2. Lung Volumes and Capacities
PFT tracings have:
Four Lung volumes: tidal
volume, inspiratory reserve
volume, expiratory reserve
volume, and residual
volume
Five capacities:, inspiratory
capacity, expiratory
capacity, vital capacity,
functional residual capacity,
and total lung capacity
Addition of 2 or more volumes comprise a capacity.
3. Lung Volumes
• Tidal Volume (TV): volume of air
inhaled or exhaled with each
breath during quiet breathing (6-
8 ml/kg)
• Inspiratory Reserve Volume
(IRV): maximum volume of air
inhaled from the end-inspiratory
tidal position.(1900-3300ml)
• Expiratory Reserve Volume
(ERV): maximum volume of air
that can be exhaled from resting
end-expiratory tidal position.(
700-1000ml).
4. Lung Volumes
• Residual Volume (RV):
– Volume of air remaining
in lungs after maximium
exhalation (20-25 ml/kg)
(1700-2100ml)
– Indirectly measured
(FRC-ERV)
– It can not be measured
by spirometry
5. Lung Capacities
• Total Lung Capacity (TLC): Sum of
all volume compartments or
volume of air in lungs after
maximum inspiration (4-6 L)
• Vital Capacity (VC): TLC minus RV
or maximum volume of air exhaled
from maximal inspiratory level.
(60-70 ml/kg) (3100-4800ml)
• Inspiratory Capacity (IC): Sum of
IRV and TV or the maximum
volume of air that can be inhaled
from the end-expiratory tidal
position. (2400-3800ml).
• Expiratory Capacity (EC): TV+ ERV
6. Lung Capacities (cont.)
• Functional Residual
Capacity (FRC):
– Sum of RV and ERV or the
volume of air in the lungs at
end-expiratory tidal
position.(30-35 ml/kg) (2300-
3300ml).
– Measured with multiple-
breath closed-circuit helium
dilution, multiple-breath
open-circuit nitrogen
washout, or body
plethysmography.
– It can not be measured by
spirometry)
7. VOLUMES, CAPACITIES AND THEIR
CLINICAL SIGNIFICANCE
1) TIDAL VOLUME (TV):
VOLUME OF AIR INHALED/EXHALED IN EACH BREATH
DURING QUIET RESPIRATION.
N – 6-8 ml/kg.
TV FALLS WITH DECREASE IN COMPLIANCE, DECREASED
VENTILATORY MUSCLE STRENGTH.
2) INSPIRATORY RESERVE VOLUME (IRV):
MAX. VOL. OF AIR WHICH CAN BE INSPIRED AFTER A
NORMAL TIDAL INSPIRATION i.e. FROM END INSPIRATION PT.
N- 1900 ml- 3300 ml.
8. CONTINUED………
3) EXPIRATORY RESERVE VOLUME (ERV):
MAX. VOLUME OF AIR WHICH CAN BE EXPIRED AFTER A
NORMAL TIDAL EXPIRATION i.e. FROM END EXPIRATION
PT.
N- 700 ml – 1000 ml
4) INSPIRATORY CAPACITY (IC) :
MAX. VOL. OF AIR WHICH CAN BE INSPIRED AFTER A
NORMAL TIDAL EXPIRATION.
IC = IRV + TV
N-2400 ml – 3800 ml.
9. CONTINUED………..
4) VITAL CAPACITY: COINED BY JOHN
HUTCHINSON.
MAX. VOL. OF AIR EXPIRED AFTER A MAX.
INSPIRATION .
MEASURED WITH VITALOGRAPH
VC= TV+ERV+IRV
N- 3.1-4.8L. OR 60-70 ml/kg
VC IS COSIDERED ABNORMAL IF ≤ 80% OF
PREDICTED VALUE
10. FACTORS INFLUENCING VC
• PHYSIOLOGICAL :
physical dimensions- directly proportional to ht.
SEX – more in males : large chest size, more
muscle power, more BSA.
AGE – decreases with increasing age
STRENGTH OF RESPIRATORY MUSCLES
POSTURE – decreases in supine position
PREGNANCY- unchanged or increases by 10% (
increase in AP diameter In pregnancy)
13. 5) Depression of respiration : opioids
6) Abdominal splinting – abdominal binders,
tight bandages, hip spica.
