Interpretation of ABGs and

Spirometry

Presenter - Dr. Shivashankar Swamy

PGIMER, Delhi

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

Normal components of ABG report

Parameters Normal range

pH 7.35 – 7.45

PaCO2 35 – 45

PaO2 80- 100

HCO3 22-26

SaO2 >95%

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

A Stepwise

Approach to

Solving

Acid-Base

Disorders

Step 1:Assessment of validity of test

results

• Assess the internal consistency of the values using the Henderseon-

Hasselbach equation

• 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

[H+] in nmol/L = 24 × PaCO₂/HCO₃

pH is inversely related to [H+]; a pH change of

1.00 represents a 10-fold change in [H+]

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

Relation b/w pH & H+ conc.

Step 1: Assessment of validity of test results

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

Four primary acid-base disorders

Metabolic acidosis

Metabolic alkalosis

Respiratory acidosis

Respiratory alkalosis

STEP -3 : Identify the primary disorderSee 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

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

Prediction of compensation

Metabolic acidosis PaCO2= (1.5 x HCO3-) + 8 ± 2

Metabolic alkalosis

PaCO2 will↑ 0.75 mmHg per mmol/L ↑ in

[HCO3-] or

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

Example 1

• Step 1: Check validity- H+ = 24 (48/34) = 33.8 (7.50 -32)

• Step 2: check pH = alkalemia

• Step 3: check Paco 2 >40 metabolic alkalosis

• Step 4: expected comp. Paco2 = 40 + {0.7(HCO3- -24)}

= 40 + 0.7 (10) = 47

• Appropriate resp. compensation

pH Paco2 HCO3

7.50 48 34

Example 2

• Step 1: Check validity- H+ = 24 (32/10) = 76.8 (7.10 - 79)

• Step 2: check pH = acedemia

• Step 3: check Paco 2 <40 metabolic acidosis

• Step 4: expected comp. PaCO2= (1.5 x HCO3-) + 8 ± 2

= (1.5 x 10) +8 ± 2 = 23 ± 2

• Inappropriate resp. compensation ( 32 ≠ 23)

• PaCO2 is higher than predicted so 2° disorder is resp acidosis

pH Paco2 HCO3

7.12 32 10

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 Serum Anions

Unmeasured anions- unmeasured cations= Na+] – {[Cl-]+[HCO3-]}

Anion gap = [Na+] - [Cl-]-[HCO3-]

Up to 12 is normal anion gap

• 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}

Causes of High AG Met Acidosis

• A useful mnemonic for the most common causes is GOLD

MARRK

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:

Cowen – Woods classification of lactic acidosis

Type A - hypoxic Type B — nonhypoxic

(septic shock, mesenteric

ischemia, hypoxemia,

hypovolemic shock, carbon

monoxide

poisoning, cyanide)

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

CAUSES OF NORMAL ANION GAP

METABOLIC ACIDOSIS

1. HCO3 loss:

GIT

Diarrhoea

Pancreatic or biliarydrainage

Urinary diversions (ureterosigmoidostomy)

Renal Proximal (type 2) RTA

Ketoacidosis (during therapy)

Post-chronic hypocapnia

2. Impaired renal acid excretion:

Distal (type 1) RTA

Hyperkalemia (type 4) RTA

Hypoaldosteronism

Early uremic acidosis

3. Misc:

Acid Administration (NH4Cl)

Hyperalimentation

Primary issue GI tract Renal

Gain of H+ Hyperalimentation Distal (type 1) RTAHyperkalemia (type4)

RTAHypoaldosteronismEarly uremic acidosis

Loss of HCO3 Diarrhoea Pancreatic or biliarydrainage Urinary diversions (ureterosigmoidostomy)Cholestyramine

Renal Proximal (type

2) RTA

Infusion of normal saline

NORMAL ANION GAP METABOLIC ACIDOSIS

• It occurs when the decrease in HCO3- corresponds with an

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.

