acid-base disorders and the abg - mans · evidence of an underlying acid-base disturbance (2). to...
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AcidAcid--Base Disorders and the ABGBase Disorders and the ABG
BYBYDr/Magda abdelDr/Magda abdel--salamsalam
Lecturer of Thoracic MedicineLecturer of Thoracic Medicine
The Goal :The Goal :
To provide Bedside approach to ABG analysis
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The Blood Gas Report: normals…
pH 7.40 +
0.05
PaCO2
40 +
5 mm Hg
PaO2
80 -
100
mm Hg
HCO3
24 +
4
mmol/L
O2 Sat
>95
Always mention and see FIO2
The essentials
HCO3
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DefinitionsHypoxiadiminished availability of oxygen to the body and tissues
Hypoxemiadeficient oxygenation of the blood
Ischemiadeficiency of blood supply to a part of the body (tissue/organ)
anoxia is ultimate ischemia
Definitions
Acidany substance which donates H+HCl-, H2CO3, CO2Base/Buffersubstances that accept H+minimize changes in pHbicarbonate; proteins; phosphates & hemoglobin
Acidosis
excess H+ in tissuespH: degree of acidity or alkalinitynegative logarithm of hydrogen ion concentrationthe higher the acidity, the lower the pH↑
H+ = ↓
pH
< 7.35
Alkalosis
Excess base (bicarbonate/HCO3)pH: degree of acidity or alkalinitynegative logarithm of hydrogen ion concentrationthe more alkaline, the higher the pH↓
H+ = ↑
pH
> 7.45
Serum bicarbonateSerum bicarbonateThe standard/corrected bicarbonateThe standard/corrected bicarbonateis the value obtained at CO2 levels of is the value obtained at CO2 levels of 40mm Hg and at room temperature.40mm Hg and at room temperature.
The actual bicarbonateThe actual bicarbonate
is the value is the value calculated from the blood gas sample. calculated from the blood gas sample. The. It gives a better estimate of the The. It gives a better estimate of the metabolic problem causing acid base metabolic problem causing acid base imbalanceimbalance
The base deficit/excessThe base deficit/excess is the is the
amount of deviation of the standard amount of deviation of the standard bicarbonate from the normal { actual bicarbonate from the normal { actual bicarbonate}. The metabolic problem could bicarbonate}. The metabolic problem could either be a low (base deficit or metabolic either be a low (base deficit or metabolic acidosis) or high (base excess or acidosis) or high (base excess or metabolic alkalosis) standard bicarbonate metabolic alkalosis) standard bicarbonate normally normally (+/-
2).
HendersonHenderson--Hasselbalch EquationHasselbalch Equation
Primary AcidPrimary Acid--Base DisordersBase Disorders
As dictated by the HendersonAs dictated by the Henderson--Hasselbalch equation, Hasselbalch equation, disturbances in either the respiratory component (pCOdisturbances in either the respiratory component (pCO22
) or ) or metabolic component (HCOmetabolic component (HCO33
--) can lead to alterations in pH.) can lead to alterations in pH.
