

Classification [BE]

Meq/L

[HCO3]

Meq/L

Normal metabolic component

0 + 2 24 + 2

Metabolic acidosis

< -2 < 22

Metabolic alkalosis

> +2 > 26


STEP THREE: COMPENSATED?

When a patient develops an acid-base imbalance, the body attempts to compensate.

Remember that the lungs and the kidneys are the primary buffer response systems in the body.

The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the pH into the normal range.


Compensatory Mechanismsex. In acidemia

1. Extracellular buffering primarily by HCO3-

(immediate)2. Respiratory compensation by an increase in

alveolar ventilation (minutes to hours)

3. Intracellular buffering primarily by proteins and phosphates (2 to 4 hours)

4. Renal compensation by an ↑ in H+ excretion and ↑HCO3

- reabsorption (hours to days)

Na+

Regulatory Response to Acidemia

Cl-

H+

Protein- PO4

=,SO4=

Organic acids

normal anion gap

URINE

HCO3-

NH4+ H2PO4

-

PCT

DT


Once the primary disorder is identified, compute the expected value of the compensating buffering system


Disorder Primary disorder

Compensated response

Degree of change

Metabolic acidosis

Low HCO3

Low pCO2 ΔpCO2 = 1.2 ΔHCO3

Metabolic alkalosis

High HCO3

High pCO2 ΔpCO2 = 0.7 ΔHCO3


SAMPLE

77/F diagnosed case of ESRD who missed her dialysis session twice admitted for decreased responsiveness


SAMPLE

pH 7.28pCO2 32HCO3 15

Step 1: Acidosis Step 2: Metabolic

Δ pCO2/ 40 = 20%Δ HCO3/24 = 38%

Step 3:Expected compensation Δ pCO2 = 1.2 ΔHCO3 = 1.2(9) = 10.8Expected pCO2 = 29.2therefore uncompensated metabolic acidosis


Respiratory acidosis

Primary disorder


Degree of change

Acute High PCO2

High HCO3 ΔHCO3 = 1/10 ΔPCO2

Chronic High PCO2

High HCO3 ΔHCO3 = 3/10 ΔPCO2


Respiratory alkalosis

Primary disorder


Degree of change

Acute Low PCO2

Low HCO3 ΔHCO3 = 2/10 ΔPCO2

Chronic Low PCO2

Low HCO3 ΔHCO3 = 4/10 ΔPCO2


58/F chronic COPD admitted for elective breast surgery


SAMPLE

Step 1: Slight Acidosis Step 2: Respiratory

Δ pCO2/ 40 = 20%Δ HCO3/24 = 17%

Step 3:Expected compensation Δ HCO3 = 3/10 Δ pCO2 = 3/10 (8) = 2.4Expected HCO3 = 26.4therefore compensated respiratory acidosis


STEP THREE: COMPENSATED

A patient can then be in a fully compensated, partially compensated, uncompensated state.


HINT


Uncompensated States


STEP FOUR: ANION GAP?

If with metabolic acidosis, check for other existing metabolic derangements; compute for the anion gap

AG = Na – (Cl + HCO3) = normal 10-12

Represents unmeasured anions in the plasma

Na136

Cl100

AG 12

HCO3

24

NORMAL

Unmeasured anionsProtein-

PO4=,SO4

= Organic

acids

Na136

Cl100

AG 12

HCO3

24

NORMAL

Na136

Cl100

AG 26

HCO3 10

HIGH GAP METABACIDOSIS

Increased when acidosis due toIncrease in fixed acids (HCO3 actsas buffer so it is depleted and theunmeasured anions increase to preserve neutrality)

Na136

Cl114

AG 12

HCO3 10

NORMAL GAP METABACIDOSIS

Gap is normal if metab acidosis due to loss of base (when HCO3 lost,Cl- anions increased to maintain Neutrality)