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

Approach to normal anion gap metabolic acidosis

yes

no

yes

Neg UAG high UAG pH>5.5 K very high

In patients receiving saline infusion, stop & switch to RL

Did acidosis resolve

Is GFR < 40 Renal failure

Excess NaCl

Hyperkalemia

(type4) RTA

Asses serum K, UAG & U.pH

Distal RTA Diarrhea Pancreatic drainage Urinary diversions

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)

= (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

Delta gap= ( AG) - (24 –measuredHCO3)

±6 = ( AG) - 24 + HCO3) ±6 +24 = ( AG) + HCO3) AG + HCO3 = 18 -30

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

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

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

PLASMA OSMOLAR GAP

Calculated Plasma Osmolarity = 2[Na+] + [Gluc]/18 + [BUN]/2.8

Normal Measured Plasma Osmolarity > Calculated Plasma Osmolarity

(upto 10 mOsm/L)

Measured Plasma Osmolarity - Calculated Plasma Osmolarity > 10

mOsm/kg indicates presence of abnormal osmotically active substance

Ethanol

Methanol

Ethylene glycol

METABOLIC

ALKALOSIS

CAUSES OF METABOLIC ALKALOSIS

1. HCO3 loss:

GIT

Diarrhoea

Pancreatic or biliarydrainage

Urinary diversions (ureterosigmoidostomy)

Renal Proximal (type 2) RTA

Ketoacidosis (during therapy)

Post-chronic hypocapnia

2. Impaired renal acid excretion:

Distal (type 1) RTA

Hyperkalemia (type 4) RTA

Hypoaldosteronism

Early uremic acidosis

3. Misc:

Acid Administration (NH4Cl)

Hyperalimentation

Primary issue GI tract Renal

Loss of H+ Vomitting

Gastric aspiration

Congenital chloridorrhea

Villous adenoma

DiureticsGitelmanBartter Mineralocorticoid

excess

Gain of HCO3 Milk alkali syndromeIngestion of NaHCO3

Contraction alkalosis

METABOLIC ALKALOSIS

Assess volume status Low

Asses BP and S. potassium

Contraction alkalosisVomittingNG suctionDiureticsGitelman, Bartter ,

Exogenous alkalimilk alkali syndrome

Mineralocorticoidexcess

hypokalemia

normal

High BPNormal BP n K

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???

Algorithm for assessing acid base status

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.

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-) + 8 ± 2 = 29 ± 2

appropriate resp. comp

• Step 5: calculate anion gap = Na – HCO3 – Cl- = 128-94-14= 20

high anion gap met. Acidosis

• Step 6: delta ratio = (AG) / ( HCO3- ) =(20-12)/10 = 0.8.

pH 7.29 Na 128

PCO₂ 30 K 3.2

HCO₃ 14 Cl 94

High anion gap metabolic acidosis + normal

anion gap metabolic acidosis

Case 2

• A 32 yr old woman with schizophrenia found unconscious and her HR

– 130 n BP is 104/70, SaO2 - 88% on RA

• 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

• Step 6: delta ratio = (AG) / ( HCO3- ) =(30-12)/12 = 1.5

• Calculate Plasma Osmolarity = 2(132) +24/2.8+74/18= 277

pH 6.96 Na 132

PCO₂ 60 K 3.4

HCO₃ 12 Cl 95

BUN 24 Glu 74

Alb 1.9

Lactate 0.8mmol/l

ketones negative

s.creat 1.1

Measured Osm= 310

Presumed ingestion of toxic alcohol leading

to high anion gap metabolic acidosis and resp

acidosis. Cannot rule out ingestion of

additional resp depressant

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.

• 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- ) =(38-12)/(24-13) = 2.2

pH 7.39 Na 139

PCO₂ 22 K 3.1

HCO₃ 13 Cl 88

High anion gap metabolic acidosis +

metabolic alkalosis + resp. alkalosis

Analyse the adequacy of oxygenation..

• Causes of hypoxia

– Hypoxemia

– Anemia

– Dyshemoglobenemia

– Histotoxic hypoxia

A-a gradient

PAO2 is always calculated based on FIO2, PaCO2, and barometric

pressure. - alveolar gas equation.