Metabolic AcidosisMetabolic Acidosis(Too little HCO(Too little HCO33
--))Metabolic AlkalosisMetabolic Alkalosis(Too much HCO(Too much HCO33
--))
Respiratory Acidosis Respiratory Acidosis (Too much CO(Too much CO22
))Respiratory AlkalosisRespiratory Alkalosis
(Too little CO(Too little CO22
))
Respiratory Acidosis
•
decreased respiratory exchange with retention of CO2 results in increased pCO2 which thencauses renal retention of bicarbonate
Respiratory Acidosis: CausesDecreased respiratory exchangeCNS Depressiontrauma/infections/tumorcerebrovascularhydrothoraxpneumothoraxLung disorderbronchial obstructionemphysema (chronicobstructive airwaydisease)severe pulmonary edemaaccidentsdrug overdoseNeuromuscular disordersMyopathiesThoracic disorders
Respiratory Acidosis: Compensation
Problem: increased pCO2 and this results in a decreased blood pH (high H+)[H+] stimulates kidney to generate and retain bicarbonaterespiratory acidosis.is compensated for by the development of a metabolic alkalosis
Respiratory Acidosis: Compensation
Compensation is complete ([HCO3] levelsout) in 2-4 daysFinal HCO3 can be calculated from thefollowing equation:HCO3 mmol/L = 0.44 X pCO2 mmHg + 7.6 (+/-
2).Limit of compensation is a HCO3 of 45mmol/L
Respiratory Alkalosis
increased respiratory exchange with loss of CO2results in a decreased pCO2 which then stimulates renal excretion of bicarbonate
Respiratory Alkalosis: Causes
Increased respiratory exchangeCNS disturbancesPsychogenic (anxiety)PregnancyHypoxiaDrug toxicity /overdosePulmonary disordersEmbolismEdemaAsthmaPneumonia
Respiratory Alkalosis: Compensation
Problem: decreased pCO2 causing increased blood pH(low H+)Increased pH stimulates the kidney to excrete bicarbonaterespiratory alkalosis is compensated for by the development of a metabolic acidosis
Respiratory Alkalosis: Compensation
If the condition has been present for 7 days or more full compensation may occurCompensation is complete ([HCO3] levels out) in 7-10 daysThe limit of compensation is a HCO3 of 12mmol/L.
Metabolic Acidosis
Increased production or renal retention of H+results in a low pH which stimulates respiration to decreased the pCO2
Metabolic Acidosis: Causes
High Anion GapRenal failuretoxinsketoacidosisNormal anion gap(hyperchloremic)Hyperkalemiaobstructive uropathydiarrhearenal tubular acidosisSome medications
Metabolic Acidosis: Compensation
Problem: decreased [HCO3] causing decreased blood pH(high H+).[H+] stimulates respiration which lowers the blood pCO2metabolic acidosis is compensated for by the development of a respiratory alkalosis
Metabolic Acidosis: Compensation
Compensation is complete (pCO2 levelsout) in 12-24 hours.The final pCO2 can be calculated from thefollowing equation:pCO2 mmHg = 1.5 x [HCO3] (mmol/L) + 8 (+/-
2).The limit of compensation is a pCO2 of 10mmHg
Anion GapAnion Gap
Law of electroneutrality:Law of electroneutrality:––
Blood plasma contains an = number of + and Blood plasma contains an = number of + and ––
charges.charges.The major cation is NaThe major cation is Na++..––
Minor cations are KMinor cations are K++, Ca, Ca2+2+
, Mg, Mg2+2+. .
The major anions are HC0The major anions are HC033
--
and Cland Cl--..
(Routinely measured.)(Routinely measured.)––
Minor anions include albumin, phosphate, sulfate Minor anions include albumin, phosphate, sulfate (called unmeasured anions).(called unmeasured anions).
––
Organic acid anions include lactate and Organic acid anions include lactate and acetoacetate,.acetoacetate,.
Anion Gap
Anion gap = Na -
( Cl + HCO3)Normally, 10-14 mEq/L.Greater than 20 = metabolic acidosis definitely present.Normally due to serum proteins with normal negative chargeAbnormal acids (anions) cause elevated anion gap.Decreased anion gap: hypoalbuminemia,myeloma
Anion gap (cont.)
•
Even
if no apparent metabolic acidosis, aniongap > 20 means a metabolic acidosis is present,just hidden by another disorder.
•
If high anion gap
metabolic acidosis only,DHCO3 should approximate the D anion gapExample, HCO3 20 (D=5) and anion gap 19 (D=5).
•
If not, another disorder is altering the HCO3-
in addition to the metabolic acidosis.
