ABG

INTERPRETATIONDebbie

Sander

PAS-II1Objectives2What’sanABG?Understanding

Acid/Base

RelationshipGeneral

approach

to

ABG

InterpretationClinical

causes

Abnormal

ABG’sCase

studiesTake

homeWhat

is

an

ABG3Arterial

Blood

GasDrawn

from

artery-

radial,

brachial,

femoralIt

is

an

invasive

procedure.Caution

must

be

taken

with

patient

onanticoagulants.Helps

differentiate

oxygen

deficiencies

from

primaryventilatory

deficiencies

from

primary

metabolic

acid-baseabnormalitiesWhat

Is AnABG?pH

[H+]PCO2

Partial

pressure

CO2PO2

Partial

pressure

O2HCO3

BicarbonateBE Base

excessSaO2Oxygen

Saturation4Acid/Base

Relationship5

This

relationship

is

critical

for

homeostasis

Significant

deviations

from

normal

pH

ranges

arepoorly

tolerated

and

may

be

life

threatening

Achieved

by

Respiratory

and

Renal

systemsCase

Study No.

1660

y/o

male

comes

ER

c/o

SOB.Tachypneic,

tachycardic,

diaphoretic

andCyanotic.

Dx

acute

resp.

failure

and

ABG’sShow

PaCO2

well

below

nl,

pH

above

nl,PaO2

is

very

low.

The

blood

gas

documentResp.

failure

due

to

primary

O2

problem.Case

Study No.

2760

y/o

male

comes

ER

c/o

SOB.Tachypneic,

tachycardic,

diaphoretic

andCyanotic.

Dx

acute

resp.

failure

and

ABG’sShow

PaCO2

very

high,

low

pH

and

PaO2is

moderately

low.

The

blood

gas

documentResp.

failure

due

to

primarily

ventilatoryinsufficiency.

There

are

two

buffers

that

work

in

pairs8

H2CO3CarbonicacidNaHCO3base

bicarbonate

These

buffers

are

linked

to

the

respiratory

andrenal

compensatory

systemBuffersRespiratory

Component9

function

of

the

lungs

Carbonic

acid

H2CO3Approximately

98%

normal

metabolites

are

in

theform ofCO2CO2

+

H2O

H2CO3

excess

CO2

exhaled

by

the

lungsMetabolic

Component10

Function

of

the

kidneys

base

bicarbonate Na

HCO3Process

of

kidneys

excreting

H+into

the

urine

and

reabsorbingHCO

-

intothe

blood

from

the

renaltubules3active

exchange

Na+

for

H+

between

the

tubularcells

and

glomerular

filtratecarbonic

anhydrase

is

an

enzyme

that

accelerates hydration/dehydration

CO2

in

renal

epithelial

cellsH2O

+

CO2

H2CO311

HCO3

+H+Acid/Base

RelationshipNormal

ABG

values12pH 7.35

–7.45PCO2

35–45

mmHgPO2

80–100mmHgHCO3

22

–26mmol/LBE -2-+2SaO2

>95%Acidosis13AlkalosispH <

7.35PCO2

>

45HCO3

<

22pH >

7.45PCO2

<

35HCO3

>

26Respiratory

Acidosis14

Think

of

CO2

as

an

acid

failure

of

the

lungs

to

exhale

adequate

CO2pH<

7.35

PCO2

>

45

CO2

+

H2CO3

pHCauses

of

Respiratory

Acidosis15

emphysema

drug

overdose

narcosis

respiratory

arrest

airway

obstructionMetabolic

Acidosis16

failure

of

kidney

function

blood

HCO3

which

results

in

availability

of

renaltubularHCO3

forH+

excretion

pH

<

7.35

HCO3

<

22Causes

of Metabolic

Acidosis17

renal

failure

diabetic

ketoacidosis

lactic

acidosis

excessive

diarrhea

cardiac

arrestRespiratory

Alkalosis18

too

much

CO2

exhaled

(hyperventilation)

PCO2,

H2CO3

insufficiency

=

pH

pH

>

7.45

PCO2

<

35Causes

of

Respiratory

Alkalosis19

hyperventilation

panic

d/o

pain

pregnancy

acute

anemia

salicylate

overdoseMetabolic

Alkalosis20

plasma

bicarbonate

pH

>

7.45

HCO3

>

26Causes

of Metabolic

Alkalosis21

loss

acid

from

stomach

or

kidney

hypokalemia

excessive

alkali

intakeHow

to Analyze

an

ABGPO2

NL

= 80–100mmHgpH

NL

= 7.35

–7.45AcidoticAlkalotic<7.35>7.453.

PCO2NL

= 35–45mmHgAcidoticAlkalotic>45<354.

