capnography vs plethysmography
TRANSCRIPT
CAPNOGRAPHY VS
PLETHYSMOGRAPHY
PRESENTED BY DR. SOURAV MONDAL
MODERATED BY DR. ARVIND RATHIYA, MD
DEPT OF ANAESTHESIOLOGY, S.S.M.C , REWA , M.P.
CAPNOGRAPHY
• Capnography was first introduced by KARL LUFT, a German bioengineer, in 1943 with an infrared CO2 measuring device he called URAS, or “Ultra Rot Absorption Schreiber”
• It was big, heavy and very impractical to use
• Today capnography is successfully used in almost all areas of health care.
HISTORY
CAPNOGRAPHY(Quantitative ETCO2 Detectors)
• Capnography is a recording of CO2 concentration versus time.
• It is a form of noninvasive monitoring of the end-tidal carbon dioxide (ETCO2) levels in the patient’s exhaled breath.
CAPNOGRAPHY PROVIDES INSTANTANEOUS INFORMATION ABOUT
• VENTILATION– How effectively CO2 is being eliminated by the pulmonary
system
• PERFUSION– How effectively CO2 is being transported through the
vascular system
• METABOLISM– How effectively CO2 is being produced by cellular metabolism
STANDARD REQUIREMENTS OF CAPNOMETER
• The CO2 reading shall be within ±12% of the value or ±4mm of Hg (0.53kPa) , whichever is greater .
• The manufacturer must disclose any interference caused by ethanol, acetone , methane , helium , tetrafluoroethane, and dicholorodifluromethane as well as commonly used halogenated anesthetic agents.
• The capnometer must have a high CO2 alarm for both inspired and exhaled CO2.
• An alarm for low exhaled CO2 is required.
METHODS OF MONITORING Mass Spectrometry
Infrared Spectrography – Most commonly used in the hospital setting
Chemical Colorimetric Analysis – Most commonly used in the pre-hospital setting
MASS SPECTROMETRY• It is a technique by which
concentrations of gas in a sample can be determined according to charge- mass ratio.
• A gas sample is passed through an ionizer and molecules become positively charged ions. Because all of the ions generated carry the same positive charge , this allows separation based solely on mass.
• A detector then counts the no.of ions of each mass & the results are translated into concentrations.
INFRARED SPECTROGRAPHYPRINCIPAL: Carbon Dioxide selectively absorbs a known amount of infrared light of a specific wavelength (4.26 µm). The amount of light absorbed is directly proportional to the concentration of carbon dioxide molecules.A predetermined amount of infrared light is sent from the emitting side of the sensor through a gas sample and collected on the receiving side of the sensor. The infrared light received is compared to the infrared light transmitted. The difference is then converted by calculations into either partial pressure or percentage of total gas concentration that we see on the monitor.
Infrared Spectrography
CHEMICAL COLORIMETRIC ANALYSIS
CHEMICAL COLORIMETRY ANALYSIS
CO2 NOT PRESENT CO2 PRESENT
•Consists of pH-sensitive indicator enclosed in housing. When the indicator is exposed to carbonic acid it becomes more acidic & changes colour. During inspiration the colour returns to resting state.
•The inlet and outlet ports are 15mm, so the device can be placed between patient and the breathing system or resusciation bag
TECHNOLOGY
HYGROSCOPIC – contains hygroscopic filter paper that is impregnated with a colourless base & an indicator that changes colour as a function of pH. A purple/mauve colour indicates low (<0.5%) CO2 level, beige colour indicates a moderate (0.5%- 2%) level while a yellow colour indicates high(>2%) level
HYDROPHOBIC – shows a colour change from blue to green to yellow when exposed to CO2. Liquid water may cause improper functioning of the device.
TYPES OF CAPNOMETERS
The 2 Types of Capnometers
Mainstream Sidestream
The infrared sensor is in the direct path of the gas source, and connected to the monitor by an electrical wire.
The sample of gas is aspirated into the monitor via a lightweight airway adapter and a 6ft length of tubing. The actual sensor is inside the monitor.
MAINSTREAM CAPNOMETERS ADVANTAGES
Mainstream – Advantages
Mainstream
No sampling tube to become obstructed.
No variation due to barometric pressure changes.
No variation due to humidity changes.
Direct measurement means waveform and readout are in ‘real-time’. There is no sampling delay.
Suitable for pediatrics and neonates.
MAINSTREAM CAPNOMETERSDISADVANTAGES
Mainstream – Disadvantages
Mainstream
The airway adapter sensor puts weight at the end of the endotracheal tube that often needs to be supported.
