2. Acid base disorders
• Acid–base homeostasis is fundamental for maintaining life
• The hydrogen-ion concentration is tightly regulated because
changes in hydrogen ions alter virtually all protein and
membrane functions.
• The three major methods of quantifying acid–base disorders are
– The physiological approach-isohydric principle
– The base-excess approach
– The physicochemical approach –Stewart method
3. Normal components of ABG report
Parameters Normal range
pH 7.35 – 7.45
PaCO2 35 – 45
PaO2 80- 100
HCO3 22-26
SaO2 >95%
4. Check for ERRORS
Have the required parameters been correctly
fed..???
Patient’s Temperature
Fi O₂ : specially if patient is in ventilator
Hemoglobin : some machines may not measure it
Barometric pressure : some machines may not measure it
6. Step 1:Assessment of validity of test
results
• Assess the internal consistency of the values using the Henderseon-
Hasselbach equation
[H+] in nmol/L = 24 × PaCO₂/HCO₃
• If there is a discripancy between the 2 results, the blood should be
reanalyzed.
• HCO3 should be within 1-3 mEq/L of Total CO2 (electrolyte). A
difference of > 4 mEq/L = technical error
7. Step 1: Assessment of validity of test results
Relation b/w pH & H+ conc.
pH [H+] in nanomoles/L
7.00 100
7.10 80
7.30 50
7.40 40
7.52 30
7.70 20
8.00 10
pH is inversely related to [H+]; a pH change of
1.00 represents a 10-fold change in [H+]
8. STEP -2: Acidemia or alkalemia..???
See the pH (<7.35 or >7.45)
Acidemia –pH less than 7.35
Acidosis – A process that would cause acidemia, if not
compensated
Alkalemia–pH greater than 7.45
Alkalosis – A process that would cause alkalemia if not
compensated
10. STEP -3 : Identify the primary disorder
See the change in PaCo2 & HCO3
-
If the PaCo2 is deranged in the same direction of pH then the primary
disorder is metabolic
If the PaCo2 is deranged in the opposite direction of pH then the
primary disorder is respiratory
pH PaCo2 HCO3
7.25 60 26
Respiratory
acidosis
11. Step 4: COMPENSATION
It is secondary adaptive response to mitigate the change in
arterial pH – so acid base homeostasis is maintained
Compensation doesnot return the pH to complete normal and
never over compensate.
Resp. compensation occurs in hours but Full renal compensation
takes 2-5 days
If given patient is not compensating as predicted, then second (or
third) acid base disorder must be present
12. Prediction of compensation
Metabolic acidosis PaCO2= (1.5 x HCO3
-) + 8 ± 2
Metabolic alkalosis
Pawill↑ 0.75 mmHg per mmol/L ↑ in
CO2 [HCO-] or
3
PaCO2= 40 + {0.7(HCO3
- - 24)}
Respiratory
acidosis
Acute
[HCO3
-] will ↑ 1 mmol/L per 10 mmHg
in PaCO2
Chronic
[HCO3
-] will ↑ 4 mmol/L per 10 mmHg
in PaCO2
Respiratory
alkalosis
Acute
[HCO3
-] will ↓ 2 mmol/L per 10 mmHg
↓ in PaCO2
Chronic
[HCO3
-] will ↓ 4 mmol/L per 10 mmHg
↓in PaCO2
15. STEP -5 : Calculate anion gap
Calculation of the anion gap is useful in the initial evaluation of
metabolic acidosis.
An elevated anion gap usually indicates the production of pathologic
acid (unmesured anion).
Total Serum Cations = Total SerumAnions
Unmeasured anions- unmeasured cations= Na+] – {[Cl-]+[HCO3-]}
Anion gap = [Na+] - [Cl-]-[HCO3-]
Up to 12 is normal anion gap
16. • Albumin is the major unmeasured anion
• The anion gap should be corrected if there are gross changes in
serum albumin levels.