CAUSES OF METABOLIC ACIDOSISCAUSES OF METABOLIC ACIDOSIS

INCREASED ANION GAPINCREASED ANION GAP

• KetoacidosisKetoacidosis

DiabeticDiabetic

AlcoholismAlcoholism

StarvationStarvation• Lactic AcidosisLactic Acidosis• UremiaUremia• ToxinsToxins

NORMAL ANION GAPNORMAL ANION GAP

• Associated w/ K lossAssociated w/ K loss

DiarrheaDiarrhea

RTARTA• Interstitial nephritisInterstitial nephritis• Early renal failureEarly renal failure• Urinary tract obstrxnUrinary tract obstrxn• Drug-inducedDrug-induced

Na+

States of Systemic Acidosis

Cl-

High anion gap

H+

Protein- PO4

=,SO4=

Organic acids

HCO3-

M- methanol U- uremia D- DKA P- paraldehyde I- iron, INH L- lactic acidosis E- ethylene glycol S- salicylates


Compute for delta delta value to determine co-existing metabolic derangements

Na136

Cl100

AG 12

HCO3

24

NORMAL

Na136

Cl94

AG 22

HCO3

20

COMBINED AGMET. ACIDOSIS& MET. ALKALOSIS

AG HCO3

= 10 4

Na136

Cl106

AG 22

HCO3 8

COMBINED AG& NAG MET. ACIDOSIS

AG HCO3

= 1016

Na136

Cl100

AG 22

HCO3

14

SIMPLE AGMETABOLICACIDOSIS

AG HCO3

= 1010

For High Gap: DELTA AnionGap/DELTA HCOFor High Gap: DELTA AnionGap/DELTA HCO33


HAGMA

Δ AG = Δ HCO3 pure HAGMA

Δ AG < Δ HCO3 HAGMA + NAGMA

Δ AG > Δ HCO3 HAGMA + metabolic alkalosis

Na136

Cl100

AG 12

HCO3

24

NORMAL

Na134

Cl110

AG 10

HCO3

14

SIMPLE NAGMETABOLICACIDOSIS

Cl HCO3

= 1010

Na128

Cl110

AG 10

HCO3 8

COMBINED NAG & AG MET. ACIDOSIS

Cl HCO3

= 1016

Na140

Cl110

AG 10

HCO3

20

COMBINED NAGMET. ACIDOSIS& MET. ALKALOSIS

Cl HCO3

= 10 4

For Normal Gap: DELTA Chloride/DELTA HCOFor Normal Gap: DELTA Chloride/DELTA HCO33


NAGMA

Δ Cl = Δ HCO3 pure NAGMA Δ Cl < Δ HCO3 NAGMA + HAGMA Δ Cl > Δ HCO3 NAGMA + metabolic

alkalosis


Looking at Base excess to check internal consistency of blood gas data

For every change in [BE] of 5 meq/l, pH changes by 0.1 unit. (assume PaCO2 of 40 mmHg)

pH BE (meq/L)

7.00 -20

7.11 -15

7.22 -10

7.33 -5

7.40 0

7.48 +5

7.55 +10

7.60 +15

7.66 +20

Case 2Case 2

A 30 year old male with a history of A 30 year old male with a history of

epilepsy has a grand mal seizure. epilepsy has a grand mal seizure.

Laboratory tests taken immediately after Laboratory tests taken immediately after

the seizure has stopped reveal:the seizure has stopped reveal:

Arterial pH = 7.14Arterial pH = 7.14

pCOpCO22 = 45 mm Hg = 45 mm Hg

Plasma [NaPlasma [Na++] = 140 meq/L] = 140 meq/L

[K[K++] = 4.0 meq/L] = 4.0 meq/L

[Cl[Cl--] = 98 meq/L] = 98 meq/L

[HCO[HCO33--] = 17 meq/L] = 17 meq/L

AG = 25AG = 25


pHpH

< 7.4< 7.4 >7.4>7.4acidemiaacidemia alkalemiaalkalemia

HCO3 < 24 pCO2 > 40HCO3 < 24 pCO2 > 40 HCO3 > 24HCO3 > 24 pCO2 < 40 pCO2 < 40 metabolicmetabolic respiratoryrespiratory metabolic metabolic respiratory respiratory

acidosisacidosis alkalosisalkalosis

3. Determine the primary disorder.3. Determine the primary disorder.

HCOHCO33

2424pCOpCO22

4040vs.vs.

