ABG INTERPRETATION

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ABG INTERPRETATION

• Debbie Sander PAS-II

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Objectives

• Whats an ABG?
• Understanding Acid/Base Relationship
• General approach to ABG Interpretation
• Clinical causes Abnormal ABGs
• Case studies
• Take home

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What is an ABG
Arterial Blood Gas Drawn from artery- radial,
brachial, femoral It is an invasive
procedure. Caution must be taken with patient on
anticoagulants. Helps differentiate oxygen
deficiencies from primary ventilatory
deficiencies from primary metabolic
acid-base abnormalities
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What Is An ABG?
pH H PCO2 Partial pressure CO2 PO2
Partial pressure O2 HCO3 Bicarbonate BE
Base excess SaO2 Oxygen Saturation
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Acid/Base Relationship

• This relationship is critical for homeostasis
• Significant deviations from normal pH ranges are
  poorly tolerated and may be life threatening
  
• Achieved by Respiratory and Renal systems

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Case Study No. 1

• 60 y/o male comes ER c/o SOB.
• Tachypneic, tachycardic, diaphoretic and
• Cyanotic. Dx acute resp. failure and ABGs
• Show PaCO2 well below nl, pH above nl,
• PaO2 is very low. The blood gas document
• Resp. failure due to primary O2 problem.

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Case Study No. 2
60 y/o male comes ER c/o SOB. Tachypneic,
tachycardic, diaphoretic and Cyanotic. Dx acute
resp. failure and ABGs Show PaCO2 very high, low
pH and PaO2 is moderately low. The blood gas
document Resp. failure due to primarily
ventilatory insufficiency.
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Buffers

• There are two buffers that work in pairs
• H2CO3 NaHCO3Carbonic acid base
  bicarbonate
• These buffers are linked to the respiratory and
  renal compensatory system

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Respiratory Component

• function of the lungs
• Carbonic acid H2CO3
• Approximately 98 normal metabolites are in the
  form of CO2
• CO2 H2O ? H2CO3
• excess CO2 exhaled by the lungs

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Metabolic Component

• Function of the kidneys
• base bicarbonate Na HCO3
• Process of kidneys excreting H into the urine
  and reabsorbing HCO3- into the blood from
  the renal tubules 1) active exchange Na for H
  between the tubular cells and
  glomerular filtrate 2) carbonic anhydrase is an
  enzyme that accelerates
  hydration/dehydration CO2 in renal epithelial
  cells

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Acid/Base Relationship

H2O CO2 ? H2CO3 ? HCO3
H
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Normal ABG values
pH 7.35 7.45 PCO2 35 45 mmHg PO2 80
100 mmHg HCO3 22 26 mmol/L BE -2 -
2 SaO2 gt95
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Acidosis Alkalosis
pH gt 7.45 PCO2 lt 35 HCO3 gt 26
pH lt 7.35 PCO2 gt 45 HCO3 lt 22
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Respiratory Acidosis

• Think of CO2 as an acid
• failure of the lungs to exhale adequate CO2
• pH lt 7.35
• PCO2 gt 45
• CO2 H2CO3 ? ? pH

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Causes of Respiratory Acidosis

• emphysema
• drug overdose
• narcosis
• respiratory arrest
• airway obstruction

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Metabolic Acidosis

• failure of kidney function
• ? blood HCO3 which results in ? availability of
  renal tubular HCO3 for H excretion
• pH lt 7.35
• HCO3 lt 22

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Causes of Metabolic Acidosis

• renal failure
• diabetic ketoacidosis
• lactic acidosis
• excessive diarrhea
• cardiac arrest

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Respiratory Alkalosis

• too much CO2 exhaled (hyperventilation)
• ? PCO2, H2CO3 insufficiency ? pH
• pH gt 7.45
• PCO2 lt 35

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Causes of Respiratory Alkalosis

• hyperventilation
• panic d/o
• pain
• pregnancy
• acute anemia
• salicylate overdose