AG (CORRECTED) = AG + { (4 – [ALBUMIN]) × 2.5}
17. Causes of High AG Met Acidosis
• A useful mnemonic for the most common causes is GOLD
Cowen –Woods classification of lactic acidosis
MARRK
Type A - hypoxic Type B — nonhypoxic
(septic shock, mesenteric
ischemia, hypoxemia,
hypovolemic shock, carbon
monoxide
poisoning, cyanide)
G - Ethylene Glycol
O - 5-oxoproline [pyroglutamic acid]
L -Lactic Acidosis – metformin ?
D – d lactate – bacterial overgrowth syndrome
M – Methanol
A- Aspirin
R- Renal Failure
R- Rhabdomyolsis
K - Ketoacidosis:
B1 – 2nd to
Hepatic failure
Renal failure
malignancy
B2:
Thiamine def, seizure
Toxins - salicylate, ethylene
glycol, propylene glycol,
methanol, paraldehyde
Drugs - metformin, propofol,
niacin, isoniazid, iron or NNRTI
B3 – inherited syndromes
18. CAUSES OF NORMAL ANION GAP
METABOLIC ACIDOSIS
Primary issue GI tract Renal
Gain of H+ Hyperalimentation Distal (type 1) RTA
1. HCO3 loss:
GIT
Diarrhoea
Pancreatic or biliary
drainage
Urinary diversions
(ureterosigmoidostomy)
Renal Proximal (type 2) RTA
Ketoacidosis (during therapy)
Post-chronic hypocapnia
2. Impaired renal acid
excretion:
Hyperkalemia (type4)
Distal (type 1) RTA
Hyperkalemia (type 4)
Hypoaldosteronism
Early uremic acidosis
RTA
RTA
Hypoaldosteronism
Early uremic acidosis
3. Misc:
Acid Administration
(NH4Cl)
Hyperalimentation
Loss of HCO3 Diarrhoea
Pancreatic or biliary
drainage
Urinary diversions
(ureterosigmoidostomy)
Cholestyramine
Renal Proximal (type
2) RTA
Infusion of normal saline
19. NORMAL ANION GAP METABOLIC ACIDOSIS
- corresponds with an
• It occurs when the decrease in HCO3
increase in Cl- to retain electroneutrality - hyperchloremic
metabolic acidosis.
• Leads to increased renal excretion of NH4.
• Measurement of urinary NH4 can be used to differentiate
between renal and extrarenal causes.
• Urinary anion gap and urinary osmolal gap are often used as
surrogate measures of urinary ammonium.
20. NORMAL ANION GAP METABOLIC ACIDOSIS
• UAG
= [Na+ + K+]u – [Cl–]u
• Hence a -ve UAG seen in GI causes while +ve value seen in renal causes
• The urinary osmolal gap
= (2 × [Na+] + 2 × [K+]) + (urine urea nitrogen ÷ 2.8) + (urine glucose ÷ 18)
• Osmolal gap below 40 mmol/L indicates renal cause
• Urine pH
– If urine pH > 5.5 : Type 1 RTA
– If urine pH < 5.5 : Type 2 or Type 4 RTA
21. Approach to normal anion gap metabolic acidosis
yes
In patients receiving saline
infusion, stop & switch to RL
Did acidosis resolve
no
Excess NaCl
Is GFR < 40 Renal failure
yes
Neg UAG high UAG pH>5.5 K very high
Hyperkalemia
(type4) RTA
Asses serum K, UAG & U.pH
Distal RTA
Diarrhea
Pancreatic drainage
Urinary diversions
22. STEP -6 : Calculate the delta gap/ delta ratio
To diagnose a high anion-gap acidosis with concomitant metabolic
alkalosis or normal anion-gap acidosis
Delta gap =(measuredAG- normAG) – (norm.HCO3 – measuredHCO3)
±±66A +=G2 ( 4+ =AH (GC)O A-3G2=)4 ++18HH CC-3OO033))
Delta gap= ( AG) - (24 –
measuredHCO3)
= ( AG) - ( HCO3
- )
Usual range: -6 to +6 mmol/L ; should be 0
> 6 mmol/l - concomitant metabolic alkalosis,.
