MONITORING IN ANAESTHESIA

MODERATOR : Dr RAVINDRA NATHPRESENTOR : Dr RAMESH REDDY

Contents

• Introduction.• History.• Terminology.• Physics.• Analyzers.• Types of Capnograms.• Capnogram Waveform Patterns.

Introduction• Presence of co2 in exhaled air reflects fundamental physiological process of ventilation ,pulmonary blood flow ,aerobic metabolism.

History

• John Scott Haldane – 1905 –First person to measure co2 in a mixture of gases.

• Luft – 1937 – First to use infra red waves to measure co2 concentration.

• Holland -1978-First country to use capnogram as integral part of anaesthesia monitoring

• ISA made desirable monitor in anaesthesia in 1999.

Terminology

•Capnometry: Measurement and numerical display of co2 concentration in a gas mixture.•Capnometer: Device that

performs measurement and displays the readings in numerical form.•Capnography: Graphic display of

instantaneous co2 concentration versus time or expired volume •Capnograph: Device that

generates wave form and displays the graph.

Physics

• Co2 in exhaled gases can be measured by:

1.Infra red spectrography 2.Raman spectrography3.Mass spectrography4.Photo spectrography5.Chemical colorometric .

Infra Red Spectrography• -Most commonly used -Cost effective • Warm objects produces

infra red waves and are absorbed by nonelementary gases like CO2 ,N2O etc.

• Particular gases absorbs particular wave lengths and produces absorption bands.

• Co2 shows strong absorption at 4.3 nm wave length.

PhysicsRaman Spectrography: Co2 analysis sample is illuminated with

high intensity argon laser beam and excited to rotational energy states(Raman scattering).

• Raman scattering signals are then measured.All type of molecules are analysed by this method.

Mass Spectrography : Based on weight : charge ratio it separates molecules.• It measures in volume percent not in partial pressures.Photo Acoustic method: Instead of optical methods acoustic

methods are used.Colorimetric Methods: Chemically treated foam indicator placed

at ETT tip and colour changes when exposed to co2.

Types of Capnographs

• Depending on the flow types two types of capnographs.

1. Side Stream / Diverting Capnography.

2. Main Stream/ Non Diverting Capnography.

Side Stream Analyzer

Side Stream Analyzer

• Diverting Stream Capnography: Small pump aspirates gas from the patients end through a 6 feet long tube to the main unit.

• Optimal rate of sampling is 50-200 ml/min.• Water droplets and secretions from the breathing

system can enter the sampling system and increases the resistance and effects reading.

• To avoid this traps ,filters ,hydrophobic membranes, special tubings are used.

Side Stream AnalyzerAdvantages:• Calibration and zeroing are usually automatic.• Dead space is minimal.• Cross infection between patients is low.• Several gases can be measured simultaneously.• Sample port can be used for bronchodilators.• Useful when monitor placed away from patient like

MRI.

Side Stream AnalyzerDisadvantages:• Sampling system problems like leaks ,blocks, wrong

connections,kinking etc.• Aspirated sample should be routed to scavanger system

or rebreathing system.• Delay time unavoidable.• Calibration gas must be available.• More disposable items used.• Produces more variable difference between arterial and

Etco2 levels than main stream monitoring.

Main Stream Analyzer

Main Stream AnalyzerMain Stream Monitoring :• Sensor placed directly between the breathing circuit and

the ETT.Advantages:• No delay time .• No gas removed from circuit, no need of scavanger system.• Water and secretions are not problem.• Sample contamination with fresh gas is less.• Standard gas not required for calibration.• Less disposable items than side stream monitoring.

Main Stream AnalyzerDisadvantages:• Sensor add weight to the breathing system and cause

traction.• Adaptor between patient and breathing system increases

dead space.• Only co2 and o2 measured.• Potential source of cross cotamination between patients.• Thermal burns can occur when sensor placed in direct

contact with skin.