In older models there were minor facial burns reported.
The sensor windows can become obstructed with secretions and water rainout.
Sensor and airway adapter can be positional – difficult to use in unusual positions (prone, etc).
SIDESTREAM CAPNOMETERSADVANTAGES
Sidestream – Advantages
Sampling capillary tube and airway adapter is easy to connect.
Can be used with patient in almost any position (prone, etc).
Can be used in awake patients via a special nasal cannula.
CO2 reading is unaffected by oxygen flow through the nasal cannula.
SIDESTREM CAPNOMETERSDISADVANTAGES
Sidestream – Disadvantages
The sampling capillary tube can easily become obstructed by water or secretions.
Water vapor pressure changes within the sampling tube can affect CO2 measurement.
Delay in waveform and readout due to the time it takes the gas sample to travel to the sensor within the unit.
PHYSIOLOGY OF CAPNOGRAPHY• During cellular respiration, small
amounts of CO2 produced , is excreted via exhalation
• When no cellular respiration is occurring, even if ventilation is, there will be no CO2 exhaled– In poor perfusion states
(cardiac arrest) no CO2 is transported to the lungs to be exhaled, so a low reading will occur
– In poor ventilation states (hypoventilation) CO2 is retained, so a high reading will occur
END-TIDAL CO2 The peak partial pressure of CO2
during exhalation (the highest level of expired CO2 reached during exhalation) is known as the end-tidal CO2 (ETCO2).– Normally occurs at the end
of the alveolar plateau
ETCO2 is a reflection of alveolar ventilation, CO2 production and pulmonary blood flow.
Normal value is 35-45 mmHg
CLINICAL APPLICATION OF ETC02
• Verification of endotracheal tube placement
• Continuous monitoring of tube location during transport
• Gauging the effectiveness of resuscitation and prognosis during cardiac arrest
• Titrating ETC02 levels in patients with suspected increases in intracranial pressure
• Determining adequacy of ventilation
WAVEFORM DISPLAYS(Quantitative Device)
The waveform is divided into 4 phases.
Phases I, II and III occur during and reflect the three phases of exhalation.
Phase IV occurs during and reflects inspiration
CAPNOGRAM: PHASE I
Phase I (A-B) occurs during exhalation of air from the anatomic dead space, which normally contains no CO2.
This part of the curve is normally flat, providing a steady baseline.
CAPNOGRAM: PHASE II
Phase II (B-C) occurs during alveolar washout and recruitment, with a mixture of dead space and alveolar air being exhaled.
Phase II normally consists of a steep upward slope.
CAPNOGRAM: PHASE III Phase III (C-D) is the alveolar
plateau, with expired gas coming from the alveoli.
In patients with normal respiratory mechanics, this portion of the curve is flat, with a gentle upward slope.
The highest point on this slope (D) represents the ETCO2 value.
CAPNOGRAM: PHASE IV
Phase IV (D-E) occurs during inspiration, where the ETCO2 level normally drops rapidly to zero.
Unless CO2 is present in the inspired air, as occurs when expired air is rebreathed , this part of the waveform is a steep, downward slope.
VOLUME CAPNOGRAM It is a graphic display of CO2 concentation/ partial pressure
versus exhaled volume. The inspiratory phase in not defined in volume capnogram.
ADVANTAGES
Allows estimation of relative contributions of anatomical dead space and alveolar dead space
More sensitive Allows for determination of total mass of CO2 exhaled during a
breath & provides estimation of VCO2
ABNORMAL CAPNOGRAMS
CAUSES OF AN ELEVATED ETCO2
• ↑CO2 PRODUCTION & DELIVERY TO LUNGS
↑metabolic rate Fever Sepsis Seizures Malignant hypothermia Thyrotoxicosis ↑Cardiac output (during
CPR) Bicarbonate administration
• ↓ALVEOLAR VENTILATION Hypoventilation Respiratory centre depression Partial muscular paralysis Neuromuscular disease COPD
• EQUIPMENT MALFUNCTION Rebreathing Exhausted C02 absorber Leak in ventilator circuit Faulty inspiratory/ expiratory valve
CAUSES OF A DECREASED ETCO2
• ↓ CO2 PRODUCTION AND DELIVERY TO LUNGS
Hypothermia Pulmonary hypoperfusion Cardiac arrest Pulmonaryembolism Haemorrhage Hypotension
• ↑ALVEOLAR VENTILATION Hyperventilation
• EQUIPMENT MALFUNCTION Ventilator disconnect Esophageal intubation Complete airway obstruction Poor sampling Leak around ETT cuff
INCOMPETENT INSPIRATORY UNIDIRECTIONAL VALVE
• The waveform shows a prolonged plateau & a slanting inspiratory downstroke
• The inspiratory phase is shortened & the baseline may or may not reach zero.
