CAPNOMETRY AND PULSE OXIMETRY

CAPNOGRAPHIC DEVICES

Infrared Absorption Photometry Molecular Correlation Spectrography Colorimetric Devices Mass Spectrometry Raman Scattering

INFRARED

First developed in 1859 Based on the Beer-Lambert law, which describes

the absorption of infrared light by CO2 The higher the CO2 concentration, the higher the

absorption N2O, H2O, and CO can also absorb infrared light at

the wavelength used Two types: mainstream and side stream More compact and less expensive than the other

types of capnometers Requires sampling gas flow of ~150ml/min thru the

unit

SIDE STREAM

Gas is sampled through a small tube that pulls it out of the main gas stream

Analysis is performed in a separate chamber Very reliable Time delay of 1-60 seconds Less accurate at high rates Sampling tube is prone to plugging by water/secretions Ambient air leaks affect reading Connector is lightweight and doesn’t pull on airway Easy to use when patient is in an unusual position, such as

prone

MAINSTREAM

Sensor is located in the airway Response time as quick as 40 msec Very accurate Difficult to calibrate without disconnecting Reading more prone to being affected by moisture Larger and heavier than sidestream…can kink the

ETT Adds deadspace to the airway Bigger chance of being damaged by mishandling Sensor window can be clogged with secretions Difficult to use in unusual positions, such as prone

Molecular Correlation Spectrography Uses an infrared emission that precisely

matches the absorption spectrum of CO2 Allows for the use of very small samples at

very low flow rates Samples are measured every 25 msecs and

uses a flowrate of 50 ml/min

COLORIMETRIC

Contains a pH sensitive dye which undergoes a color change in the presence of CO2

The dye is usually metacresol purple and it changes to yellow in the presence of CO2

Portable and lightweight Low false positive rate…higher false negative rate Acidic solutions (eg-lidocaine, epi, atropine) will

permanently change the color Deadspace high for a neonate – can’t use for long

MASS SPECTROMETRY

Separates and counts ionized molecules to determine the concentration of gas

A gas sample is aspirated into a vacuum chamber when an electron beam ionizes and fragments the components of the sample

The ions are accelerated into a final chamber which has a magnetic field that allows for determination of the components of the gas and the concentration of each component

Very expensive and bulky, but have the advantage of being able to monitor multiple patients at a time (eg-OR)

RAMAN SCATTERING

Raman scattering occurs when light hits a molecule and it scatters the light…most of the scattered light is the same wavelength as the laser source, but a small amount of light scattered is at a different wavelength

The different wavelength produced gives information about the molecule

An argon laser is shone through a gas sample and the CO2 in the sample will scatter it…the amount of scattering is related to the CO2 level

NORMAL CAPNOGRAM

Phase I: the beginning of exhalation…CO2 level is zero

Phase II: alveolar gas begins to mix with the deadspace gas and the CO2 rises rapidly

Phase III: elimination of CO2 from the alveoli…usually has a slight upward slope

Phase IV: end exhalation Phase 0: inspiration

THE NORMAL CAPNOGRAM

ABNORMALITIES

Increased Phase 3 slope: Obstructive lung dx

Phase 3 dip: Spont resp Curare cleft

Horizontal Phase 3 with large ET-art gradient: Pulm. Embolism Decreased CO hypovolemia

Sudden decrease to 0 Ventilator malfunction ETT disconnect ET obstruction Extubation

Sudden decrease Partial obstruction Air leak

Exponential decrease Severe hyperventilation CP event

ABNORMALITIES

Gradual decrease Hyperventilation Decreased T Gradual decrease in

volume Sudden increase

Bicarb administration Release of limb

tourniquet

Gradual increase Fever Hypoventilation

Increased baseline Rebreathing CO2 Exhaused CO2 absorber

PaCO2-PetCO2 GRADIENT

Usually <6 mm Hg PetCO2 is usually less than arterial Difference depends on the number of

underperfused alveoli Tend to mirror each other if the slope of

Phase 3 is horizontal or minimal Decreased CO will increase the gradient

LIMITATIONS

Critically ill patients often have rapidly changing deadspace and V/Q mismatch

Higher rates and small Vt can increase the amount of deadspace ventilation

High mean airway pressures and PEEP restrict alveolar perfusion leading to falsely decreased readings