Capnogram• Capnograms are 2 types 1.Time capnogram 2.Volume capnogramTime Capnogram:• Exhaled co2 recorded aganist time.• Most commonly used type.• Both inspiratory and expiratory

phases• Two angles alpha and beta .• Expiratory phase has 3 distinct

components.• Phase1:Co2 exhalation from central

conducting system

Time Capnogram• Phase 2 : Trasitional gas between airways and alveoli• Phase 3 : Alveolar gas component

Time Capnogram

• Homogenous/Ideal V/Q lungs – flat.• Heterogenus distribution of V/Q lungs (Asthma,COPD,ALI

etc..) – upsloping.• Bronchodilators, PEEP makes phase 3 flat.•Phase 0: Inspiratory effort/rapid down sloping.•Phase 4: Sharp upstroke observed at the very end of phase 3.• Occurs due to closure of some lung units at relatively low

pco2 and allows for regions of higher co2 to contribute more for ET CO2.

Time CapnogramAlpha angle: • Angle between phase 2 and phase 3.• Increases with increase in phase 3 slope.• Normal value between 100-110 degrees.Beta angle: • Angle between phase3 and descending limb • Normal angle around 90 degrees.• Increases with rebreathing.

ETCO2• Final value of the exhaled CO2 curve at the very end of the

expiratory phase.

• It is not universal. Varies with capnograph in use like:

1. Pco2 value just before inspiration.2. Largest Pco2 value during single expiratory cycle.3.Averaged Pco2 value at a specified time across several

breaths.

ETCO2

Volume Capnogram

• Capnogram with Co2 partial pressure versus exhaled volume .

• Inspiratory phase not defined.• 3 phases in expiration just like

time capnogram.

Volume Capnogram

Volume Capnogram

Advantages:• Partition of tidal volume into air way dead space and effective

alveolar tidal volume.• More sensitive than the time capnogram in detecting subtle changes

in dead space.• Estimates total mass of CO2 exhaled.

Capnogram Study Approach

• 1.Co2 wave form present/not.• 2.Respiratory base line – Rebreathing/not.• 3.Expiratory upstroke –steep/slope/prolonged• 4.Plateu-flat/slope/notch/prolonged.• 5.Etco2-increased/decreased.• 6.ABG –Compare the Etco2 with Paco2.

Capnogram Waveform Patterns

The slope the expiratory plateau is increased as a normal physiological variation in pregnancy.

Prolonged Inspiratory Descending Limb

• Due to dispersion gases in the sampling line or as well as prolonged response time of the analyzer. Seen in children who have faster respiratory rates.

Base line elevation

Base Line is elevated in :

• Inadequate fresh gas flow.• Accidental administration of CO2.• Rebreathing.• Insp / exp valve malfunction.• Exhausted CO2 absorber.

Elevation of base line

Expiratory valve malfunction

•Expiratory valve malfunction can result in prolonged abnormal phase 2 and phase 0

Inspiratory valve malfunction

• Elevation of the base line, prolongation of down stroke, prolongation of phase III

Rebreathing in Bain’s circuit

• Inspiratory base line and phase I are elevated above the zero due to rebreathing. Note the rebreathing wave during inspiration.

Hypoventilation

•Gradual elevation of the height of the capnogram, base line remaining at zero.

Hyperventilation

•Gradual decrease in the height of the capnogram, base line remaining at zero.

Oesophageal Intubation

Oesophageal Intubation

Cardiogenic Oscillations

• Ripple effect, superimposed on the plateau and the descending limb, resulting from small gas movements produced by pulsations of the aorta and heart.

Airway obstruction (eg., bronchospasm). Phase II and phase III are prolonged and alpha angle (angle between phase II and phase III) is increased.

Bronchospasm

During Bronchospasm After relief

Curare Effect

Malignant Hyperpyrexia

Hypothermia• A gradual decrease in end tidal carbon dioxide -hypothermia, -reduced metabolism, -hyperventilation, - leaks in the sampling system

Kyphoscoliosis

•The CO2 waveform has two humps resulting in a compression of the right lung.

Spontaneous Ventilation

• Capnogram during spontaneous ventilation in adults.

Spontaneous Ventilation

• Sampling problems such as air or oxygen dilution during nasal or mask sampling of carbon dioxide in spontaneously breathing patients.

Detection of Pulmonary Air Embolism

• A rapid decrease of PETCO2 in the absence of changes in blood pressure, central venous pressure and heart rate indicates an air embolism without systemic hemodynamic consequences.