7)Abdominal pain – decreases by 50% & 75% in
lower & upper abdominal Surgeries
respectively.
8) Posture – by altering pulmonary Blood
volume.
14. CONTINUED…….
6) TOTAL LUNG CAPACITY :
Maximum volume of air attained in lungs after
maximal inspiration.
N- 4-6 l or 80-100 ml/kg
TLC= VC + RV
7) RESIDUAL VOLUME (RV):
Volume of air remaining in the lungs after
maximal expiration.
N- 1570 – 2100 ml OR 20 – 25 ml/kg.
15. CONTINUED……
8) FUNCTIONAL RESIDUAL CAPACITY (FRC):
Volume of air remaining in the lungs after
normal tidal expiration, when there is no airflow.
N- 2.3 -3.3 L OR 30-35 ml/kg.
FRC = RV + ERV
Decreses under anaesthesia
• with spontaneous Respiration – decreases by
20%
• With paralysis – decreases by 16%
16. FACTORS AFFECTING FRC
• FRC INCREASES WITH
• Increased height
• Erect position (30% more than in supine)
• Decreased lung recoil (e.g. emphysema)
• FRC DECREASES WITH
• Obesity
• Muscle paralysis (especially in supine)
• Supine position
• Restrictive lung disease (e.g. fibrosis, Pregnancy)
• Anaesthesia
• FRC does NOT change with age.
17. FUNCTIONS OF FRC
• Oxygen store
• Buffer for maintaining a steady arterial po2
• Partial inflation helps prevent atelectasis
• Minimise the work of breathing
• Minimise pulmonary vascular resistance
• Minimised v/q mismatch
• Keep airway resistance low
18. Pulmonary Function Tests
• The term encompasses a wide variety of
objective tests to assess lung function
• Provide objective and standardized
measurements for assessing the presence and
severity of respiratory dysfunction.
19. GOALS
To predict the presence of pulmonary
dysfunction
To know the functional nature of disease
(obstructive or restrictive. )
To assess the severity of disease
To assess the progression of disease
To assess the response to treatment
To identify patients at increased risk of morbidity
and mortality, undergoing pulmonary resection.
20. To wean patient from ventilator in icu.
Medicolegal- to assess lung impairment as a
result of occupational hazard.
Epidemiological surveys- to assess the
hazards to document incidence of disease
To identify patients at perioperative risk of
pulmonary complications
GOALS, CONTINUED……..
21. BED SIDE PFT
1) Sabrasez breath holding test:
• Ask the patient to take a full but not too deep breath & hold it as
long as possible.
>25 SEC.-NORMAL Cardiopulmonary Reserve (CPR)
15-25 SEC- LIMITED CPR
<15 SEC- VERY POOR CPR (Contraindication for elective surgery)
25- 30 SEC - 3500 ml VC
20 – 25 SEC - 3000 ml VC
15 - 20 SEC - 2500 ml VC
10 - 15 SEC - 2000 ml VC
5 - 10 SEC - 1500 ml VC
22. BED SIDE PFT
2) Single breath count:
After deep breath, hold it and start counting till the
next breath.
N- 30-40 COUNT
Indicates vital capacity
23. BED SIDE PFT
3) SCHNEIDER’S MATCH BLOWING TEST:
MEASURES Maximum Breathing Capacity.
Ask to blow a match stick from a distance of 6”
(15 cms) with-
Mouth wide open
Chin rested/supported
No purse lipping
No head movement
No air movement in the room
Mouth and match at the same level
24. BED SIDE PFT
• Can not blow out a match
– MBC < 60 L/min
– FEV1 < 1.6L
• Able to blow out a match
– MBC > 60 L/min
– FEV1 > 1.6L
• MODIFIED MATCH TEST:
DISTANCE MBC
9” >150 L/MIN.
6” >60 L/MIN.
3” > 40 L/MIN.
25. BED SIDE TEST
4) COUGH TEST: DEEP BREATH F/BY COUGH
ABILITY TO COUGH
STRENGTH
EFFECTIVENESS
INADEQUATE COUGH IF: FVC<20 ML/KG
FEV1 < 15 ML/KG
PEFR < 200 L/MIN.
VC ~ 3 TIMES TV FOR EFFECTIVE COUGH.
A wet productive cough / self propagated paraoxysms
of coughing – patient susceptible for pulmonary
Complication.
