abg lecture dr lenora fernandez
TRANSCRIPT
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ARTERIAL BLOOD GAS INTERPRETATIONARTERIAL BLOOD GAS INTERPRETATION
Lenora C. Fernandez, MD FPCCP
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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
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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
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COMPONENTS OF AN ABG
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%.
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COMPONENTS OF AN ABG
PaCO2 The amount of carbon dioxide dissolved
in arterial blood. The normal range is 35 to 45 mm Hg.
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COMPONENTS OF AN ABG
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.)
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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.
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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
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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
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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
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ANALYSIS OF RESULTS
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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
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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
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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
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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.
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STEP TWO: Respiratory vs. Metabolic
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
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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
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STEP TWO: Respiratory vs. Metabolic
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.
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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
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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.
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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)
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Na+
Regulatory Response to Acidemia
Cl-
H+
Protein- PO4
=,SO4=
Organic acids
normal anion gap
URINE
HCO3-
NH4+ H2PO4
-
PCT
DT
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Once the primary disorder is identified, compute the expected value of the compensating buffering system
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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
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SAMPLE
77/F diagnosed case of ESRD who missed her dialysis session twice admitted for decreased responsiveness
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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
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Respiratory acidosis
Primary disorder
Compensated response
Degree of change
Acute High PCO2
High HCO3 ΔHCO3 = 1/10 ΔPCO2
Chronic High PCO2
High HCO3 ΔHCO3 = 3/10 ΔPCO2
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Respiratory alkalosis
Primary disorder
Compensated response
Degree of change
Acute Low PCO2
Low HCO3 ΔHCO3 = 2/10 ΔPCO2
Chronic Low PCO2
Low HCO3 ΔHCO3 = 4/10 ΔPCO2
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58/F chronic COPD admitted for elective breast surgery
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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
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STEP THREE: COMPENSATED
A patient can then be in a fully compensated, partially compensated, uncompensated state.
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HINT
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Uncompensated States
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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
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Na136
Cl100
AG 12
HCO3
24
NORMAL
Unmeasured anionsProtein-
PO4=,SO4
= Organic
acids
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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)
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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
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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
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Compute for delta delta value to determine co-existing metabolic derangements
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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
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HAGMA
Δ AG = Δ HCO3 pure HAGMA
Δ AG < Δ HCO3 HAGMA + NAGMA
Δ AG > Δ HCO3 HAGMA + metabolic alkalosis
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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
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NAGMA
Δ Cl = Δ HCO3 pure NAGMA Δ Cl < Δ HCO3 NAGMA + HAGMA Δ Cl > Δ HCO3 NAGMA + metabolic
alkalosis
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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
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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
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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.
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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.
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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.
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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.
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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
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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
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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%)
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Philippine General Hospital
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
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Philippine General Hospital
REVIEW
Step 1: Acidosis vs Alkalosis Step 2: Respiratory vs. Metabolic Step 3: Compensated? Step 4: Anion Gap? Step 5: Oxygenation
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QUESTIONS ?
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Philippine General Hospital
THANK YOU!
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Philippine General Hospital
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