LEAK IN SAMPLING LINE DURING PPV
• Will result in upswing at the end of Phase III.
• The brief peak is caused by the next inspiration , when positive pressure pushes undiluted end-tidal gas through the sampling line.
PULSE OXIMETRY
(PHOTOPLETHYSMOGRAPHY+ OXIMETRY)
PULSE OXIMETRY
Pulse oximeters combines the principle of oximetry and plethysmography to noninvasively measure oxygen saturation in arterial blood
HISTORY
• In early 1940 GLEN MALKIKAN coined the term oximeter.
• MATHEES- father of oximetry 20 papers in1934 –1944• HERTZMAN 1937 –use of photoelectric finger
plethsmography• 1975 –concept of pulse oximetry –Japan• In 2008 modification continued and term High
Resolution Pulse Oximetry come into existence
INTRODUCTION• Also called the fifth vital sign
• Low SpO2 provide warning of hypoxemia before other signs such as cyanosis or a change in heart rate are observed.
• Until the 1980s, noninvasive oximeters, known oximeters, were large, expensive, and cumbersome. They required “arterialization”.
• Technical advances, including LEDs, miniaturized photodetectors,
and microprocessors, allowed the creation of a new generation of oximeters.
• These differentiate the absorption of light by the pulsatile arterial component from the static components, so they are called Pulse Oximeters.
OXYGEN SATURATION
• Saturation is defined as ratio of O2 content to oxygen capacity of Hb - expressed as percentage.
• Desaturation leads to Hypoxemia – a relative deficiency of O2 in arterial blood. (PaO2 < 80mmHg – hypoxemia)
• (SaO2< than 76% is life threatening.)
FRACTIONAL SATURATION
This is the ratio of oxygenated Haemoglobin to sum of all haemoglobin species in blood.
Fractional saturation = HbO2 - --------------------------
HbO2+ Hb+ Met Hb +CO Hb
FUNCTIONAL SATURATION
This is the of ratio between HbO2 to all the functional haemoglobin
PLETHYSMOGRAPHY• Pulse oximeters show pulsatile change in
absorbance in a graphical form. This is called the “plethysmographic trace” or “pleth”
• Because absorption of light is proportional to the amount of blood between the transmitter & the photodetector , changes in the blood volume are reflected in pulse oximetry trace. Hence pulse oximeter can also be used as a photoplethysmograph
PRINCIPLES • All atom and molecules absorb specific wavelength of light. This
property is the basis for an optical technique known as SPECTROPHOTOMETRY.
BEER-LAMBERT LAW It states that if a known intensity of light illuminates a chamber of
known dimensions , then the concentration of a dissolved substance can be determined if the incident and transmitted light is measured :
It = Iie –dcα
Solved for C,C= (1/dα)ln[Ii/It]
Substances have a specific pattern of absorbing specific wavelength – EXTINCTION COEFFICIENT
Uses two lights of wavelengths660nm –deoxy Hb absorbs ten times as oxy Hb940 nm – absorption of oxyHb is greater
Lab oximeters use 4 wavelengths to measure 4 species of haemoglobin
OPERATING PRINCIPLES
• The pulse oximeter computes the ratio between the two signals and relates this ratio to the arterial oxygen saturation, using an empirical algorithm.
• Pulse oximeters discriminate between arterial blood and other components.
• The oximeter pulses the red and infrared LEDs ON and OFF several hundred times per second
• The rapid sampling rate allows recognition of the peak and trough of
each pulse wave.
• At the trough, the light is transmitted through a vascular bed that contains mainly capillary and venous blood as well as intervening tissue.
• At the peak, it shines through all these plus arterial blood.
• A photodiode collects the transmitted light and converts it into electrical signals.
• The emitted signals are then amplified, processed, and displayed on the monitor.
ACCURACY
• A clinically acceptable level of arterial oxygenation(SaO2 above 70%),the oxygen
saturation recorded by pulse oximeters (SpO2) differs by less than 3% from the actual saturation.
• Pulse oximetry also show a high degree consistency of repeated measurements.
PHYSIOLOGY OF PULSE OXIMETRY• Pulse Oximetry uses light to work
out oxygen saturation. Light is emitted from light sources which goes across the pulse oximeter probe and reaches the light detector.