Low CO will decrease the reading

USES

Metabolic Assess energy expenditure

Cardiovascular Monitor trend in cardiac output Can use as an indirect Fick method Measure of effectiveness in CPR

Diagnosis of pulmonary embolism: measure the gradient

PULMONARY USES

Effectiveness of bronchodilator therapy Monitor gradient Worsening indicated by rising Phase 3 w/o

plateau Find optimal PEEP by following the gradient

…should be lowest at optimal PEEP level Can predict successful extubation…Vd/Vt >

0.6 predicts failure Limited pulm usefulness if CV unstable

CAPNOMETRY

Measures and displays a numerical value of the CO2 level 30-43 mm Hg 4.0-5.6%

ESOPHAGEAL INTUBATION/ DISCONNECTION FROM VENTILATOR/TOTALLY OBSTRUCTED ETT

Air Leak/Loose Connection between sampling tube and capnograph

Increasing Temperature/Metabolism

Hypothermia/Reduced Metabolism/ Hyperventilation/Decreased CO Cause a gradual decrese in end-tidal CO2

Cardiac Oscillations

Bronchospasm/COPD/obstructed ETT Slanting and prolonged phase 2 and

increased slope of phase 3 Sometimes there’s a reverse phase 3 slope

seen in patients with emphysema

Ventilator IMV breath during spontaneous ventilation

Sticking Inspiratory Valve

Hypoventilation

Leak/Partial Disconnect in Circuit/ETT too high

Pulmonary Embolism/Pneumonia/ Hypovolemia/ Hyperventilation

Curare Cleft

Spontaneous Breathing

Rebreathing of CO2

CAUSES OF INCREASED PetCO2 Increased CO2 production and delivery to the lungs

Fever Sepsis Bicarb administration Increased metabolic rate Seizures

Decreased alveolar ventilation Respiratory center depression Muscular paralysis Hypoventilation COPD

Equipment malfunction Rebreathing Exhausted CO2 absorber Leak in ventilator circuit

CAUSES OF DECREASED PetCO2 Decreased CO2 production and delivery to the lungs

Hypothermia Pulmonary hypoperfusion Cardiac arrest Hemorrhage Hypotension

Increased alveolar deadspace Decreased CO Pulmonary embolism High PEEP levels

Increased alveolar ventilation Hyperventilation

Equipment malfunction Ventilator disconnect Esophageal intubation Complete airway obstruction Poor sampling Leak around ETT cuff Water in sampling line Air entrainment into sampling line Inadequate tidal volume

CAUSES OF INCREASED P(a-et)CO2 Pulmonary hypoperfusion Pulmonary embolism Cardiac arrest Positive pressure ventilation, especially with

PEEP High rate/low tidal volume ventilation

PULSE OXIMETRY

Uses spectrophotometry based on the Beer-Lambert law

Differentiates oxy from deoxy Hb by the differences in absorption of light at 660 nm and 940 nm

Minimizes tissue interference by separating out the pulsatile signal

Estimates HR by measuring cyclic changes in light transmission

Estimates functional Hb by comparing amounts of oxy and deoxy Hb

SOURCES OF ERROR

Sensitive to motion Sats below 85% have increased error Calibration is performed by company on

normal patients breathing various gas mixtures, so cal is accurate only down to 80%

Low perfusion state increases error Ambient light interferes with reading Delay in reading of about 12 seconds

SOURCES OF ERROR

Skin pigmentation Darker color may make the reading more variable

due to optical shunting Dark nail polish has the same effect, especially

black, blue, and green…red is OK Hyperbilirubinemia has no effect

Methylene blue and indigo carmine (dyes) cause underestimation of the saturation

SOURCES OF ERROR

Dysfunctional hemoglobin Carboxyhemoglobin leads to overestimation of

sats because it absorbs at 660 nm like oxyHb does

MetHb can mask the true saturation because it absorbs at both wavelengths used…sats are overestimated