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Metabolic Alkalosis

• ? plasma bicarbonate
• pH gt 7.45
• HCO3 gt 26

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Causes of Metabolic Alkalosis

• ? loss acid from stomach or kidney
• hypokalemia
• excessive alkali intake

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How to Analyze an ABG

• PO2 NL 80 100 mmHg
• 2. pH NL 7.35 7.45
• Acidotic lt7.35
• Alkalotic gt7.45
• PCO2 NL 35 45 mmHg
• Acidotic gt45
• Alkalotic lt35
• HCO3 NL 22 26 mmol/L
• Acidotic lt 22
• Alkalotic gt 26

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Four-step ABG Interpretation

• Step 1
• Examine PaO2 SaO2
• Determine oxygen status
• Low PaO2 (lt80 mmHg) SaO2 means hypoxia
• NL/elevated oxygen means adequate oxygenation

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Four-step ABG Interpretation

• Step 2
• pH acidosis lt7.35
• alkalosis gt7.45

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Four-step ABG Interpretation

• Step 3
• study PaCO2 HCO 3
• respiratory irregularity if PaCO2 abnl HCO3 NL
• metabolic irregularity if HCO3 abnl PaCO2 NL

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Four-step ABG Interpretation
Step 4 Determine if there is a compensatory
mechanism working to try to correct the pH. ie
if have primary respiratory acidosis will have
increased PaCO2 and decreased pH. Compensation
occurs when the kidneys retain HCO3.
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PaCO2 pH Relationship
80 7.2060 7.3040 7.4030 7.5020 7.60

