Oxygen Measurements + Oximetry Flashcards
Define Oxygen capacity
the max quantity of O2 which can combine with Hb in a unit volume of blood
Normal Hb levels for male:
160g/Litre
Normal Hb levels for female:
135g/Litre
How much can 1g of Hb combine with O2
1.34ml of O2
maximum amount body can carry
201ml/L
Explain Oxygen saturation
(SO2)
when O2 is fully saturated, 4 binding sites on HB all have O2 bound to them (100)%
normal O2 sat is 95-98%
Can be measured by an oximeter
What is SaO2 and SpO2
arterial oxygen saturation
peripheral oxygen saturation
Define oxygen content
is the total volume of oxygen carried by the blood
units ml per litre
Oxygen content equation
O content= o2 carried by Hb + O2 in solution
Calculate CaO2
(1.34xhb)x SaO2/100 +(0.225 xPaO2)
=201+3
=204ml of O2 per litre of blood
Factors affecting O2 content
- partial P of inspired O2
- efficiency of ventilation and gas exchange
- Hb level and affinity of Hb for O2
PaO2 (partial arterial pressure) determines o2 saturation of Haemoglobin
anemia:
The partial pressure of O2 is normal, low hemoglobin, has normal oxygen saturation, as sites that are available are saturated, just less sites for binding
Define hypoxia, hypoxemia and cyanosis
Hypoxemia- lack of O2 in the blood
Hypoxia- reduced O2 to the tissues
Cyanosis- blue coloration of skin and mucus membranes resulting from raised levels of deoxyhemoglobin caused by hypoxemia
Causes of Hypoxemia:
- Hypoventilation (not breathing efficiently): causes raised CO2 in the blood (hypercapnia)
- diffusion limitation: e.g., lung fibrosis, problem getting O2 across alveoli membrane, CO2 is more soluble
- Shunt: blood that has gone through the lungs, but has not been ventilated
- V/Q mismatch: Ventilation and perfusion,
diseases cause large mismatch causing so areas to not be ventilated, but still perfused, leading to hypoxemia
pH of blood:
7.35-7.45
Standard partial pressure of CO2
4.8-6.0 kPa
standard Partial pressure of O2
10.0-13.5
Standard O2 saturation
95-98%
Standard HCO3- concentration
22.0-26.0 mmol/L
What is respiratory failure?
Where the patient is severely hypoxemic, PaO2 below 10kPa, below 8kPa is called respiratory failure
Vital signs associated with oximetry
Temperature
Oxygen saturation
Pulse
Blood pressure
Respiratory rate
What does arterial blood measure?
pH, PaO2, PaCO2, SaO2%, and HCO3- concentration
as well as creatine, hb, electrolytes, Hb, lactate and glucose levels
How is arterial blood received and how is it analyzed
Arterial blood is collected from an artery in the wrist
It is measured by a blood gas analyzer
Disadvantages of arterial blood gas sampling
The machines are very expensive and do not give instant results. There is a delay between taking the blood and analysing the results.
Individual components are costly,
For patients, it is an invasive and painful process
It needs to be carried out by specially trained staff
Define and explain earlobe capillary blood gas sample
A sample of blood is taken from the earlobe, using a scalpel and a capillary tube. The ear needs to be well perfused (rubbed) to circulate blood flow, rub with a paper towel
Advantages of the earlobe gas sample
It is less invasive and less painful
Disadvantages of earlobe gas sampling
Needs specially trained staff
It is slightly painful
Not the most accurate form of blood sampling, tend to give an underestimate of O2 saturation,
Too much compression may cause interstitial fluid to be collected alongside blood
What is a pulse oximeter
It consists of a small probe which measures O2 saturation and pulse rate, it is not invasive nor painful
Abnormal O2 saturation range
94-98%
If it falls 55% or lower then..
Person is at risk of losing consciousness, this could lead to death, brain damage or organ failure
oxygen saturation target for respiratory patients
88-92%
Explain type 1 respiratory failure
This is a failure in oxygenation, there is low levels of oxygen in the blood with normal/ reduced PaCO2
Explain type 2 respiratory failure
This is a failure in ventilation, this involves hypoxemia as well as hypercapnia, as movement of air out of the lungs is reduced, prevents CO2 from being eliminated from blood, so builds up, affecting blood pH
Advantages of oximeters?
Non-invasive
not painful
no specialist training needed
cheaper
easier to use
compact and portable
can be used for continuous monitoring
When would pulse oximeters be used?
Pre-op
acute emergencies
Hypoxia challenges
excercise and sleep studies
spot check measurements
What is a spot check oximeter measurement?
used for a single test, measures O2 saturation and pulse rate. It can sometimes show us volume changes in blood,
What is a bedside oximeter needed for?
Needed for continuous monitoring e.g., overnight
What is a recording oximeter used for?
To measure oxygen saturation and pulse rate, over a period of time as it is able to store data, used mostly in sleep and exercise studies
Limitations of Oximetry
Only measures O2 sat & PR, ignores pH, PaO2, PaCO2, HCO3- conc, Hb levels, can lack accuracy and precision as oximeters differ between manufacturers
May suffer electromagnetic interference and light interference
Artefacts may be produced if patient it moving
Cannot distinguish between HbO2 and HbCO, this may give a falsely high reading, as CO binds better than O2,
Skin pigmentation may affect reading, those with darker skin may get a false reading
Probe applied incorrectly e.g., upside down
How to perform an oximetry
- wash your hands & apply PPE, introduce yourself, give patient details and check their identity, explain the procedure
- obtain verbal consent & allow the patient to sit comfortably for 5+ mins
- remove any nail polish, rings etc from the left index finger
- make sure hands are warm & well perfused
- Select probe and apply it correctly, note down date, time & site of probe
- If patient on O2, record flow rate & make note of it
- check pulsatile signal, if abnormal check patient’s pulse on the wrist
- compare against blood gas sample, allow reading to stabilize
- record results, remove probe, clean probe
- tell patient results & produce a written report which includes: date, time, your name & title, type of oximeter, SaO2 and pulse rate
