Monitors Flashcards
What are the standard ASA monitors?
Pulse Ox Capnography O2 analyzer Vent disconnect alarms Body temperature EKG
When is continuous ETCO2 monitoring required?
GA
Moderate sedation
Deep sedation
When is EKG monitoring required?
ALL anesthetics
What is the Beer-Lambert law?
the intensity of transmitted light is inversely proportional to the concentration of the substance through which the light passes
More decay through more dense tissue
What is functional saturation (SpO2)?
SpO2 = O2Hb / (O2Hb + RHb) x 100%
RHb = deoxyhemoglobin, 940nm O2Hb = oxyhemoglobin, 660nm
What is fractional saturation (SaO2)?
SaO2 = O2Hb / (O2Hb + RHb + carboxyhemoglobin (COHb), + methemoglobin (MetHb)) x 100
requires more wavelengths of light to detect different types of hemoglobin and is typically performed by a laboratory co-oximeter on an arterial blood gas
By what 2 principles does a pulse oximeter report SpO2?
plethysmography
absorption spectrophotometry
With normal hemoglobin at an SpO2 of 90%, what is the PaO2?
60 mmHg
What are the x and y axes of the plethysmography waveform?
y = light absorption x = time
What factors affect pulse ox:
- signal strength
- pulse amplitude
- probe application
Signal strength - poor perfusion, high venous pressures, electromagnetic interference, or poorly applied probes.
Pulse amplitude - reduced by decreased stroke volume, hypothermia, occlusion, and/or vasopressors.
Probe application - affected by location, edema, motion, or light interference.
Pulse oximeters can’t distinguish between normal and abnormal hemoglobins. Explain what happens with:
- COHb
- MethHb
- SulfHb
- Sickle cell disease, fetal Hb
- Hct
COHb absorbs light at 660nm –> falsely elevated SpO2
MetHb absorbs light at both 660 and 940 nm –> saturation to trend towards 85%, which does not reflect the true O2Hb saturation.
SulfHb –> falsely high MetHb measurement and thus cause a falsely low SpO2.
Sickle cell –> resulting in a rightward shift of the O2Hb dissociation curve but SpO2 readings remain accurate.
Fetal hemoglobin = do not alter pulse oximetry reading
Anemia with a hematocrit underestimation error.
How do the following affect SpO2 readings:
- indocyanine green, indigo carmine, and methylene blue
- Fluorescein and bilirubin
- bright artificial ambient light
- temporary falsely low SpO2 readings
- do NOT cause SpO2 changes, as they do not absorb light at 660 or 940nm.
- SpO2 may be falsely high when ambient light flicker frequency matching diode flashing frequency
What is the formula for mean arterial pressure (MAP)?
MAP = DBP + 1/3 (SBP-DBP)
What is the appropriate width and length of a blood pressure cuff bladder?
width ~40-50% the circumference of the extremity
length - strap section should be long enough to overlap securely
What correction factor is used when the site of BP measurement is above or below the heart?
adding or subtracting 0.7 mmHg for each centimeter (approximately 2 mmHg for each inch) that the cuff is above or below the level of the heart
What types of errors result in falsely elevated or falsely low blood pressure readings?
falsely high
- cuff is too small
- loosely applied
- extremity is below the level of the heart
falsely low
- cuff is too large
- extremity is above the level of the heart
- cuff is deflated too rapidly
What blood pressure can be measured using the palpation technique?
Systolic blood pressure can be determined by palpating when a peripheral pulse is reestablished after it is occluded by a cuff
tends to underestimate SBP bc of the insensitivity of touch and the delay b/w flow under the cuff and distal pulsations
Describe how an oscillometric NIBP cuff works
sensor detects oscillations produced by movement of the arterial wall
MAP = directly measured = maximum amplitude signal (most accurate)
systolic and diastolic pressures are calculated by various algorithms that vary from manufacturer to manufacturer from the MAP
systolic pressure = onset of oscillations
diastolic pressure = offset of oscillations
What is the most important factor in obtaining an accurate reading from an oscillometric NIBP cuff?
WIDTH ~40-50% the circumference of the extremity
As an NIBP cuff is placed progressively more distally on an extremity, what happens to the reported values for systolic and diastolic blood pressure?
systolic pressure increases
diastolic pressure decreases
MAP relatively unchanged
Which reported value from an oscillometric NIBP cuff is most affected by rhythms other than normal sinus?
Atrial fibrillation - actual systolic, mean, and diastolic pressures vary with each heartbeat
NIBP assumes uniform pressure with each beat
Partial compensation - more frequent measurements, e.g. averaging 2 or more back-to-back measurements.
What two wavelengths of light are most commonly used in pulse oximetry?
Red light (660 nm) is well absorbed by oxyhemoglobin Infrared light (940 nm) is well absorbed by deoxyhemoglobin
What are some factors that can interfere with the accuracy and reliability of pulse oximetry?
inaccurate at low SpO2 dyshemoglobins dyes (methylene blue, indocyanine green, and indigo carmine) nail polish ambient light light-emitting diode variability motion artifact low perfusion states (e.g., low cardiac output, profound anemia, hypothermia, increased systemic vascular resistance) malpositioned sensor venous pulsations in a dependent limb bypassing the arterial bed background noise
Which dyshemoglobinemia would result in an Sp02 of 85%?
Methemoglobinemia
- absorbs red and infrared wavelengths of light in a 1:1 ratio corresponding to an SpO2 of approximately 85%
- cyanosis resolved w/in 15-30 min and substantial inc in SpO2 seen in 30-60min