Metabolic Alkalosis
increased production or renal retention of HCO3results in a high pH which inhibitsrespiration to increase the pCO2
Metabolic Alkalosis: Causes
decreased Urinary chlorideGut H+ lossVomiting, suctionRenal H+ lossDiuretic therapyContraction alkalosisincreased Urinary chlorideMineralocorticoidExcess Diuretic therapy
Metabolic Alkalosis: Compensation
Problem: increased [HCO3] causing increased blood pH. (low H+)
Low [H+] suppresses respiration which increases blood pCO2
metabolic alkalosis is compensated for by the development of a respiratory acidosis
Metabolic Alkalosis: Compensation
Compensation is complete (pCO2 levels out) in 12-24 hours
The final pCO2 can be calculated from the following equation:pCO2 mmHg = 0.9 X [HCO3] (mmol/L) + 9 (+/-2)
The limit of compensation is a pCO2 of 60
CompensationCompensation
When a primary acidWhen a primary acid--base disorder exists, the base disorder exists, the body attempts to return the pH to normal via body attempts to return the pH to normal via the the ““other halfother half””
of acid base metabolism.of acid base metabolism.
Primary metabolic disorder Primary metabolic disorder Respiratory compensationRespiratory compensation
Primary respiratory disorder Primary respiratory disorder Metabolic compensationMetabolic compensation
Compensation (continued)Compensation (continued)Primary DisorderPrimary Disorder Compensatory MechanismCompensatory Mechanism
Metabolic acidosisMetabolic acidosis Increased ventilationIncreased ventilation
Metabolic alkalosisMetabolic alkalosis Decreased ventilationDecreased ventilation
Respiratory acidosisRespiratory acidosis Increased renal reabsorption of HCOIncreased renal reabsorption of HCO33
--
in the proximal tubulein the proximal tubuleIncreased renal excretion of H in the Increased renal excretion of H in the
distal tubule distal tubule Respiratory alkalosisRespiratory alkalosis Decreased renal reabsorption of HCODecreased renal reabsorption of HCO33
--
in the proximal tubulein the proximal tubuleDecreased renal excretion of HDecreased renal excretion of H++
in the in the
distal tubule distal tubule
CO
MP
EN
SIO
N LIM
ITS
METABLIC ACIDOSISPaCO2 = Up to 10 ?
METABOLIC ALKALOSISPaCO2 = Maximum 6O
RESPIRATORY ACIDOSISBICARB = Maximum 40
RESPIRATORY ALKALOSISBICARB = Up to 10
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The Arterial Blood Gas (ABG)The Arterial Blood Gas (ABG)
pH, pCO2
, pO2
–
Measured directly
HCO3-, O2
saturation (usually) –
Calculated from pH, pCO2
, and pO2
Practical ApproachPractical Approach
1. Check the pH1. Check the pH
If the pH < 7.35, acidemia (and at least 1 acidosis) is present.If the pH < 7.35, acidemia (and at least 1 acidosis) is present.
If the pH > 7.45, If the pH > 7.45, alkalemiaalkalemia
(and at least 1 alkalosis) is present. (and at least 1 alkalosis) is present.
Practical ApproachPractical Approach
2. Check the pCO2. Check the pCO22
pH < 7.35 and pCOpH < 7.35 and pCO22
< 40 < 40 metabolic acidosismetabolic acidosispH < 7.35 and pCOpH < 7.35 and pCO22
> 40 > 40 respiratory acidosisrespiratory acidosis
pH > 7.45 and pCOpH > 7.45 and pCO22
< 40 < 40 respiratory alkalosisrespiratory alkalosispH > 7.45 and pCOpH > 7.45 and pCO22
> 40 > 40 metabolic acidosismetabolic acidosis
Practical ApproachPractical Approach3. Choose the appropriate compensation formula3. Choose the appropriate compensation formula
Most prominent Most prominent disorderdisorder
Compensation formulaCompensation formula
Metabolic acidosisMetabolic acidosis pCOpCO22
≈≈
1.5 [HCO1.5 [HCO33
--] + 8 ] + 8
Metabolic alkalosisMetabolic alkalosis pCOpCO22
≈≈
0.9 [HCO0.9 [HCO33
--] + 9 ] + 9
Respiratory acidosisRespiratory acidosis For every 10 For every 10 ΔΔ
in pCOin pCO22
, pH decreases by:, pH decreases by:0.08 (in acute resp. acidosis)0.08 (in acute resp. acidosis)0.03 (in chronic resp. acidosis) 0.03 (in chronic resp. acidosis)
Respiratory alkalosisRespiratory alkalosis For every 10 For every 10 ΔΔ
in pCOin pCO22
, pH increases by:, pH increases by:0.08 (in acute resp. alkaloses)0.08 (in acute resp. alkaloses)0.03 (in chronic resp. alkalosis) 0.03 (in chronic resp. alkalosis)
Practical ApproachPractical Approach
4. Determine if the degree compensation is 4. Determine if the degree compensation is appropriateappropriate
(If it isn(If it isn’’t, a second acidt, a second acid--base disorder is likely present)base disorder is likely present)
Practical ApproachPractical Approach
5. Calculate the anion gap 5. Calculate the anion gap
The difference between [NaThe difference between [Na++] and the sum of [HC0] and the sum of [HC033
--] and [] and [ClCl-- .].]
