abg for pgs

8/9/2019 abg for pgs

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Dr Mobin

Approach to acid

base problems


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“Life is a struggle, not against sin,not against the Money Power, notagainst malicious animal magnetism,

but against hydrogen ions." H.L. MENCEN


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COMPONENTS OF THE ABG

pH: Measrement o! acidit" or al#alinit"$ based on the h"dro%en &H'( )*+, - )*.,Pao/ The partial pressre o0"%en that is dissol1ed in arterial blood*

234533 mm H%*

PCO/: The amont o! carbon dio0ide dissol1ed in arterial blood*

+,- ., mmH%HCO+

: The calclated 1ale o! the amont o! bicarbonate in the blood

// - /6 mmol78B*E:

The base e0cess indicates the amont o! e0cess or ins9cient le1el o! bicarbonate* 4/ to '/mE78&A ne%ati1e base e0cess indicates a base de;cit in blood(

SaO/: The arterial o0"%en satration*


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Step?ise approach to ABG

! Step 1: Acidemic or Alkalemic? ! Step 2: Asses for Abnormalities in PCo2? ! Step 3. Asses to Pa O2. ! Step 4. Identify abnormalities in bicarb

! Step . Assess t!e normal compensation by t!erespiratory system for a metabolic dist"rbance

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@s there acidemia oral#alemia


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STEPS TO AN ABG @NTEPETAT@ON• Step:5

•

Assess the pH -acidotic7al#alotic• @! abo1e )*, - al#alotic

• @! belo? )*+, - acidotic


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5* acidemia or

al#alemia

A. Whatever side of 7.4 the pH is on, the process that caused it

to shift to that side is the primary abnormality.

B. The body does not fully compensate for primary acid base

disorders.

C. After this step one should search for the underlying primary

disorder. What processes, metabolic or respiratory broughtthe pH to either side of 7.4.


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! Step 2: Asses for Abnormalities in PCo2?

• Identify whether the PCO2 is normal, inappropriate, and whether there• is a respiratory acidosis or alkalosis• For acute increases in PCO2 of 10 mm Hg there should be a rise of 0.08•

in the pH.• For acute decreases in PCO2 of 10 mm Hg there should be a fall of 0.08• in the pH.• At this point one should decide whether the change in pH could be• explained by your PCO2.• If not, consider ordering a set of electrolytes• to review the causes for a metabolic acidosis One would evaluate the• bicarbonate level• In addition, evaluating the anion gap can direct you toward the cause• for an acidosis.


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Step 2*

• @! pH and paCO/ mo1es in oppositedirection - primar" respirator"problem*


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Step3: Evaluation of oxygenation

• If pH change is appropriate for PCO2

change then go to the evaluation of

oxygenation


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Step .: @denti!"abnormalities in

bicarbonate*

1. Normal bicarbonate is 22-24

2. If the value is lower than 22, there is a metabolic acidosis.3. If the value is higher than 24, then there is a metabolic alkalosis.

4. Metabolic Acidosis is commonly divided into anion gap and non-anion

gap acidosis.

5. If the Bicarbonate value is less than 22 one must calculate the anion

gap; if the anion gap is greater than 17, it suggests anionic gap acidosis.

6. Normal Anion Gap 12 +/- 2. If AG > 17, a metabolic acidosis is

probably present.


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Step . cont

1. Calculate the excess anion gap. The excess anion gap = total anion gap

minus the normal anion gap (12 mmol per liter). Add the excess anion gapto the measured bicarbonate concentration;

2. if the sum is greater than a normal serum bicarbonate (> 30 mmol per

liter) there is an underlying metabolic alkalosis;

3. if the sum is less than a normal bicarbonate (< 23) there is an underlying

nonanion gap metabolic acidosis

4. If the value is not less than 23 or greater than 30 then additional process is

going on.


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Step . cont

1. If there is no anion gap but the bicarbonate is low, a urinary anion gap

can help differentiate renal vs. gut as loss of bicarbonate.

2. (Na + K)- Cl-correlates with ammonium excretion.