• If a finger is placed in between the light source and the light detector, the light will now have to pass through the finger to reach the detector. Part of the light will be absorbed by the finger and the part not absorbed reaches the light detector.
The amount of light absorbed depends on the following:
concentration of the light absorbing substance.
length of the light path in
the absorbing substance
TYPES OF OXIMETRYTRANSMISSION PULSE OXIMETRY• light beam is transmitted through
a vascular bed and is detected on opposite side of that bed
REFLECTANCE PULSE OXIMETRY• relies on light that is reflected to
determine oxygen saturation. The probe have the emitters & detectors on the same side.
advantage - its signal in low perfusion is better.
limitations -The probe design must eliminate light that is passed directly to the probe or is scattered in the outer surface of the skin. The signals are weaker.
EQUIPMENT
PROBES
• The probe (sensor, transducer) comes in contact with the patient.
• It contains one or more LEDs (photodiodes) that emit light at specific wavelengths and a photodetector.
• The LEDs provide monochromatic light. • Probes may be reusable or disposable.
• Self-adhesive probes are less likely to come off if the patient moves.
• Probes are available in different sizes.
• The photocell should be aligned with the probe
• Contamination should be reduced.
CABLE• The probe is connected to the oximeter by an
electrical cable.
CONSOLE• Many different consoles are available . Most
oximeters that are used in the operating room are part of a physiologic monitor.
• Most stand-alone units are line operated but will work on batteries, making them useful during transport.
• Most instruments provide an audible tone whose pitch changes with the saturation.
• Alarms are commonly provided for low and high pulse rates & for low and high saturation.
• ASA standards for Basic Anesthetic Monitoring require that the variable pitch pulse tone and low threshold alarm be audible.
OXIMETER STANDARDS
• There must be a means to limit the duration of continuous operation at temperature above 41°C .
• The accuracy must be stated over the range of 70% to 100% SpO2.
• There must be an indication when the SpO2 or pulse rate data is not current.
• It must be provided with an alarm system
• There must be an alarm for low SpO2 that is not less than 85% SpO2 .
• An indication of signal inadequacy must be provided if the SpO2 or pulse rate value displayed is potentially incorrect.
SITES OF PROBE PLACEMENTFingers ToeEar Nose Tongue Cheek Esophagus Forehead
MISC• Pharyngeal pulse oximetry by using a pulse oximeter
attached to a laryngeal mask may be useful in patients with poor peripheral perfusion.
• Flexible probes may work through the palm, foot, penis, ankle, lower calf, or even the arm in infants
• Pulse oximetry may be used to monitor fetal oxygenation during labor by attaching a reflectance pulse oximetry probe to the presenting part . A disadvantage is that the probe has to be placed blindly and may be positioned over a subcutaneous vein or artery, which will affect the reliability of the readings
USES• Monitoring oxygenation:
PACU, transport.• Detect inadvertent bronchial
intubation • Managing one lung
anaesthesia• Weaning from artificial
ventilation• Controlling 02 administration• Monitoring peripheral
circulation• Determining systolic blood
pressure
• Locating vessels ( eg: axillary artery)
• Avoiding hyperoxaemia• Monitoring vascular
volume• Monitoring sympathetic
tone(dicrotic notch)• Pulse rate• Arrhythmias• Neonatal care(one element
of suggesting congenital heart diseases)
ADVANTAGES
• Accuracy• Independence from gases and vapours• Fast response time• Non invasive• Continuous Measurements• Separate Respiratory and circulatory variables• Convenience• Fast start time
• Tone modulation• User –friendliness• Light weight & compactness• No heating required• Battery operated• Probe variety• Economy
LIMITATIONS AND DISADVANTAGESFailure to Determine the Oxygen Saturation Factors that are reported to contribute to higher failure rates
include ASA physical status III , IV or V patients, orthopedic, vascular, and cardiac surgery; electrosurgery use; hypothermia; hypotension; low hematocrit and motion
Poor Function with Poor Perfusion Readings may be unreliable or unavailable if there is loss or
diminution of the peripheral pulse (proximal blood pressure cuff inflation, external pressure, improper positioning, hypotension, hypothermia, Raynaud's phenomenon, low cardiac output, hypovolemia, peripheral vascular disease).
Erratic Performance with Dysrhythmias Double- or triple-peaked arterial pressure
waveform that confuses the pulse oximeter, so it may not provide a reading
Carboxyhaemoglobin COHb has an absorption spectrum similar to
that of 02Hb at 660nm, so most pulse oximeters give falsely elevated readings.