HCO3NL

= 22–26mmol/LAcidoticAlkalotic<

22>

2622Four-step ABG

Interpretation23Step

1:

Examine

PaO2

&

SaO2

Determine

oxygen

status

Low

PaO2

(<80

mmHg)

&

SaO2

means

hypoxia

NL/elevated

oxygen

means

adequate

oxygenationStep

2:

pHacidosisalkalosis<7.35>7.45Four-step ABG

Interpretation24Step

3:

study

PaCO2

&

HCO

3

respiratoryirregularity

if

PaCO2

abnl

&

HCO3

NL

metabolic

irregularity

if

HCO3

abnl

&

PaCO2

NL25Four-step ABG

InterpretationStep

4:Determine

if

there

is

a

compensatory

mechanism

workingto

try

to

correct

the

pH.ie: if

have

primary

respiratory

acidosis

will

have

increasedPaCO2

and

decreased

pH. Compensation

occurswhenthe

kidneys

retain

HCO3.26Four-step ABG

Interpretation~

PaCO2

–

pH

Relationship27807.20607.30407.40307.50207.60CompensatedRespiratoryAcidosisCO2More

AbnormalRespiratoryAcidosisCO2ExpectedMixedRespiratoryMetabolicAcidosisCO2Less

AbnormalCO2

Changec/wAbnormalityMetabolicMetabolicAcidosisCO2NormalCompensatedMetabolicAcidosisCO2

ChangeopposesAbnormalityAcidosisABG

Interpretation28CompensatedRespiratoryAlkalosisCO2More

AbnormalRespiratoryAlkalosisCO2ExpectedMixedRespiratoryMetabolicAlkalosisCO2Less

AbnormalCO2

Changec/wAbnormalityMetabolicAlkalosisCO2NormalCompensatedMetabolicAlkalosisCO2

ChangeopposesAbnormalityAlkalosisABG

Interpretation29Respiratory

Acidosis30pH

7.30PaCO2

60HCO3

26Respiratory

Alkalosis31pH

7.50PaCO2

30HCO3

22Metabolic

Acidosis32pH

7.30PaCO2

40HCO3

15Metabolic

Alkalosis33pH

7.50PCO2

40HCO3

30What

are

thecompensations?34Respiratory

acidosis

metabolic

alkalosisRespiratory

alkalosis

metabolic

acidosisIn

respiratory

conditions,

therefore,

the

kidneys

willattempt

to

compensate

and

visa

versa.In

chronic

respiratory

acidosis

(COPD)

the

kidneysincreasethe

eliminationofH+

andabsorbmore

HCO3.

The

ABG

willShow

NL

pH,

CO2and

HCO3.Buffers

kick

in

withinminutes. Respiratory

compensationis

rapid

and

starts

within

minutes

and

complete

within

24hours.

Kidney

compensation

takes

hours

and

up

to

5

days.Mixed

Acid-Base

Abnormalities35Case

Study

No.

3:56

yo

neurologic

dz

required

ventilator

support

for

severalweeks. She

seemed

most

comfortable

when

hyperventilatedto

PaCO2

28-30mmHg. She

required

daily

doses

of

lasix

toassure

adequate

urine

output

and

received

40

mmol/L

IV

K+each

day.

On

10th

day

ofICU

her

ABG

on24%

oxygen

&

VS:ABG

Results36pH7.62BP115/80

mmHgPCO230mmHgPulse88/minPO285mmHgRR10/minHCO330mmol/LVT1000mlBE10mmol/LMV10LK+2.5mmol/LInterpretation:

Acute

alveolar

hyperventilation(resp.

alkalosis)

and

metabolic

alkalosis

with

correctedhypoxemia.Case

study No.

43727

yoretarded

with

insulin-dependent

DM

arrivedat

ERfrom

the

institution

where

he

lived. On

room

air

ABG

&

VS:pH7.15BP180/110

mmHgPCO222mmHgPulse130/minPO292mmHgRR40/minHCO3

9mmol/LBE -30mmol/LVT

800mlMV

32LInterpretation:Partly

compensated

metabolic

acidosis.Case

study No.

53874

yo

with

hx

chronic

renal

failure

and

chronic

diuretictherapywas

admitted

to

ICU

comatose

and

severelydehydrated.

On40%

oxygen

her

ABG

&

VS:pH7.52BP130/90

mmHgPCO255mmHgPulse120/minPO292mmHgRR25/minHCO342mmol/LVT150mlMV

3.75LPartly

compensated

metabolic

alkalosis

withBE 17mmol/LInterpretation:corrected

hypoxemia.Case

study No.

643

yo

arrives

in

ER

20

minutes

after

a

MVA

in

which

heinjured

his

face

on

thedashboard. He

is

agitated,

has

mottled,cold

and

clammy

skin

and

has

obvious

partial

airwayobstruction.An

oxygen

mask

at

10

L

is

placed

on

hisface. ABG

&

VS:pH7.10BP150/110

mmHgPCO260mmHgPulse150/minPO2125mmHgRR45/minHCO318mmol/LVT?

mlBE-15mmol/LMV?