< −6 mmol/l - concomitant normal anion-gap metabolic acidosis
23. Easier alternative
Result
( AG + HCO3)
Metabolic disorder
< 18 High anion gap + normal anion gap
metabolic acidosis
18- 30 High anion gap acidosis only
>30 High anion gap acidosis + metabolic
alkalosis
24. Delta ratio
• It is calculation that compares the increase in anion gap to the
decrease in HCO3
Delta ratio = ( AG) / ( HCO3
- )
• Delta ratio depends on cause of elevated anion gap
Pathologic process Expected delta ratio
Lactic acidosis 1-2
ketoacidosis 0.8 - 1.2
25. Delta ratio
Delta ratio Metabolic disorder
Less than expected range High anion gap + normal anion gap
metabolic acidosis
Within expected range High anion gap acidosis only
Higher than expected
range
High anion gap acidosis +
metabolic alkalosis
29. METABOLIC ALKALOSIS
Assess volume status Low
normal
Asses BP and S.
potassium
Contraction alkalosis
Vomitting
NG suction
Diuretics
Gitelman, Bartter ,
Exogenous alkali
milk alkali syndrome
Mineralocorticoid
excess
hypokalemia
High BP
Normal BP n K
30. Algorithm for assessing acid base status
STEP -1 :check for validity
STEP -2 : Acidosis or alkalosis..???
See the pH (<7.35 or >7.45)
STEP -3 : Identify the primary disorder
See the change in PCo2 & pH
STEP -4 : Calculate the compensatory response
Is adequately compensated???
31. STEP -5 : Calculate anion gap
STEP -6 : Calculate the delta gap (unmask hidden mixed
disorders)
STEP -7 : Acquire additional relevant diagnostic data for each
identified disorder and generate differential diagnosis.
32. Case 1
• A 75 yr old woman presents with profuse diarrhea and fever her HR –
130 n BP is 60/40
• Step 1: Check validity- H+ = 24 (30/14) =51
(7.30 -50)
• Step 2: check pH = acidemia
• Step 3: check Paco 2 <40 metabolic acidosis
• Step 4: expected comp. Paco2 = (1.5 x HCO3
pH 7.29 Na 128
PCO₂ 30 K 3.2
HCO₃ 14 Cl 94
-) + 8 ± 2 = 29 ± 2
appropriate resp. comp
• Step 5: calculate anion gap = Na – HCO3 – Cl- = 128-94-14= 20
high anion gap met. Acidosis
High anion gap metabolic acidosis + normal
anion gap metabolic acidosis
• Step 6: delta ratio = ( AG) / ( HCO3
- ) =(20-12)/10 = 0.8.
33. Case 2
pH 6.96 Na 132
PCO₂ 60 K 3.4
HCO₃ 12 Cl 95
BUN 24 Glu 74
Alb 1.9
Measured Osm= 310
• A 32 yr old woman with schizophrenia found unconscious and her HR
– 130 n BP is 104/70, SaO2 - 88% on RA
Lactate 0.8mmol/l
ketones negative
s.creat 1.1
• Step 1: Check validity- H+ = 24 (60/13) = 110 (6.95 -112)
• Step 2: check pH = acidemia
• Step 3: check Paco 2 >40 respiratory acidosis
• Step 4: expected comp. HCO3 = ↑ 1 mmol/L per 10 mmHg in PaCO2
no. Comp. Metabolic alkalosis.