26. BED SIDE TEST
5) FORCED EXPIRATORY TIME:
After deep breath, exhale maximally and
forcefully & keep stethoscope over trachea &
listen.
N FET – 3-5 SECS.
OBS.LUNG DIS. - > 6 SEC
RES. LUNG DIS.- < 3 SEC
27. BED SIDE PFT
6) WRIGHT PEAK FLOW METER: Measures PEFR (Peak Expiratory Flow
Rate)
N – MALES- 450-700 L/MIN.
FEMALES- 350-500 L/MIN.
<200 L/ MIN. – INADEQUATE COUGH EFFICIENCY.
7) DEBONO WHISTLE BLOWING TEST: MEASURES PEFR.
Patient blows down a wide bore tube at the end of which is a
whistle, on the side is a hole with adjustable knob.
As subject blows → whistle blows, leak hole is gradually increased
till the intensity of whistle disappears.
At the last position at which the whistle can be blown , the PEFR can
be read off the scale.
28. MEASUREMENT OF TV & MV
8)Wright respirometer : measures tv, mv
• Simple and rapid
• Instrument- compact, light and portable.
• Disadvantage: It under- reads at low flow rates and over- reads at high flow
rates.
• Can be connected to endotracheal tube or face mask
• Prior explanation to patients needed.
• Ideally done in sitting pos.
• MV- instrument record for 1 min. And read directly
• TV-calculated and dividing MV by counting Respiratory Rate.
• USES: 1)BED SIDE PFT
• 2) ICU – WEANIG PTS. FROM Ventilation.
30. BED SIDE PFT
9) MICROSPIROMETERS – MEASURE VC.
10) BED SIDE PULSE OXIMETRY
11) ABG.
31. CATEGORIZATION OF PFT
1) MECHANICAL VENTILATORY FUNCTIONS OF
LUNG / CHEST WALL:
A) STATIC LUNG VOLUMES & CAPACITIES – VC, IC, IRV,
ERV, RV, FRC.
B) DYNAMIC LUNG VOLUMES –FVC, FEV1, FEF 25-75%,
PEFR, MVV, RESP. MUSCLE STRENGTH
C) VENTILATION TESTS – TV, MV, RR.
32. CATEGORIZATION OF PFT
2) GAS- EXCHANGE TESTS:
A) Alveolar-arterial po2 gradient
B) Diffusion capacity
C) Gas distribution tests- single breath
N2 test.
- Multiple Breath N2 test
- Helium dilution method.
- Radio Xe scinitigram.
D) ventilation – perfusion tests
A) ABG
B) single breath CO2 elimination test
C) Shunt equation
33. CATEGORIZATION OF PFT
3) CARDIOPULMONARY INTERACTION:
A) Qualitative tests:
- History , examination
- Abg
- Stair climbing test
B) Quantitative tests
- 6 min. Walk test (gold standard)
www.anaesthesia.co.in
34. STATIC LUNG VOLUMES AND
CAPACITIES
• SPIROMETRY : CORNERSTONE OF ALL
PFTs.
• John hutchinson – invented spirometer.
• “Spirometry is a medical test that
measures the volume of air an individual
inhales or exhales as a function of time.”
• Measures VC, FVC, FEV1, PEFR.
• CAN’T MEASURE – FRC, RV, TLC.
35. PREREQUISITIES
• Prior explanation to the patient
• Not to smoke /inhale bronchodilators 6 hrs
prior or oral bronchodilators 12hrs prior.
• Remove any tight clothings/ waist belt/
dentures
• Pt. Seated comfortably
If obese, child < 12 yrs- standing
36. PREREQUISITES
• Nose clip to close nostrils.
• Exp. Effort shld last ≥ 4 secs.
• Should not be interfered by coughing, glottic
closure, mechanical obstruction.
• 3 acceptable tracings taken & largest value is
used.
37. SPIROMETER
• Double walled cylinder with water to maintain
water tight seal
• Inverted bell attached to pulley which carries
a counterweight and pen – moves up and
down as volume of bell changes
• BREATHING ASSEMBLY i.E. Unidirectional
breathing valves with mouth piece.