[Na[Na++] ] –– ([HC0([HC033 --] + [Cl] + [Cl--]) = ]) =
––
140 140 --
(24 + 105) = 11(24 + 105) = 11Normal = 12 Normal = 12 ++
2 2
Clinicians use the anion gap to identify the cause of Clinicians use the anion gap to identify the cause of metabolic acidosis.metabolic acidosis.
Variations of anion gapVariations of anion gapCauses of decreased anion gap includeCauses of decreased anion gap include
hypoalbuminaemia and severe haemodilution. hypoalbuminaemia and severe haemodilution. Rarer causes include increase in minor cation Rarer causes include increase in minor cation concentrations like calcium and magnesium concentrations like calcium and magnesium
. . Causes of a raised anion gap includeCauses of a raised anion gap include dehydration and any cause of raised dehydration and any cause of raised
unmeasurable anions, like lactate, ketones and unmeasurable anions, like lactate, ketones and renal acids, along with treatment with drugs renal acids, along with treatment with drugs given as organic acids such as penicillin, given as organic acids such as penicillin, salicylates and poisoning with methanol, ethanol salicylates and poisoning with methanol, ethanol and paraldehyde. Rarely it may be due to and paraldehyde. Rarely it may be due to decreased minor cation concentrations such as decreased minor cation concentrations such as calcium or magnesiumcalcium or magnesium
Raised anion gap metabolic Raised anion gap metabolic acidosisacidosis
As documented above, accumulation of a number of As documented above, accumulation of a number of acids can result in raised anion gap metabolic acidosis.acids can result in raised anion gap metabolic acidosis.In such cases, the reduction in serum HCO3In such cases, the reduction in serum HCO3--
matches matches
the anion gap.the anion gap.If not, a second acid base disorder should be kept in If not, a second acid base disorder should be kept in mind. When metabolic acidosis and alkalosis coexist, as mind. When metabolic acidosis and alkalosis coexist, as in vomiting and ketoacidosis, the plasma HCO3in vomiting and ketoacidosis, the plasma HCO3--
may be may be
normal, and a raised anion gap may be the initial normal, and a raised anion gap may be the initial evidence of an underlying acidevidence of an underlying acid--base disturbance (2).base disturbance (2).
To differentiate between the many causes of 'increased To differentiate between the many causes of 'increased anion gap metabolic acidosis', we measure the osmolar anion gap metabolic acidosis', we measure the osmolar gap that is the difference between the measured gap that is the difference between the measured osmolarity and the calculated osmolarityosmolarity and the calculated osmolarity
Normal anion gap Normal anion gap metabolic acidosismetabolic acidosis hyperchloraemichyperchloraemic
This usually results from conditions where in This usually results from conditions where in there is a loss of alkali (i.e.HCO3there is a loss of alkali (i.e.HCO3--) or metabolic ) or metabolic equivalent (eg, excretion of salts of organic equivalent (eg, excretion of salts of organic anions in proportionate excess of chloride) or an anions in proportionate excess of chloride) or an accumulation of HCl or metabolic equivalent (eg, accumulation of HCl or metabolic equivalent (eg, NH4Cl and chloride salts of amino acids) (2). NH4Cl and chloride salts of amino acids) (2). Loss of HCO3Loss of HCO3--
can occur either due to GI can occur either due to GI
causes or due to renal causes (renal excretion causes or due to renal causes (renal excretion or insufficient generation). In many surgical or insufficient generation). In many surgical conditions, the cause is usually obvious. conditions, the cause is usually obvious.