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ABG Interpretation
Acidosis
CO2 Change
CO2
CO2 Change
c/w
Normal
opposes
Abnormality
Abnormality
CO2
CO2
CO2
Metabolic
Compensated
Metabolic Acidosis
More Abnormal
Expected
Less Abnormal
Metabolic
Acidosis
Compensated
Respiratory
Mixed
Respiratory
Acidosis
Respiratory
Acidosis
Metabolic
Acidosis
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ABG Interpretation
Alkalosis
CO2 Change
CO2
CO2 Change
c/w
Normal
opposes
Abnormality
Abnormality
CO2
CO2
CO2
Metabolic
Compensated
More Abnormal
Expected
Less Abnormal
Alkalosis
Metabolic
Alkalosis
Compensated
Respiratory
Mixed
Respiratory
Alkalosis
Respiratory
Alkalosis
Metabolic
Alkalosis
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Respiratory Acidosis
pH 7.30 PaCO2 60 HCO3 26
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Respiratory Alkalosis
pH 7.50 PaCO2 30 HCO3 22
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Metabolic Acidosis
pH 7.30 PaCO2 40 HCO3 15
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Metabolic Alkalosis
pH 7.50 PCO2 40 HCO3 30
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What are the compensations?
Respiratory acidosis ? metabolic
alkalosis Respiratory alkalosis ? metabolic
acidosis In respiratory conditions, therefore,
the kidneys will attempt to compensate and visa
versa. In chronic respiratory acidosis (COPD)
the kidneys increase the elimination of H and
absorb more HCO3. The ABG will Show NL pH, ?CO2
and ?HCO3. Buffers kick in within minutes.
Respiratory compensation is rapid and starts
within minutes and complete within 24 hours.
Kidney compensation takes hours and up to 5 days.
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Mixed Acid-Base Abnormalities
Case Study No. 3 56 yo ? ? neurologic dz
required ventilator support for several weeks.
She seemed most comfortable when
hyperventilated to PaCO2 28-30 mmHg. She
required daily doses of lasix to assure adequate
urine output and received 40 mmol/L IV K each
day. On 10th day of ICU her ABG on 24 oxygen
VS
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ABG Results
pH 7.62 BP 115/80 mmHg PCO2 30
mmHg Pulse 88/min PO2 85 mmHg RR 10/min HCO3 30
mmol/L VT 1000ml BE 10 mmol/L MV 10L K 2.5
mmol/L
Interpretation Acute alveolar hyperventilation
(resp. alkalosis) and metabolic alkalosis with
corrected hypoxemia.
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Case study No. 4
27 yo retarded ? with insulin-dependent DM
arrived at ER from the institution where he
lived. On room air ABG VS pH 7.15 BP 180/11
0 mmHg PCO2 22 mmHg Pulse 130/min PO2 92
mmHg RR 40/min HCO3 9 mmol/L VT 800ml BE -30
mmol/L MV 32L
Interpretation Partly compensated metabolic
acidosis.
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Case study No. 5
74 yo ? with hx chronic renal failure and chronic
diuretic therapy was admitted to ICU comatose and
severely dehydrated. On 40 oxygen her ABG
VS pH 7.52 BP 130/90 mmHg PCO2 55
mmHg Pulse 120/min PO2 92 mmHg RR 25/min HCO3 42
mmol/L VT 150ml BE 17 mmol/L MV 3.75L
Interpretation Partly compensated metabolic
alkalosis with corrected hypoxemia.
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Case study No. 6
43 yo ? arrives in ER 20 minutes after a MVA in
which he injured his face on the dashboard. He
is agitated, has mottled, cold and clammy skin
and has obvious partial airway obstruction. An
oxygen mask at 10 L is placed on his face. ABG
VS pH 7.10 BP 150/110 mmHg PCO2 60
mmHg Pulse 150/min PO2 125 mmHg RR 45/min HCO3 1
8 mmol/L VT ? ml BE -15 mmol/L MV ? L .
Interpretation Acute ventilatory failure (resp.
acidosis) and acute metabolic acidosis with
corrected hypoxemia
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Case study No. 7
17 yo, 48 kg ? with known insulin-dependent DM
came to ER with Kussmaul breathing and irregular
pulse. Room air ABG VS pH 7.05 BP 140/90
mmHg PCO2 12 mmHg Pulse 118/min PO2 108
mmHg RR 40/min HCO3 5 mmol/L VT 1200ml BE -30
mmol/L MV 48L
Interpretation Severe partly compensated
metabolic acidosis without hypoxemia.
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Case No. 7 contd
This patient is in diabetic ketoacidosis. IV
glucose and insulin were immediately
administered. A judgement was made that severe
acidemia was adversely affecting CV function and
bicarb was elected to restore pH to ?
7.20. Bicarb administration calculation Base
deficit X weight (kg) 4 30 X 48
360 mmol/L Admin 1/2 over 15 min 4
repeat ABG
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Case No. 7 contd
ABG result after bicarb pH 7.27 BP 130/80
mmHg PCO2 25 mmHg Pulse 100/min PO2 92
mmHg RR 22/min HCO3 11 mmol/L VT 600ml BE -14
mmol/L MV 13.2L
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Case study No. 8
47 yo ? was in PACU for 3 hours s/p
cholecystectomy. She had been on 40 oxygen and
ABG VS pH 7.44 BP 130/90 mmHg PCO2 32
mmHg Pulse 95/min, regular PO2 121
mmHg RR 20/min HCO3 22 mmol/L VT 350ml BE -2
mmol/L MV 7L SaO2 98 Hb 13 g/dL
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Case No. 8 contd
Oxygen was changed to 2L N/C. 1/2 hour pt. ready
to be D/C to floor and ABG VS pH 7.41 BP 130
/90 mmHg PCO2 10 mmHg Pulse 95/min,
regular PO2 148 mmHg RR 20/min HCO3 6
mmol/L VT 350ml BE -17 mmol/L MV 7L SaO2 99 Hb
7 g/dL
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Case No. 8 contd
What is going on?
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Case No. 8 contd
If the picture doesnt fit, repeat ABG!! pH 7.
45 BP 130/90 mmHg PCO2 31 mmHg Pulse 95/min PO2
87 mmHg RR 20/min HCO3 22 mmol/L VT 350ml BE -2
mmol/L MV 7L SaO2 96 Hb 13 g/dL
Technical error was presumed.
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Case study No. 9
67 yo ? who had closed reduction of leg fx
without incident. Four days later she experienced
a sudden onset of severe chest pain and SOB.
Room air ABG VS pH 7.36 BP 130/90
mmHg PCO2 33 mmHg Pulse 100/min PO2 55
mmHg RR 25/min HCO3 18 mmol/L BE -5
mmol/L MV 18L SaO2 88
Interpretation Compensated metabolic acidosis
with moderate hypoxemia. Dx PE
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Case study No. 10
76 yo ? with documented chronic hypercapnia
secondary to severe COPD has been in ICU for 3
days while being tx for pneumonia. She had been
stable for past 24 hours and was transferred to
general floor. Pt was on 2L oxygen ABG
VS pH 7.44 BP 135/95 mmHg PCO2 63
mmHg Pulse 110/min PO2 52 mmHg RR 22/min HCO3 42
mmol/L BE 16 mmol/L MV 10L SaO2 86 .
Interpretation Chronic ventilatory failure
(resp. acidosis) with uncorrected hypoxemia
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Case No. 10 contd
She was placed on 3L and monitored for next
hour. She remained alert, oriented and
comfortable. ABG was repeated pH 7.36 BP 140/
100 mmHg PCO2 75 mmHg Pulse 105/min PO2 65
mmHg RR 24/min HCO3 42 mmol/L BE 16
mmol/L MV 4.8L SaO2 92 .
Pts ventilatory pattern has changed to more
rapid and shallow breathing. Although still
acceptable the pH and CO2 are trending in the
wrong direction. High-flow oxygen may be better
for this pt to prevent intubation
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Take Home Message