3. As ammonium excretion increases the urinary gap narrows. So with

diarrhea (gap is negative), ammonium excretion unimpaired and increases

with acidosis.

4. With RTA (gap is positive) the gap widens since they have lost the abilityto absorb bicarb and form ammonium.


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Step 5: Assessment of Oxygenation

1. PaO2 of < 60 and Saturation < 90 is hypoxia.

2. A-a gradient is important to evaluate. One can estimate the Alveolar O2 bymultiplying 5 X the FiO2.

3. Alveolar O2 = (760-47)FIO2 - PCO2/.8 or 5 X FIO2 .

4. If the predicted PaO2 is much different than the true PaO2 than further

evaluation for causes should be done.

5. The main lesson here is to understand that by looking at the ABG one can

assess if there is an inappropriate FiO2 requirement.6. Normally the A-a is < 60 - 100. If it is larger the ability of the patient to

oxygenate is compromised.


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Step 5

The differentiation between A/C & CHR.respiratorydisorders is based on whether there is associated

acidemia / alalemia. !f the chan"e in paco2 is associated with the chan"ein pH# the disorder is ac$te.

!n chronic process the compensatory processbrin"s the pH to within the clinically acceptable

ran"e % .'( ) .5(*


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normal+etabolic

Alalosis

normal+etabolic

Acidosis

normalRespiratory

Alalosis

normalRespiratory

acidosis

HC(',aCo2pH


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Venous Blood Gas

• In patients with normal cardiac output, central venous pH is lower than• arterial by an average of 0.03 units, with venous PCO2 being higher by about 6 mm

Hg.• With severe circulatory failure, the pH differs by 0.1 unit with pCO2 difference of

24.• With a cardiopulmonary arrest, a venous blood gas could have a pH difference of

0.35 and a CO2 difference of 56 when compared to an arterial blood gas


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@ts not ma%ic nderstandin%ABGs$ it st ta#es a littlepractice


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i 5

pH 7.34

pCO2 33.9

HCO3 18.2

B.E. -6.2pO2 85.2


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i /

pH 7.34

pCO2 40.3

HCO3 21.4

B.E. -3.6pO2 41.0


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i .

pH 7.17pCO2 69.3

HCO3 21.0

B.E. -5.5

pO2 40.9


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i ,

pH 7.07pCO2 11.4

HCO3 3.1

B.E. -26.3

pO2 115.1


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i 6

pH 7.25pCO2 74.3

HCO3 12.4

B.E. -13.4

pO2 29.1


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i )

pH 7.45pCO2 27.0

HCO3 19.1

B.E. -3.5

pO2 65.5


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i 2

pH 7.28pCO2 79.5

HCO3 37.1

B.E. +8.4

pO2 30.0


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i =

pH 7.51pCO2 39.4

HCO3 31.3

B.E. +7.5

pO2 77.3


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i 53

pH 7.39pCO2 39.0

HCO3 23.4

B.E. -1.0

pO2 61.2


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i 55

pH 7.31

pCO2 58.5HCO3 26.1

B.E. +1.6

pO2 74.5


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i 5/

pH 7.46pCO2 34.0

HCO3 26.0

B.E. +2.0

pO2 43.8


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i 5+

pH 7.44pCO2 27.8

HCO3 19.2

B.E. -4.0

pO2 100


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i 5.

pH 7.36pCO2 75.1

HCO3 40.6

B.E. +11.9

pO2 65.2


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i 5,

pH 7.44pCO2 48.0

HCO3 32.6

B.E. +7.3

pO2 63.6


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i 5,

pH 7.18pCO2 42.0

HCO3 15.0

B.E. -12.5

pO2 69.0


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i 56

pH 7.36pCO2 30.0

HCO3 15.0

B.E. -12.0

pO2 80.0


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i 5)

pH 7.42

pCO2 39.0

HCO3 25.4

B.E. +1.1

pO2 92.0


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i 52

pH 7.52pCO2 31.0

HCO3 26.4

B.E. +2.7

pO2 100


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i 5=

pH 7.08

pCO2 54.0

HCO3 15.0

B.E. -15.5

pO2 54.0

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