• As the size of air embolism increases, a reduction in cardiac output occurs which further decreases PETCO2 measurement. A reduced cardiac output by itself can decrease PETCO2.

Effective Circulating Blood Volume Can Reduce The Height Of Capnograms

Capnogram Waveform Patterns

Capnogram Waveform Patterns

CLINICAL USES OF CAPNOGRAPHY

• Confirmation of endotracheal intubation.• Monitoring of adequacy of ventilation in controlled or spontaneously

ventilating patients.• Noninvasive estimate of PaCO2 a. assumes the normal 2 to 5 mm Hg difference between expired

(PETCO2) & arterial (PaCO2) that exists in the awake state is present . b. The gradient between PETCO2 & PaCO2 may be increased with age

, pulmonary disease , pulmonary embolus , low cardiac output & hypovolemia .

Contd…

• Detection of patient disease :1. causes of increased CO2 production : a. fever , b. sepsis , c. malignant hyperthermia , d. hyperthyroidism , e.

shivering.2. Causes of decreased PETCO2 : a . decreased cardiac output , b. Hypothermia , c . pulmonary embolism , d. hyperventilation.3. Airway obstruction may be detected due to abnormalities in the

capnography tracing.• Detection of problems with the anaesthetic breathing system : a. rebreathing , b. incompetent valves , c. circuit disconnect , d. circuit leak.

Pulse Oximetry

1. Greatest advance in patient monitoring

2. Unique advantage of continuously monitoring the oxygen

saturation of haemoglobin in peripheral arterial blood.

3. Easy & Non-invasive

4. Provides measure of Cardio Respiratory function

HISTORY

1935 Carl Matthes Transillumination Method.

1940 J R Squire Compressing tissue

Early 1940’s

Glen Millikan

Earl Wood

Coined “Oximeter”

Added Pneumatic Cuff

1964 Robert Shaw Self-calibrating Ear Oximeter

1972 Takuyo Arogi ‘Serendipity’

After1980’s Technical

advances LED, Photo detectors, Microprocessors

Basic Principles

• Pulse Oximeters measure the arterial Oxygen saturation of

haemoglobin.

• Two basic physical principles:

• Absorption of light at two different wavelengths by haemoglobin

differs depending on the degree of oxygenation of haemoglobin.

• The light signal following transmission through the tissues has a

pulsatile component.

Ronald Pristera 2002

Terminology • SpO2 Non invasive Oxygen saturation

• SaO2 Arterial Oxygen Saturation

(Oxygen bound to the hemoglobin molecules)

• PaO2 Arterial Partial Pressure,

Oxygen dissolved in the plasma

(only about 3% of total content) or PO2

• CaO2Total amount of Oxygen in the blood or the SaO2 + PaO2

Components• Pulse Oximeters consist of:

• Peripheral Probe

• Microprocessor unit - displays a waveform, the Oxygen saturation and

the pulse rate.

• Most Oximeters also have an audible pulse tone, the pitch of which is

proportional to the Oxygen saturation

Wrist Pulse Oximeter

Physics

1. Spectrophotometry measures the O2 saturation

2.Optical Plethysmography measures pulsatile changes in

arterial blood volume at sensor site

Spectrophotometry

❖ Uses Beer-Lambert Law

I trans = I inc – A

A = DCE

I trans – intensity of transmitted light

I inc - intensity of incident light

A - absorption

D - distance light is transmitted through liquid

C - concentration of solute ( Haemoglobin)

E - extinction coefficient of the solute

BEER – LAMBERT LAW❑If there is one solute, the absorption A is product

of the path length, the concentration and the

Extinction coefficient.

❑Each solute has a specific extinction coefficient for

absorption of light at a specific wavelength.

❑If more than one solute is present, ‘A’ is the sum of

similar expressions for each solute.