• Valuable information can be gained from an
• ABG as to the patients physiologic condition
• Remember that ABG analysis if only part of the
  patient
• assessment.
• Be systematic with your analysis, start with
  ABCs as always and look for hypoxia (which
  you can usually treat quickly), then follow
  the four steps.
• A quick assessment of patient oxygenation can be
  achieved with a pulse oximeter which measures
  SaO2.

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Its not magic understanding ABGs, it just takes
a little practice!
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Any Questions?
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References

• Shapiro, Barry A., et al Clinical Application
  of BloodGases 1994
• 2. American Journal of Nursing1999Aug99(8)34-6
  
• 3. Journal Post Anesthesia Nursing1990Aug5(4)2
  64-72
• 4. Irvine, DavidABG Interpretation, A Rough
  and DirtyProduction

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Practice ABGs

• PaO2 90 SaO2 95 pH 7.48 PaCO2 32
  HCO3 24
• PaO2 60 SaO2 90 pH 7.32 PaCO2 48
  HCO3 25
• PaO2 95 SaO2 100 pH 7.30 PaCO2 40
  HCO3 18
• PaO2 87 SaO2 94 pH 7.38 PaCO2 48
  HCO3 28
• PaO2 94 SaO2 99 pH 7.49 PaCO2 40
  HCO3 30
• 6. PaO2 62 SaO2 91 pH 7.35 PaCO2
  48 HCO3 27
• PaO2 93 SaO2 97 pH 7.45 PaCO2 47
  HCO3 29
• PaO2 95 SaO2 99 pH 7.31 PaCO2 38
  HCO3 15
• PaO2 65 SaO2 89 pH 7.30 PaCO2 50
  HCO3 24
• 10. PaO2 110 SaO2 100 pH 7.48 PaCO2
  40 HCO3 30

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Answers to Practice ABGs

1. Respiratory alkalosis
2. Respiratory acidosis
3. Metabolic acidosis
4. Compensated Respiratory acidosis
5. Metabolic alkalosis
6. Compensated Respiratory acidosis
7. Compensated Metabolic alkalosis
8. Metabolic acidosis
9. Respiratory acidosis
10. Metabolic alkalosis