24 - 1724 - 17 2424

45 - 4045 - 40 4040

vs.vs.

77 2424

55 4040

>>

The primary disorder is a The primary disorder is a metabolic acidosis.metabolic acidosis.


4. Compute for the compensatory 4. Compute for the compensatory response.response.

HCOHCO33 = 24 – 17 = 7 = 24 – 17 = 7

pCOpCO22 = 7 x 1.2 = 8.4 = 7 x 1.2 = 8.4

Exp. pCOExp. pCO22 = 40 – 8.4 = 31.6 = 40 – 8.4 = 31.6 ±± 2 2

Actual pCOActual pCO22 of 45 is higher than exp. pCO of 45 is higher than exp. pCO22

This is a mixed metabolic acidosis This is a mixed metabolic acidosis and respiratory acidosis.and respiratory acidosis.


6. Use the delta-deltas to detect 6. Use the delta-deltas to detect coexisting metabolic disorders.coexisting metabolic disorders.

AG 25 – 12 13

HCO3 24 – 17 7====

This is a combined high anion gap This is a combined high anion gap metabolic acidosis and metabolic metabolic acidosis and metabolic

alkalosis.alkalosis.


STEP FIVE: Assess the PO2

Classification PaO2 (mmHg)

Hyperoxemia > 100

Normoxemia 80-100

Mild hypoxemia 60-79

Moderate hypoxemia 45-69

Severe hypoxemia < 45

For Adults


Room air, patient < 60 y.o.Room air, patient < 60 y.o.– Mild hypoxemiaMild hypoxemia paO2 < 80 mm HgpaO2 < 80 mm Hg– Moderate hypoxemiaModerate hypoxemia paO2 < 60 mm paO2 < 60 mm

HgHg– Severe hypoxemiaSevere hypoxemia paO2 < 40 mm HgpaO2 < 40 mm Hg

For each year > 60 y.o., subtract 1 mm Hg for For each year > 60 y.o., subtract 1 mm Hg for limits of mild and moderate hypoxemialimits of mild and moderate hypoxemia

At any age, a paO2 < 40 mm Hg indicates At any age, a paO2 < 40 mm Hg indicates severe hypoxemiasevere hypoxemia


Quantifying pulmonary dysfunction: Oxygenation Ratio or PF ratio (PaO2/FiO2)

Pulmonary status Oxygenation ratio

(PaO2/FiO2)

Normal 400-500

Moderate

Acute lung injury

200-390

< 300

Substantial pulmonary dysfunction

< 200

Part of ARDS criterion < 200

(equivalent to shunting > 20%)


Quantifying pulmonary dysfunction: Alveolar-arteriolar oxygen tension gradient

PAO2 = ideal/alveolar O2 tension P(A-a)O2 - quantitates efficiency of oxygen loading - increased in shunts, V/Q mismatch - Normal: < 60 yo, 10 mmHg (upper limit 20) > 60 yo, upper limit 35 mmHg When FiO2 < 60%, PAO2 = PiO2 – 1.2(PaCO2) PiO2 = (PB-PH2O) x FiO2 at sea level & room air, PiO2 ~ 150 mmHg or PiO2 = (760-47 mmHg) X 0.21 Limitations:

– Not helpful when changing FiO2– Above FiO2 60%, didn’t change anymore– Not a guide for oxygenation


REVIEW

Step 1: Acidosis vs Alkalosis Step 2: Respiratory vs. Metabolic Step 3: Compensated? Step 4: Anion Gap? Step 5: Oxygenation

QUESTIONS ?


THANK YOU!


BUFFERS IN THE BLOOD

Extracellular fluid buffers– Plasma HCO3– Plasma proteins– Inorganic phosphates

Intracellular fluid buffers– HCO3– Hb– Oxyhemoglobin– Inorganic phosphates– Organic phosphates

HCO3 buffering system (open system) responsible >50% buffering

ABG Lecture Dr Lenora Fernandez

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