6. Mixed Acid 6. Mixed Acid -- Base DisordersBase Disorders Compensated AppropriatelyCompensated Appropriatelymet acidosismet acidosis
expected pCO2 = 1.5(HCO3) + 8
met alkalosismet alkalosis
expected pCO2 = .9(HCO3) + 9Acute resp acidAcute resp acid
each increase in pCO2 of 1, pH should decr by .008
Acute resp alkAcute resp alk each decrease in pCO2 of 1, pH should incr by .008
Chronic resp acidChronic resp acid
each increase in pCO2 of 1, pH should decr by .003
Chronic resp alkChronic resp alk
each decrease in pCO2 of 1, pH should incr by .003
7.7. The Triple DisorderThe Triple Disorder Calculate the Delta GapCalculate the Delta GapIf If AGAG
acidosis is present: to determine if other acidosis is present: to determine if other
underlying condition presentunderlying condition presentTake the change in AG= (AG Take the change in AG= (AG --
12)12)
Add to HCO3Add to HCO3––
ieie
(AG (AG --
12) + Serum HCO312) + Serum HCO3
if <23if <23
= non gap acidosisif >30if >30
= metabolic alkalosis
Practical ApproachPractical Approach
8. If an elevated gap acidosis is present, calculate 8. If an elevated gap acidosis is present, calculate the deltathe delta--delta ratio, to determine if a second delta ratio, to determine if a second metabolic disorder is present.metabolic disorder is present.
DeltaDelta––Delta = Delta = Measured anion gap Measured anion gap ––
Normal anion gapNormal anion gapNormal [HCONormal [HCO33
--] ] ––
Measured [HCOMeasured [HCO33
--]]
Differential Diagnosis for AcidDifferential Diagnosis for Acid--Base DisordersBase Disorders
Summary of the Approach to Summary of the Approach to ABGsABGs
1.1.
Check the pHCheck the pH2.2.
Check the pCOCheck the pCO22
3.3.
Select the appropriate compensation formulaSelect the appropriate compensation formula4.4.
Determine if compensation is appropriateDetermine if compensation is appropriate
5.5.
Check the anion gapCheck the anion gap6.6.
If the anion gap is elevated, check the deltaIf the anion gap is elevated, check the delta--deltadelta
7.7.
If a metabolic acidosis is present, check urine pHIf a metabolic acidosis is present, check urine pH8.8.
Generate a differential diagnosisGenerate a differential diagnosis
Case 1Case 1
A 26 year old man with unknown past medical history A 26 year old man with unknown past medical history is brought in to the ER by ambulance, after friends is brought in to the ER by ambulance, after friends found him unresponsive in his apartment. He had last found him unresponsive in his apartment. He had last been seen at a party four hours prior.been seen at a party four hours prior.
ABG:ABG:
pH pH 7.257.25
Chem. 7:Chem. 7:
NaNa++
137137PPCOCO22
6060
KK++ 4.54.5
HCOHCO33
--
2626
ClCl-- 100100
PPOO22
55 55 HCOHCO33
--
2525
Case 2Case 2A 67 year old man with diabetes and early diabetic A 67 year old man with diabetes and early diabetic nephropathy (without overt renal failure) presents for a nephropathy (without overt renal failure) presents for a routine clinic visit. He is currently asymptomatic. routine clinic visit. He is currently asymptomatic. Because of some abnormalities on his routine blood Because of some abnormalities on his routine blood chemistries, you elect to send him for an ABG.chemistries, you elect to send him for an ABG.
ABG:ABG:
pH pH 7.357.35
Chem. 7:Chem. 7:
NaNa++
135135PPCOCO22
3434
KK++ 5.15.1
HCOHCO33
--
1818
ClCl-- 110110
PPOO22
92 92 HCOHCO33
--
1616CrCr
1.41.4
Urine pH:Urine pH:
5.05.0
Case 3Case 3
A 68 year old woman with metastatic colon cancer A 68 year old woman with metastatic colon cancer presents to the ER with 1 hour of chest pain and presents to the ER with 1 hour of chest pain and shortness of breath. She has no known previous shortness of breath. She has no known previous cardiac or pulmonary problems.cardiac or pulmonary problems.
