Capnography And Pulse Oximetry Flashcards
Oxygen dissociation curve landmarks
PaO2 of 60mmHg = SaO2 of 90%
PaO2 of 40mmHg = SaO2 of 75%
The P50 is 27mmHg. The Hb has a saturation of 50% at a PaO2 of 27mmHg
Oxyhemoglobin dissociation curve; affinity
The strength of noncovalent binds between two substances, as measured by the dissociation constant of the complex
Oxyhemoglobin dissociation curve; left shift
Increased affinity for O2
Low pCO2, H+, 2,3-DPG, temp
Oxyhemoglobin dissociation curve; right curve
Decreased affinity for O2
High pCO2, H+, 2,3-DPG, temp
Low pH
Haldane effect
Describes the ability of hemoglobin to carry increased amounts of carbon dioxide in the deoxygenated state
A high concentration of CO2 facilitates dissociation of oxyhemoglobin
Bohr effect
An increase in CO2 results in a decrease in blood pH, resulting in hemoglobin releasing their load of oxygen
A decrease in CO2 provokes an increase in pH, which results in hemoglobin picking up more oxygen
Three types of pulse oximetry
-Photoelectric (optical) plethysmography
-Spectrophotometry
-Light emitting diodes (LEDs)
Machine can determine a baseline absorption
Tissue, venous blood flow
Dependent on pulsatile flow
Absorption changes due to increased arterialized blood flow, determine baseline absorption, subtracting this baseline, the machine is able to determine absorption by arterial blood
Photoelectric (optical) plethysmography
-Uses light absorption to produce waveforms from the blood pulsating in the vascular beds
-The amount of light absorbed is proportional to amount of blood flow
-Maximum absorption during systole and minimum during diastole
Spectrophotometry
-uses light wavelengths to measure light absorption through a substance, in our case blood
-the two wavelengths that are used in pulse oximetry are; red light - wavelength of 660nm, infrared light - wavelength of 920nm
Light emitting diodes (LED)
2 types of oximeter sensors are used; transmission, reflective
Measurement in pulse oximetry
-The saturation that we see is calculated
-This is called a functional saturation
-Functional saturation is the ratio of HbO2 to the total Hb available for binding with O2
Indications for using oximetry
-Noninvasive
-For continuous monitoring of arterial oxyhemoglobin saturation
-To monitor the adequacy of oxyhemoglobin saturation
Advantages of pulse oximetry
-Noninvasive
-Saturations can be monitored continuously at the bedside in real time
-Little training or knowledge is required to use the equipment
-Safe and usually quite accurate
Disadvantages of pulse oximetry
-Abnormal forms of Hb can not be measured
-HbCO is measured as HbO2 therefore false SpO2 readings
-Not as accurate as co-oximetry
Appropriate oximetry sites
-Finger, toe
-Ear
-Bridge of nose
-Forehead
-Infant; across the foot or hand
Factors that effect the accuracy of pulse oximetry
-Motion
-Low perfusion
-External lights
-False nails / nail polish
-Wring types of sensor or incorrect placement
-Dysfunctional Hb, anemia
-Vascular dyes
Accuracy of pulse oximetry
-Accurate witching 2-4% until SpO2 of 80
-False high readings with SpO2 <80
PaCO2
Partial pressure of CO2 in arterial blood
EtCO2
Measurement of the concentration of CO2 at the end of exhalation
A-ADCO2
Difference between ETCO2 and PaCO2 (normally 2-5 mmHg)
Capnometry
Measurement and the numerical display of CO2 at the patient’s airway
Capnography
Measurement and waveform display of CO2 concentration at the patient’s airway
Capnogram
Waveform display of CO2 throughout respiration
Indications for ETCO2; intubated applications
-verifying ETT placement, monitoring ETT position during transport, head injury, CPR (effectiveness of cardiac compressions, earliest sign of ROSC, predictor of survival)
Indications for ETCO2; non-intubated applications
-Bronchospastic disease; asthma, COPD
-Hypoventilation states
-Shock states; sepsis, hypovolemia, anaphylaxis, cardiogenic
-Hyperventilation
Types of ETCO2 monitoring devices
-Spectrophotometry
-Colorimetric
-Mainstream
-Slidestream
-Microstream
ETCO2 Spectrophotometry
Infrared absorption
Increased CO2 = increased absorption
ETCO2 mainstream
-In-line with subjects exhaled gas via ETT
-Can pull on the airway (heavy)
-Slight increase in dead space
ETCO2 slidestream
-Sample rate -150mL / min
-Slight lag in response time
-Slight increase in dead space
-Sample line can occlude with condensation
ETCO2 microstream
-Newer units have a reduced sample rate - 50mL / min; reduces secretion aspiration
-Smaller bore sample line; quicker response time
-Incorporate nafion tubing to minimize H2O content
-Small sample cell (15uL); quick response time
ETCO2 Capnogram; phase 1
Respiratory baseline
-Flat
-No CO2 present
-Corresponds with late inspiratory / early expiration part of respiratory cycle
ETCO2 Capnogram; phase 2
Respiratory upstroke
-Mixture of dead space and alveolar gases
ETCO2 Capnogram; phase 3
Respiratory plateau
-Represents air from ventilated alveoli
-Nearly constant CO2 level
-Highest point = ETCO2
-Recorded by capnometer
ETCO2 Capnogram; phase 4
Inspiratory phase
-Sudden downstroke to baseline as atmospheric air is inspired
Normal values
PaCO2: 35-45 mmHg
ETCO2: 35-45 mmHg
Difference between PaCO2 and ETCO2 is approx 2-5 mmHg
Causes of Low ETCO2
-Mechanical; circuit disconnect, leaks
-Respiratory; airway obstruction, Bronchospasm, displaced ET tube, hyperventilation, mucous plug
-Circulatory; cardiac arrest, embolism, sudden hypovolemia
-Metabolic; hypothermia
Causes of High ETCO2
-Mechanical; excessive mechanical dead space, faulty valve
-Respiratory; COPD, respiratory depression or insufficiency
-Circulatory; increased cardiac output
-Metabolic; hyperthermia, malignant hyperthermia
Verifying ETT placement
-Sensitivity and specifics are up to 100%
-Can’t be used to detect bronchial or some hypopharyngeal misplacements
-Time to detect esophageal intubation using capnography; 1.6 +/- 2.4 sec
-Time to detection of esophageal intubation using clinical signs; 97 +/- 92 sec
ETCO2 during CPR
-Square box waveform
-ETCO2 10-15 mmHg (possibly higher) with adequate CPR
-Change rescuers if ETCO2 falls below 10 mmHg
-Will detect ROSC