Methaemoglobin Methaemoglobin absorbs light equally at the red and infrared
wavelengths that are used by most pulse oximeters. When compared with functional saturation, most pulse oximeters give falsely low readings for saturations above 85% and falsely high values for saturations below 85% .
Sulfhaemoglobin• Sulfhaemoglobinemia may be caused by drugs such as
metoclopramide , phenacetin, dapsone and sulfonamides. Sulfhaemoglobin causes the pulse oximeter to display artifactually low oxygen saturation
Mixing Probes SpO2 measurements may not be accurate if one
manufacturer's probe is used with a different manufacturer's instrument
Bilirubinemia Severe hyperbilirubinemia can cause an
artifactual elevation of metHb and carboxyhaemoglobin when using in vitro oximetry but does not affect pulse oximetry readings.
Low Saturations Pulse oximetry becomes less accurate at low oxygen
saturations . This inaccuracy is greater in patients with dark skin. It should be used with caution in patients with cyanotic heart disease.
Malpositioned Probe• Prominent pulsations of venous blood may lead to
underestimation of the SpO2. • High airway pressures during artificial ventilation may cause
phasic venous congestion, which may be interpreted by the oximeter as a pulse wave.
Nail polish and coverings• All colour of nail polishes especially black, dark blue & purple may cause
significantly lower saturation readings
Electrical Interference• Electrical interference from an electrosurgical unit can cause the oximeter
to give an incorrect pulse count . • Steps to minimize electrical interference include – keeping the oximeter
probe & console as far from the surgical field as possible; locating electrosurgery ground plate as close as possible to the surgical site ;not plugging in the electrosurgical apparatus and pulse oximeter into the same power circuit
Severe Anaemia -The pulse oximeter may overestimate SpO2, especially at low saturations, in patients with severe anemia.
Skin Pigmentation -pigmentation does not make a significant difference in pulse oximeter accuracy
PROBLEM OF MOVEMENT
• Pulse oximeters are very vulnerable to motion, such as a patient moving his hand. As the finger moves, the light levels change dramatically. Such a poor signal makes it difficult for the pulse oximeter to calculate oxygen saturation.
• Lengthening the averaging time will increase the likelihood that enough true pulses will be detected to reject the motion artifacts.
PROBLEM OF OPTICAL SHUNTING
• If the probe is of the wrong size or has not being applied properly, some of the light , instead of going through the artery, goes by the side of the artery (shunting).
• This reduces the strength of the pulsatile signal making the pulse oximeter prone to errors. It is therefore important to select the correct sized probe and to place the finger correctly in the chosen probe for best results.
PROBLEMS OF TOO MUCH AMBIENT LIGHT
• If the ambient light is too strong, the LED light signal gets "submerged" in the noise of the ambient light. This can lead to erroneous readings. Therefore, it is important to minimise the amount of ambient light falling on the detector.
PROBLEM OF NOT DETECTING HYPEROXIA
• Haemoglobin is not the only way oxygen is carried in blood. Additional oxygen can also be dissolved in the solution in which red blood cells travel (plasma).
• The problem is that the pulse oximeter cannot "see" the extra dissolved oxygen. So even if the patient’s blood contains extra oxygen, the saturation will still show 100 %.
METHODS TO IMPROVE SIGNALS
• Application of vasodilating cream• Digital nerve block• Administration of intraarterial vasodilators• Placing a gloves filled with warm water over patient
hand.• Warming cool extremities• Trying an alternative probe site.• Trying a different probe• Trying a different machine
PATIENT COMPLICATIONS
• Corneal Abrasions
• Pressure and Ischemic Injuries
• Burns
• Electric Shock
CAPNOGRAPHY VS PLETHYSMOGRAPHY –
a comparison & conclusion
• Oxygenation is monitored by pulse oximetry whereas Ventilation is monitored with capnography
• Oximeters measure saturated haemoglobin in peripheral blood and provide additional information about the adequacy of lung perfusion and oxygen delivery to the tissues. However, pulse oximetry is a late indicator of O2 supply, and is less sensitive than capnography. It does not afford a complete picture of ventilatory status.
• Capnography continuously and nearly instantaneously measures pulmonary ventilation and is able to rapidly detect small changes in cardio-respiratory function before oximeter readings change.
• Hypoventilation & hypercarbia may occur without a decrease in Hb O2 saturation, so pulse oximeter cannot be relied on to detect leaks, disconnections or esophageal intubations ; whereas capnography can be reliably used in these conditions.
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