LIn.

terpretation: Acute

ventilatory

failure

(resp.

acidosis)andacute

metabolic

acidosis

with

corrected

hypoxemia39Case

study No.

74017yo,48kg

with

knowninsulin-dependent

DM

came

to

ERwith

Kussmaul

breathing

and

irregular

pulse. Room

airABG

&

VS:pH7.05BP140/90

mmHgPCO212mmHgPulse118/minPO2108mmHgRR40/minHCO35mmol/LVT1200mlBE-30mmol/LMV48LInterpretation:

Severe

partly

compensated

metabolicacidosis

without

hypoxemia.Case

No.

7

cont’d41This

patient

is

in

diabetic

ketoacidosis.IV

glucose

and

insulin

were

immediatelyadministered.

Ajudgement

was

made

that

severe

acidemia

was

adverselyaffecting

CV

function

and

bicarb

was

elected

to

restore

pH

to

7.20.Bicarb

administration

calculation:Base

deficit

X

weight

(kg)430X

48 =

360mmol/L4Admin

1/2

over

15

min

&repeat

ABGCase

No.

7

cont’d42ABG

result

after

bicarb:pH7.27BP130/80

mmHgPCO225mmHgPulse100/minPO292mmHgRR22/minHCO311mmol/LVT600mlBE-14mmol/LMV13.2LCase

study No.

84347

yo

was

inPACU

for

3

hours

s/pcholecystectomy.

Shehad

been

on

40%

oxygen

and

ABG

&

VS:pH7.44BP130/90

mmHgPCO232mmHgPulse95/min,

regularPO2121mmHgRR20/minHCO322mmol/LVT350mlBE-2mmol/LMV7LSaO2Hb98%13

g/dLCase

No.

8

cont’d44Oxygen

was

changed

to

2L

N/C. 1/2

hour

pt.

ready

to

be

D/Cto

floor

and

ABG

&

VS:pH7.41BP130/90

mmHgPCO210mmHgPulse95/min,

regularPO2148mmHgRR20/minHCO36mmol/LVT350mlBE-17mmol/LMV7LSaO2Hb99%7g/dLCase

No.

8

cont’dWhat

is

going

on?45Case

No.

8

cont’d46If

the

picture

doesn’t

fit,

repeat

ABG!!pH7.45BP130/90

mmHgPCO231mmHgPulse95/minPO287mmHgRR20/minHCO322mmol/LVT350mlBE-2mmol/LMV7LSaO2Hb96%13

g/dLTechnical

error

was

presumed.Case

study No.

94767

yo

who

had

closed

reduction

of

leg

fx

withoutincident.Four

days

later

she

experienced

a

sudden

onset

of

severe

chestpain

and

SOB. Room

air

ABG

&

VS:pH7.36BP130/90

mmHgPCO233mmHgPulse100/minPO2HCO3BE55mmHg18mmol/L-5mmol/LRRMV25/min18LSaO288%Interpretation:

Compensated

metabolic

acidosis

withmoderate

hypoxemia.

Dx:

PECase

study No.

104876

yo

with

documented

chronic

hypercapnia

secondary

tosevere

COPD

has

been

in

ICU

for

3

days

while

being

tx

forpneumonia. She

had

been

stable

for

past

24

hours

and

wastransferred

to

general

floor. Pt

was

on

2L

oxygen

&

ABG

&VS:pHPCO27.4463mmHgBPPulse135/95

mmHg110/minPO2HCO352mmHg42mmol/LRR22/minSaO2BE +16

mmol/L86%.Interpretation:MV

10LChronic

ventilatory

failure

(resp.

acidosis)with

uncorrected

hypoxemiaCase

No.

10

cont’dShe

was

placed

on

3L and

monitored

for

nexthour.

Sheremained

alert,

oriented

and

comfortable. ABGwasrepeated:pH7.36BP140/100

mmHgPCO275mmHgPulse105/minPO2HCO3BE65mmHg42mmol/L+16

mmol/LRRMV24/min4.8LSaO292%.Pt’s

ventilatory

pattern

has

changed

to

more

rapid

andshallow

breathing. Although

still

acceptable

the

pH

andCO2are

trending

in

the

wrongdirection.

High-flowoxygen

may

be

better

for

this

pt

to

preventintubation49Take

Home

Message:50

Valuable

information

can

be

gained

from

anABG

as

to

the

patients

physiologic

condition

Remember

that

ABG

analysis

if

only

part

of

thepatientassessment.

Be

systematic

with

your

analysis,

start

with

ABC’s

as

alwaysand

look

for

hypoxia

(which

you

can

usual