38. Flow-Volume Curves and Spirograms
• Two ways to record results of FVC maneuver:
– Flow-volume curve---flow meter measures flow
rate in L/s upon exhalation; flow plotted as
function of volume
– Classic spirogram---volume as a function of time
42. Measurements Obtained from the FVC
Curve
• FEV1---the volume exhaled during the first second of the FVC
maneuver
• FEF 25-75%---the mean expiratory flow during the middle half
of the FVC maneuver; reflects flow through the small (<2 mm
in diameter) airways
• FEV1/FVC---the ratio of FEV1 to FVC X 100 (expressed as a
percent); an important value because a reduction of this ratio
from expected values is specific for obstructive rather than
restrictive diseases
43. Spirometry Interpretation: Obstructive
vs. Restrictive Defect
• Obstructive Disorders
– Characterized by a limitation
of expiratory airflow so that
airways cannot empty as
rapidly compared to normal
(such as through narrowed
airways from bronchospasm,
inflammation, etc.)
Examples:
– Asthma
– Emphysema
– Cystic Fibrosis
• Restrictive Disorders
– Characterized by reduced
lung volumes/decreased lung
compliance
Examples:
– Interstitial Fibrosis
– Scoliosis
– Obesity
– Lung Resection
– Neuromuscular diseases
– Cystic Fibrosis
46. Spirometry Interpretation: What do the
numbers mean?
• FVC
• Interpretation of %
predicted:
– 80-120% Normal
– 70-79% Mild reduction
– 50%-69% Moderate reduction
– <50% Severe reduction
FEV1
Interpretation of % predicted:
– >75% Normal
– 60%-75% Mild obstruction
– 50-59% Moderate obstruction
– <49% Severe obstruction
47. Spirometry Interpretation: What do the
numbers mean?
• FEF 25-75%
• Interpretation of %
predicted:
– >79% Normal
– 60-79% Mild
obstruction
– 40-59% Moderate
obstruction
– <40% Severe obstruction
• FEV1/FVC
• Interpretation of
absolute value:
– 80 or higher
Normal
– 79 or lower
Abnormal
48. What about lung volumes and obstructive
and restrictive disease?
49. MEASUREMENTS OF VOLUMES
• TLC, RV, FRC – MEASURED USING
Nitrogen washout method
Inert gas (helium) dilution method
Total body plethysmography
50. CONTINUED………..
1) HELIUM DILUTION METHOD:
Patient breathes in and out of a spirometer filled with 10%
helium and 90% o2, till conc. In spirometer and lung
becomes same (equilibirium).
As no helium is lost; (as it is insoluble in blood)
C1 X V1 = C2 ( V1 + V2)
V2 = V1 ( C1 – C2)
C2
V1= VOL. OF SPIROMETER
V2= FRC
C1= Conc.of He in the spirometer before equilibrium
C2 = Conc, of He in the spirometer after equilibrium
51. CONTINUED………
2) TOTAL BODY PLETHYSMOGRAPHY:
Subject sits in an air tight box. At the end of normal exhalation – shuttle of
mouthpiece closed and pt. is asked to make resp. efforts. As subject
inhales – expands gas volume in the lung so lung vol. increases and box
pressure rises and box vol. decreases.
BOYLE’S LAW:
PV = CONSTANT (at constant temp.)
For Box – p1v1 = p2 (v1- ∆v)
For Subject – p3 x v2 =p4 (v2 - ∆v)
P1- initial box pr. P2- final box pr.
V1- initial box vol. ∆ v- change in box vol.
P3- initial mouth pr., p4- final mouth pr.
V2- FRC
52. CONTINUED………
DIFFERENCE BETWEEN THE TWO METHODS:
• In healthy people there is very little difference.
• Gas dilution technique measures only the
communicating gas volume.
• Thus,
• Gas trapped behind closed airways
• Gas in pneumothorax
• => are not measured by gas dilution technique,
but measured by body plethysmograph
53. CONTINUED………
3) N2 WASH OUT METHOD:
• Following a maximal expiration (RV) or normal
expiration (FRC), Pt. inspires 100% O2 and
then expires it into spirometer ( free of N2) →
over next few minutes (usually 6-7 min.), till
all the N2 is washed out of the lungs. N2
conc. of spirometer is calculated followed by
total vol.of AIR exhaled. As air has 80% N2 →
so actual FRC/RV is calculated.