Hemoglobin Saturation 4 types of Hemoglobin

1. Oxyhemoglobin (O2Hb)

2. Reduced Hemoglobin (Hb)

3. Methemoglobin (MetHb)

4. Carboxyhemoglobin (COHb)

Principles of Pulse Oximetry

Use of two different wavelengths

Photodetector to measure the transmitted light

Measured as a cuvette containing Hemoglobin

Peripheral Probe with Microprocessor unit displaying a waveform, O2 saturation, and Pulse rate

Amount of light absorption depends on degree of Oxygenation of Hb in tissues

Pulsatality of arterial blood helps to filter the extraneous noise which is non-pulsatile.

Haemoglobin Extinction Curves

660 nm 940 nm

Ronald Pristera 2002

Components of Light Absorption

Pulse Added Absorbance

• Microprocessor first determines the AC component of absorbance at each wavelength.

• And divides this by corresponding DC component.

• PAA for wavelength 660nm= AC660

DC660

• PAA for wavelength 940nm= AC940

DC940

❑ Ratio of Ratio’s(R)

❑ If ‘R’ is equal to 1, SpO2 approx. 85%

Clinical indications

• 1.critical care – hypoxemia

•2.monitoring during Anaesthesia

•3.exercise testing

•4.sleep studies – detect hypoxemia

•5.monitoring of patients on LTOT

Sites

•Generally measured on:

•Fingers,

•The Earlobe

•Infants - the bridge of the nose

Limitations

❖Patient Factors

❖Machine Factors

❖Environmental Factors

Dyshemoglobinemia

Elevated Methemoglobin

▪ it absorbs equal amounts of Red and Infra Red

▪ R=1

▪ SpO2=85%

Carboxyhemoglobin

▪ Absorbs more light at 660nm as OxyHb

▪ Absorbs little light at 940nm

▪ Causing Pulse Oximeter to over read.

Poor Function with Poor Perfusion

▪ Proximal BP cuff inflation▪ Leaning on an extremity▪ Improper positioning▪Hypotension▪Hypovolemia▪ Low cardiac output▪Hypothermia▪ Cardiopulmonary bypass▪ Peripheral Vascular Disease▪ Vasoactive drug infusion▪ Valsalva maneuver in Laboring patients

Difficulty in Detecting High Oxygen Perfusion Pressure

•Has limited ability to distinguish high but safe levels

of arterial O2 from excess oxygenation-harmful

•Premature infants, COPD patients

Delayed Detection of Hypoxic Events

•Delay is related to sensor location

•Lag time will be increased with poor perfusion

▪Venous obstruction

▪Peripheral vasoconstriction

▪Hypothermia

▪Motion artifacts

❑Lag time may be decreased by neural block.

Local factors

Nail polish (Black,Blue,Green)

Synthetic Nails

Onychomycosis

Dirt under nail

Electrical Interference❖ Incorrect pulse count or falsely register decrease in O2

saturation

❖ Transient effect and limited to duration of Cauterization

❖ To avoid,

✔ keep the Electro surgery grounding plate as close to the

surgical field and Oximeter sensor far from the field

✔ Keep the console far from grounding plate and table

✔ Should not be plugged in the same power source

Motion Artifacts

❖During patient transportation, shivering patient

❖In children

❖Nerve stimulators

❖More with clip on probes

Other Limitations

❑Pressure on sensor

❑Hyperemia- venous blood becomes pulsatile

❑Failure to detect absence of circulation

❑Failure to detect Hypoventilation and Hypercarbia

❑Discrepancies in readings from different monitors

High Failure RatesVery young and very elderly

ASA grade 3&4

Orthopaedic and Cardiovascular surgeries

Use of electrocautery

Hypertension

Prolonged duration of intraoperative procedure

CRF

Low hematocrit

Pigmented skin

Methods to Improve Signal

•Application of Vasodilating cream

•Digital nerve blocks

•Administration of intra-arterial vasodilators

•Placing a glove filled with warm water in patient hand

Conclusion • Standard of care in all clinical situations and mandatory for all patients under

anaesthesia

• Early warning of hypoxic events helps us to take remedial action expeditiously

• ASA Standards of Basic Monitoring during Anaesthesia to adopt Pulse

Oximetry as of January1,1990.

• Most important limitation- inaccurate in patients who need them the most

• The presence of a functioning Pulse Oximeter should not be considered as

evidence of adequate tissue oxygenation or O2 delivery to organs.