• Step 5: calculate anion gap = Na – HCO3 – Cl-= 132-95-12= 25
adjusted anion gap =25 + 2.5(4-alb)=30 high anion gap met. Acidosis
Presumed ingestion of toxic alcohol leading
to high anion gap metabolic acidosis and resp
acidosis. Cannot rule out ingestion of
additional resp depressant
• Step 6: delta ratio = ( AG) / ( HCO3
- ) =(30-12)/12 = 1.5
• Calculate Plasma Osmolarity = 2(132) +24/2.8+74/18= 277
34. Case 3
• A 14 yr old girl with bulimia was brought to ER after bieng found
unconscious at her home with empty drug bottle nearby.
pH 7.39 Na 139
• Step 1:
• Step 2: check pH = normal
• Step 3: check Paco 2 <22 resp.alkalosis
• Step 4: calculate comp. 2nd – met.acidosis
• Step 5: calculate anion gap = Na – HCO3 – Cl- = 139-88-13= 38
high anion gap met. Acidosis
• Step 6: delta ratio = ( AG) / ( HCO3
PCO₂ 22 K 3.1
HCO₃ 13 Cl 88
- ) =(38-12)/(24-13) = 2.2
High anion gap metabolic acidosis +
metabolic alkalosis + resp. alkalosis
37. A-a gradient
A-a gradient = PAO2 – PaO2
PAO2 is always calculated based on FIO2, PaCO2, and barometric
pressure. - alveolar gas equation.
38. Alveolar Gas Equation
• Where PAO2 is the average alveolar PO2, and FIO2 is the partial pressure of
inspired oxygen in the trachea
•
PPPAAAOOO222===1((7P560a0 t–m-41-7.42)7x5)0(xP.2Fa1IC O-O2P 2a-)CPOaC2/O0.28/RQ
• Normal A- a gradient increase with age
Normal A- a gradient = (age/4) +4
39. A-a gradient in hypoxic patient
• If A- a gradient is normal
– Hypoventilation
– Low PI (extreme hight)
• If A- a gradient is elevated
– Shunt
– V/Q mismatch
– Imapaired diffusion
40. PaO2 / FIO2 Ratio
• Measure of severity of hypoxemia in ARDS
– Mild 200 – 300
– Moderate 100- 200
– Severe < 100
41. Saturation gap
• Saturation gap = [ SpO2 - Sa O2]
• > 5% is significant.
• Causes: methemoglobinemia
carboxyhemoglobinemia
42. Example 1
• 83 yr old woman with dementia was sent ER after she was found
tachypnic and hypoxic. She is in resp distress. Her ABG reads
pH – 7.53, PCO2- 26, PaO2- 41.
• check A-a gradient PAO2=(Patm-47)xFIO2 - PaCO2/RQ
PAO2=150 - 26/0.8 = 118
A-a gradient = PAO2 - PaO2
= 118 – 41 = 77
• Estimate normal A-a gradient = (age/4) +4 =83/4 +4 =25
43. Example 2
• A 22 yr old young male who works in printing press
presented to RML emergency with one day history of
confusional state, headache and slurring of speech. On
examination he appeared cyanosed, SpO2 -87% and ABG
revealed -
pH PaO2 SaO2
7.48 140 99
45. Learning objectives
Introduction
Types of spirometry
Understand the meaning of spirometric indices and flow
volume loop
How to use these values for diagnostic evaluations
Severity of disease based on FEV1
46. Spirometry
• Method of assessing lung function by measuring the volume of
air that the patient is able to expel from the lungs after a maximal
inspiration.
• It is a reliable method of differentiating between obstructive
airways disorders and restrictive diseases.
• Spirometry is the most effective way of determining the severity
of COPD.
47. Indications
• Diagnosis of symptomatic disease
– Obstructive
– Restrictive
– Mixed
• Screening for early asymptomatic disease
• Prognostication
• Monitor response to treatment
48. Technologies used in spirometers
• Volumetric Spirometers
– Water bell
– Bellows wedge
• Flow measuring Spirometers
50. Spirograms
• Most spirometers display the following graphs
a volume-time curve, showing volume (liters) along the Y-axis
and time (seconds) along the X-axis
a flow-volume loop, which graphically depicts the rate of airflow
on the Y-axis and the total volume inspired or expired on the X-axis
51. Volume-time curve
flow = volume / time
Maximum slope of curve = peak expiratory flow rate
52. Spirometry indices
• FVC – the total volume of air that the patient can forcibly exhale
in one breath after maximal inspiration.
• FEV1 – the volume of air that the patient is able to exhale in the
first second of forced expiration.
