Overnight Oximetry Flashcards
What’s the difference between type 1 and type 2 polysomnography?
Type 1 = attended sleep study
Type 2 = unattended sleep study
What’s the difference between type 3 and type 4 limited sleep studies?
Type 3 = at least 4 channels monitored
Type 4 = Only 1 or 2 monitored parameters
What is the clinical use of overnight oximetry?
A continuous overnight measurement during sleep, measuring oxygen saturation (SpO2) & pulse rate
Give 3 uses of OO?
1 –> Determine Frequency of O2 desaturations and SpO2
2–> Give information on pulse rate (marker for sleep arousal)
3–> Evaluation of treatments e.g. CPAP
How do you perform OO?
- Performed at home (commonly)
- Need correct size probe and correct placement
- Aware of limitations like varnish etc
- Use Instruction sheet
- Set up Oximeter with date/time/battery/clean
- Cover probe with opaque material elimination environmental light
- Taping lead to back of hand helping securing device in place
- DO NOT TAPE OVER PROBE
- Sleep Questionnaire
Which patients wouldn’t be suitable for OO?
- Severe Intellectual Disabilities
- Communication difficulties
- Physical difficulties
- Unstable home
- Discretionary (serious medico-legal)
- More extensive studies needed
Advantages of OO?
- Easy to perform
- Can be done at home
- Cheaper
Disadvantages of OO?
- Normal Study cannot exclude Sleep Apnoea
- Cannot distinguish between OSA and CSA
- Patient Co-operation
What is the ODI?
Oxygen Desaturation Index (ODI)
Define ODI?
- The number of 4% desaturations per hour of sleep
- Sometimes called RDI (Respiratory Disturbance Index)
How do we calculate ODI?
ODI =
Total no. of desaturations) / (sleep duration in hours
A worked example of ODI?
So the patient had 35 desats
and slept for 7 hours
so ODI = 35/7 = 5
units are events per hour
so 5 events per hour
What is a normal ODI?
Mild –> 5-14 events per hour
Moderate –> 15-20 events per hour
Severe–> 30+ events per hour
What are the essential features for screening equipment for OSAHS?
- Sampling Frequencies for each signal must be appt
- Portable
- Battery Powered or Rechargeable
- Data must be downloadable and editable
- Easy to clean/decontaminate
- Comply with Intl/EU standards
What is the minimal sampling frequency?
4Hz
Why can inappropriate sampling frequency lead to?
underestimate of desaturation events
What is Sample Frequency?
The number of samples per second in a sound
What is Sample Frequency measured in?
Measured in Hertz (Hz) – a unit of frequency
What does minimal Samp frequency actually mean in terms of per second?
–> minimum sample frequency for SpO2 (4Hz)
–>1 Hz = 1 sample per second
–>4 Hz = 4 samples per second
–>Sample time therefore is reduced to 0.25 sec
Relationship between SF and time?
↑sample freq = ↓ time
as in a higher frequency means there is less time between every sample
What is data averaging?
- the average of repeated SpO2 readings over a specific time interval
What is the maximum averaging times?
-3s
What is the protocol for negative studies?
- must be investigated further using a multi-channel screening system”
What is the 4 steps to interpret Interpretation and Pulse Oximetry traces?
- Quality of data (artefact/length)
- Interpretation (Look at traces/data)
- Link report to patient/referral
- Recommendations/Further actions
–> Services should have SOP’s for reporting
What can OO be important for?
Patients with suspected or known nocturnal hypoventilation.
What is nocturnal hypoventilation?
When level of alveolar ventilation is insufficient to meet metabolic needs
-Seen by a rise in CO2 (hypercapnia)
Go over interpretation of OO
Paper Copy of Slides