A-a gradient = PAO 2 – PaO2

Alveolar Gas Equation

• Where PAO2 is the average alveolar PO2, and FIO2 is the partial pressure of

inspired oxygen in the trachea

•

• Normal A- a gradient increase with age

PAO2=150 – 1.25(PaCO2)PAO2=(760-47)x0.21 - PaCO2/0.8PAO2=(Patm-47)xFIO2 - PaCO2/RQ

Normal A- a gradient = (age/4) +4

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

PaO2 / FIO2 Ratio

• Measure of severity of hypoxemia in ARDS

– Mild 200 – 300

– Moderate 100- 200

– Severe < 100

Saturation gap

• Saturation gap = [ SpO2 - Sa O2]

• > 5% is significant.

• Causes: methemoglobinemia

carboxyhemoglobinemia

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

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

Spirometry

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

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.

Indications

• Diagnosis of symptomatic disease

– Obstructive

– Restrictive

– Mixed

• Screening for early asymptomatic disease

• Prognostication

• Monitor response to treatment

Technologies used in spirometers

• Volumetric Spirometers

– Water bell

– Bellows wedge

• Flow measuring Spirometers

Types of spirometer

Pneumotachometer

Fully electronic spirometer

Incentive spirometer

Tilt-compensated spirometer

Windmill-type spirometer

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

Volume-time curve

flow = volume / timeMaximum slope of curve = peak expiratory flow rate

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 .

Flow volume loop

FVC

PEFR

Values measured by spirometry

Major

• FEV1

• FVC

• FEV1/FVC ratio

• Flow- volume loop

Minor

• PEFR

• PEF 25-75%

• Response to Bronchodilators

INTERPRETATION

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

Interpretation

low normal

Low normal low normal

Asses FEV- 1/ FVC ratio

Asses FVCAsses FVC

Normal lung mechanics

Possible restriction

Obstruction

Obstruction/ mixed

Fixed Airway obstruction

Variable extrathoracicAirway obstruction

Variable intrathoracicAirway obstruction

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%

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.

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

goldcopd.com

case 1

Pre-Bronchodilator (BD) Post- BD

Test Actual Predicted % Predicted % Change

FVC (L) 4.39 4.32 102 -1

FEV1 (L) 3.20 3.37 95 7

FEV1/FVC

(%)

73 78

FRC (L) 3.17 3.25 98

ERV (L) 0.63 0.93 68

RV (L) 2.54 2.32 109

TLC (L) 6.86 6.09 113

DLCO uncorr 25.69 31.28 82

DLCO corr 26.14 31.28 84

65 year-old man No pulmonary complaints PFT as part of a routine

health screening test Lifelong non-smoker Prior history of asbestose

exposure

His flow volume loops is as follows::

34 year – old woman With dyspnea &cough Non-smoker,with no occupational

exposures.

Case 2

Pre-Bronchodilator (BD) Post- BD

Test Actual Predicted %

Predicted

Actual %

Change

FVC (L) 3.19 4.22 76 4.00 25

FEV1 (L) 2.18 3.39 64 2.83 30

FEV1/FVC

(%)

68 80 78 4

PFT report

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

Case 2 interpretation

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.

Case 3

Pre-Bronchodilator (BD) Post- BD

Test Actual Predicted %

Predicted

Actual %

Change

FVC (L) 1.7 4.4 39 1.7

FEV1 (L) 1.6 3.7 43 1.6

FEV1/FVC

(%)

94 84 94

RV (L) 0.7 1.4 50

TLC (L) 2.5 5.7 44

RV/TLC

(%)

76 37

DLCO corr 20.73 33.43 62

PFT report

25 year-old man With dyspnea and wheezing Non smoker History of mtor

vehicle accident , hospitalization and tracheostomy 2 years ago

Case 4

His flow volume loops is as follows:

Pre-Bronchodilator (BD)

Test Actual Predicted % Predicted

FVC (L) 4.73 4.35 109

FEV1 (L) 2.56 3.69 69

FEV1/FVC (%) 54 85

PFT report

Case 4 interpretation

Flow volume loop: Flattened inspiratory &expiratory

limb

Decreased FEV1 , FEV1/FVC moderate

obstruction

Dx: Fixed UAWO

Take home messages…

• 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

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

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)

• 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

pH 7.50 Na 138

PCO₂ 48 K 3.2

HCO₃ 36 Cl 92

Post hypocapnic metabolic alkalosis