• FEV1 /FVC – the ratio of FEV1 to FVC expressed as a fraction
(previously this was expressed as a percentage).
• MEF25-75 This is the mid expiratory flow rate between 25-75%
of an expired air .
56. Patterns of Spirometric Curves
Interpretation FVC FEV1 FEV1/FVC%
(Tiffeneau index)
Healthy person Normal
(>80%)
Normal
(>80%)
Normal
(>0.7)
Airway
obstruction
Low/normal Low Low
Restrictive Low Low/ normal Normal/
increased(>0.7)
Mixed Low Low Low
57. Interpretation
Asses FEV- 1/ FVC ratio
low normal
Asses FVC
Asses FVC
Low normal low normal
Normal lung
mechanics
Possible
restriction
Obstruction
Obstruction/
mixed
62. Staging of COPD based on FEV1
GOLD staging FEV1 compared to
predicted
Stage 1 > 80%
Stage 2 50% < FEV1 <80%
Stage 3 30% < FEV1 <50%
Stage 4 <30%
63. Bronchodilator Reversibility
• Administer salbutamol in four separate doses of 100 μg through
a spacer
• FEV1/FVC should be measured before and 15-20 minutes after
bronchodilator
• An increase in FEV1 and/or FVC >12% of control and >200 mL
constitutes a positive bronchodilator response.
• It is important to determine whether fixed airway narrowing is
present. In patients with COPD, post-bronchodilator FEV1/FVC
remains < 0.7.
64. Limitations of test
• Highly dependent on patient cooperation and effort, - FVC may
be underestimated
• Not suitable for unconscious, heavily sedated, or have limitations
that would interfere with vigorous respiratory efforts.
• Many intermittent or mild asthmatics have normal spirometry
between acute exacerbation
• Normal results in pulmonary vascular disorders
70. Case 2 interpretation
Flow volume loop: decreased PEFR and coving of 2nd phase of
exp loop
Decreased FEV1 ,FVC & FEV1/FVC moderate airflow
obstruction
BD response
Dx: obstructive disease
71. Case 3
32 year-old animal trainer presents With progressive
dyspnea and dry cough over last 2 months.
RR – 28, sa02 – 88% on RA,
RS - fine B/l basal crepts.
73. Case 4
25 year-old man With dyspnea and wheezing Non smoker History of mtor
vehicle accident , hospitalization and tracheostomy 2 years ago
His flow volume loops is as follows:
77. • ABG and spirometry are very useful diagnostic tools for our day
to day practice.
• Approach to interpret should be step wise & in a systematic
manner.
• Any abnormal result should be analyzed cautiously in light of
clinical context.
• Appropriate use of these tools using clinical judgment is of
paramount importance
78. Bibliography
• HARRISON’S principles of internal medicine, 18th edition.
• Disorders of Fluids and Electrolytes, Julie R. Ingelfinger, M.D.,
NEJM, 0ct-9, 2014.
• Spirometry for health care providers, GOLD, 2010.
• ‘‘ATS/ERS TASK FORCE: STANDARDISATION OF LUNG
FUNCTION TESTING’’ V. Brusasco, R. Crapo and G. Viegi,
Eur Respir J 2005; 26: 948–968
• Vijayan ; Spirometry in South Indian children Indian J Chest Dis
Allied Sci 2000; 42: 147–156
79.
80.
81. Case
• A 56 yr old woman with copd presents with shortness of breath since
3hr. Her HR – 130 n BP is 110/70, SaO2-90%
• Step 1: Check validity- H+ = 24 (48/36) =32
(7.50 -30)
pH 7.50 Na 138
PCO₂ 48 K 3.2
HCO₃ 36 Cl 92
• Step 2: check pH = alkalemia
• Step 3: check Paco 2 <40 metabolic alkalemia
• Step 4: expected comp. Paco2 = 40+{0.7(36-24)=48
• appropriate resp. comp
• Step 5: calculate anion gap = Na – HCO3 – Cl- = 138-92-36= 10
Post hypocapnic metabolic alkalosis