54. MEASUREMENT OF DYNAMIC LUNG
VOLUMES
• TIMED EXPIRED SPIROGRAMS
FEF25–75% = forced
expiratory flow during
expiration of 25 to 75% of
the FVC; FEV1 = forced
expiratory volume in the
first second of forced vital
capacity maneuver; FVC =
forced vital capacity (the
maximum amount of air
forcibly expired after
maximum inspiration).
55. FORCED VITAL CAPACITY (FVC)
Max vol. Of air which can be expired out as forcefully and
rapidly as possible, following a maximal inspiration to
TLC.
Exhaled volume is recorded with respect to time.
Indirectly reflects flow resistance property of airways.
Normal healthy subjects have VC = FVC.
Prior instruction to patients, practice attempts as it
needs patient cooperation and effect.
Exhalation should take at least 4 sec and should not be
interrupted by cough, glottic closure or mechanical
obstruction.
56. FORCED VITAL CAPACITY IN 1 SEC.
(FEV1)
Forced expired vol. In 1 sec during fvc
maneuver.
Expressed as an absolute value or % of fvc.
N- FEV1 (1 SEC)- 75-85% OF FVC
FEV2 (2 SEC)- 94% OF FVC
FEV3 (3 SEC)- 97% OF FVC
57. CONTINUED……
CLINICAL RANGE (FEV1)
• 3 - 4.5 L
• 1.5 – 2.5 L
• <1 L
• 0.8 L
• 0.5 L
PATIENT GROUP
• NORMAL ADULT
• MILD – MOD.OBSTRUCTION
• HANDICAPPED
• DISABILITY
• SEVERE EMPHYSEMA
58. CONTINUED……
FEV1 – Decreased in both obstructive &
restrictive lung disorders.
FEV1/FVC – Reduced in obstructive disorders.
NORMAL VALUE IS 75 – 85 % (FEV1/FVC)
< 70% OF PREDICTED VALUE – MILD OBST.
< 60% OF PREDICTED VALUE – MODERATE OBST.
< 50% OF PREDICTED VALUE – SEVERE OBST.
59. CONTINUED……
DISEASE STATES FVC FEV1 FEV1/FVC
1) OBSTRUCTIVE NORMAL ↓ ↓
2) STIFF LUNGS ↓ ↓ NORMAL
3 ) RESP. MUSCLE
WEAKNESS
↓ ↓ NORMAL
60. PEAK EXPIRATORY FLOW RATE (PEFR)
- It is the max. Flow rate during fvc maneuver in the initial 0.1
sec.
-PEFR DETERMINED BY : 1) Function of caliber of airways
2) Expiratory muscle strength
3) Pt’s coordination & effort
- Estimated by 1) drawing a tangent to steepest part of FVC
spirogram (error prone)
2) average flow during the litre of gas expired after
initial 200 ml during fvc maneuver.
- .
61. FORCED MID-EXPIRATORY FLOW RATE
(FEF25%-75%):
• Maximum Mid expiratory Flow rate
• Max. Flow rate during the mid-expiratory part of FVC maneuver.
• Effort independent
• Misnomer, as FEF25-75% decreased by
1) marked reduction in exp. Effort
2)submaximal inspiration b4 maneuver → ↓FVC → ↓ FEF25-
75%
• It may decrease with truly max. Effort as compared to slightly
submaximal effort as dynamic airway compression occurs with
maximal effort.
• N value – 4.5-5 l/sec. Or 300 l/min.
• Upto 2l/sec- acceptable.
• CLINICAL SIGNIFICANCE: SENSITIVE & IST INDICATOR OF
OBSTRUCTION OF SMALL DISTAL AIRWAYS
62. MAXIMUM BREATHING CAPACITY:
(MBC/MVV)
• MAX. VOLUNTARY VENTILATION
Largest volume that can be breathed per minute by voluntary
effort , as hard & as fast as possible.
N – 150-175 l/min.
Estimate of max. Ventilation available to meet increased
physiological demand.
Measured for 12 secs – extrapolated for 1 min.
MVV = FEV1 X 35
64. RESPIRATORY MUSCLE STRENGTH
Evaluated by measuring max. Static resp. Pressure with
anaeroid gauge
• Pressures are generated against occluded airway
during a max. Forced insp/exp. Effort
MAX STATIC INSP. PRESSURE: (PIMAX)-
• Measured when inspiratory muscles are at their
optimal length i.e. at RV
• PI MAX = -125 CM H2O
• CLINICAL SIGNIFICANCE:
IF PI MAX< 25 CM H2O – Inability to take deep breath.