ABG:ABG:
pH pH 7.497.49
ChemChem
7:7:
NaNa++
133133PPCOCO22
2828
KK++ 3.93.9
HCOHCO33
--
2121
ClCl-- 102102
PPOO22
5252
HCOHCO33
--
2222
Case 4Case 4
A 6 year old girl with severe gastroenteritis is admitted A 6 year old girl with severe gastroenteritis is admitted to the hospital for fluid rehydration, and is noted to to the hospital for fluid rehydration, and is noted to have a high [HCOhave a high [HCO33
--] on hospital day #2. An ABG is ] on hospital day #2. An ABG is ordered:ordered:
ABG:ABG:
pH pH 7.477.47
ChemChem
7:7:
NaNa++
130130PPCOCO22
4646
KK++ 3.23.2
HCOHCO33
--
3232
ClCl-- 8686
PPOO22
96 96 HCOHCO33
--
3333
Urine pH:Urine pH:
5.85.8
Case 5Case 5A 75 year old man with morbid obesity is sent to the A 75 year old man with morbid obesity is sent to the ER by his skilled nursing facility after he developed a ER by his skilled nursing facility after he developed a fever of 103fever of 103°°
and rigors 2 hours ago. In the ER he is and rigors 2 hours ago. In the ER he is
lucid and states that he feels lucid and states that he feels ““terribleterrible””, but offers no , but offers no localizing symptoms. His ER vitals include a heart rate localizing symptoms. His ER vitals include a heart rate of 115, and a blood pressure of 84/46.of 115, and a blood pressure of 84/46.
ABG:ABG:
pH pH 7.127.12
ChemChem
7:7:
NaNa+ + 138138PPCOCO22
5050
KK++ 4.24.2
HCOHCO33
--
1313
ClCl-- 9999
PPOO22
5252
HCOHCO33
--
1515
Urine pH:Urine pH:
5.05.0
Case 6Case 6
A 25 year old man with type I diabetes presents to the A 25 year old man with type I diabetes presents to the ER with 24 hours of severe nausea, vomiting, and ER with 24 hours of severe nausea, vomiting, and abdominal pain.abdominal pain.
ABG: pH ABG: pH 7.157.15
ChemChem
7:7:
NaNa++
138138PPCOCO22
3030
KK++ 5.65.6
HCOHCO33
--
1010
ClCl--
8888PPOO22
8888
HCOHCO33
--
1111CrCr
1.11.1
Urine pH:Urine pH:
5.05.0
Case 7Case 7A 62 year old woman with severe COPD comes to the A 62 year old woman with severe COPD comes to the ER complaining of increased cough and shortness of ER complaining of increased cough and shortness of breath for the past 12 hours. There are no baseline breath for the past 12 hours. There are no baseline ABGsABGs
to compare to, however, her HCOto compare to, however, her HCO33
--
measured measured during a routine clinic visit 3 months ago was 34 during a routine clinic visit 3 months ago was 34 mEqmEq/L./L.
ABG:ABG:
pH pH 7.217.21
ChemChem
7:7:
NaNa++
135135PPCOCO22
8585
KK++ 4.04.0
HCOHCO33
--
3333
ClCl-- 9090
PPOO22
47 47 HCOHCO33
--
3434
Urine pHUrine pH
5.55.5
Case 8Case 8
A 36 year old man with a history of alcoholism is A 36 year old man with a history of alcoholism is brought to the ER after being found on the floor of his brought to the ER after being found on the floor of his apartment unresponsive, soiled with vomit, and with an apartment unresponsive, soiled with vomit, and with an empty pill bottle nearby.empty pill bottle nearby.
ABG:ABG:
pH pH 7.037.03
ChemChem
7:7:
NaNa++
134134PPCOCO22
7575
KK++ 5.25.2
HCOHCO33
--
1919
ClCl--
9090PPOO22
48 48 HCOHCO33
--
2020
Urine pHUrine pH
5.05.0