65. CONTINUED…….
• MAX. STATIC EXPIRATORY PRESSURE (PEMAX):
Measured after full inspiration to TLC
N VALUE OF PEMAX IS =200 CM H20
PEMAX < +40 CM H20 – Impaired cough ability
Particularly useful in pts with NM Disorders during
weaning
66. MEASUREMENT OF AIRWAY
RESISTANCE
1) Raw- Body plethysmography
2) Forced expiratory maneuvers:
Peak expiratory flow (PEF)
FEV1
3) Response to bronchodialtors (FEV1)
67. Spirometry Pre and Post Bronchodilator
• Obtain a flow-volume loop.
• Administer a bronchodilator.
• Obtain the flow-volume loop again a minimum of 15
minutes after administration of the bronchodilator.
• Calculate percent change (FEV1 most commonly
used---so % change FEV 1= [(FEV1 Post-FEV1
Pre)/FEV1 Pre] X 100).
• Reversibility is with 12% or greater change.
68. FLOW VOLUME LOOPS
• Do FVC maneuver and then inhale as rapidly
and as much as able.
• This makes an inspiratory curve.
• The expiratory and inspiratory flow volume
curves put together make a flow volume loop.
70. How is a flow-volume loop helpful?
• Helpful in evaluation of air flow limitation on inspiration and
expiration
• In addition to obstructive and restrictive patterns, flow-
volume loops can show provide information on upper airway
obstruction:
– Fixed obstruction: constant airflow limitation on inspiration and
expiration—such as in tumor, tracheal stenosis
– Variable extrathoracic obstruction: limitation of inspiratory flow,
flattened inspiratory loop—such as in vocal cord dysfunction
– Variable intrathoracic obstruction: flattening of expiratory limb; as in
malignancy or tracheomalacia
71. TESTS FOR GAS EXCHANGE FUNCTION
1) ALVEOLAR-ARTERIAL O2 TENSION GRADIENT:
Sensitive indicator of detecting regional V/Q inequality
N value in young adult at room air = 8 mmhg to upto
25 mmhg in 8th decade (d/t decrease in PaO2)
AbN high values at room air is seen in asymptomatic
smokers & chr. Bronchitis (min. symptoms)
PAO2 = PIO2 – PaCo2
R
73. CONTINUED……
3) DIFFUSING CAPACITY OF LUNG: defined as the
rate at which gas enters into bld. divided by its
driving pr.
DRIVING PR: gradient b/w alveoli & end capillary
tensions.
Fick’s law of diffusion : Vgas = A x D x (P1-P2)
T
D= diffusion coeff= solubility
√MW
74. CONTINUED…….
• DL IS MEASURED BY USING CO, COZ:
A) High affinity for Hb which is approx. 200
times that of O2 , so does not rapidly build
up in plasma
B) Under N condition it has low bld conc ≈ 0
C) Therefore, pulm conc.≈0
75. SINGLE BREATH TEST USING CO
• Pt inspires a dilute mixture of CO and hold the
breath for 10 secs.
• CO taken up is determined by infrared
analysis:
• DlCO = CO ml/min/mmhg
• PACO – PcCO
• N range 20- 30 ml/min./mmhg.
• DLO2 = DLCO x 1.23
77. TESTS FOR CARDIOPLULMONARY
INTERACTIONS
• Reflects gas exchange, ventilation, tissue O2,
CO.
• QUALITATIVE- history, exam, ABG, stair
climbing test
• QUANTITATIVE- 6 minute walk test
78. CONTINUED…….
• 1) STAIR CLIMBING TEST:
• If able to climb 3 flights of stairs without stopping/dypnoea at
his/her own pace- ↓ed morbidity & mortality
• If not able to climb 2 flights – high risk
• 2) 6 MINUTE WALK TEST:
- Gold standard
- C.P. reserve is measured by estimating max. O2 uptake during
exercise
- Modified if pt. can’t walk – bicycle/ arm exercises
- If pt. is able to walk for >2000 feet during 6 min pd,
- VO2 max > 15 ml/kg/min
- If 1080 feet in 1 min : VO2 of 12ml/kg/min
- Simultaneously oximetry is done & if Spo2 falls >4%- high risk
