1. Pulse oximetry in general practice By Søren Brorson, MD, GP

2. Introduction to pulse oximetry
Introduced in the early 1980s
Measures the percentage of haemoglobin which is saturated with oxygen (SaO2)
Non-invasive, simple, valid, low-cost method of monitoring the oxygen saturation of patients blood
Detect hypoxia before the patient becomes clinically cyanosed (SaO2<80%)
The pulse oximeter is a vital tool in secondary care
A routine vital sign in the emergency medicine
Provides continuous monitoring of oxygenation in anaesthesiology, critical care and transport
Viability of limbs after surgery e.g. vascular grafting
Overnight sleep studies e.g. obstructive sleep apnoea
Exercise testing e.g. shuttle walk test

3. How does an oximeter work
Consist of a probe which is attached to the patient’s finger and a computerized unit with a display showing SaO2 and pulse rate
Possesses two light-emitting diodes (LEDs), one red and one infrared, and a detector, which derives the oxygen saturation from the intensity of transmitted light during a pulse beat
Plethysmographic waveform display which are useful in assessing the quality of the signal and the effects of artefact
Calibrated during manufacture and automatically check their internal circuits when switched on

4. Pulse oximeter calibrator

5. Limitations of pulse oximetry
The full picture is not provided, only SaO2
Arterial CO2 can rise to dangerous levels with still acceptable oxygen saturations
It does not replace clinical judgement
The gold standard for SaO2 is arterial blood gas analysis (PaO2, PaCO2, acid-base balance), but is invasive, painful, time consuming and costly
Pulse oximeters are most accurate at saturations of 70-99% (+/- 2%)

6. CASE 1 45 year-old man brought in to emergency department
Unconscious for 10 min. during BBQ in the patio where he had set up the grill
Physician finds normal cognitiv function, normal neurology, BT 130/80, normal EKG, SAT 97%
Diagnose?

7. Limitations of pulse oximetry
Pulse oximetry cannot distinguish between different forms of haemoglobins e.g. methaemoglobin and carboxyhaemoglobin absorb light at similar wavelength
Other sources of error lead to signal losses og under-/overestimations of oxygen saturation
Reduced perfusion e.g. cold, cardiac failure
Nail vanish and paint
Ambient light e.g. bright overhead lights
Not SKUB-tested

8. Does pulse oximetry influence the patient managment ?
Mower ét al. Chest 1995
Physicians were significantly more likely to change the medical treatment of patients with SaO2<95% compared with patients with SaO2>95%
The oximetry results altered management because the physicians failed to recognize underlying cardiopulmonary difficulties or because they did not realize the severity of the illness
Anderson ét al. Ped Emerg Care 1991
SaO2 measurements changed the previously assessed degree of illness in 53% of the patients
13% were deemed more ill and 37% less ill than at the initial assessment
17% had their management plan changed, 8% were treated more aggressively and 11% less aggressively

9. CASE 2 5 year-old boy, no history of astma or atopi
2 days with coughing and difficulty in breathing, no fever
A little pale, quiet, tachypnoea, use of accessoric muscles, CRP<10, puls 90, St.p.: rhonchi bilat. SaO2 86%
O2 10 L/min, Ventoline inhalations SaO2 rises to 96%. Admited to hospital.
CXR: Pneumonia dxt.

10. Recognition of hypoxia
Respiratory rate is not a reliable screen for hypoxia and is not closely correlated with pulse oximetry measurements
Studies shows that observers have difficulty detecting hypoxaemia undtil SaO2<80%
In one study the pulse oximeter measured SaO2 in 50 pt.’s in A&E. The oximeter identified 21 pt.’s (42%) with clinically unsuspected hypoxia
Pulse oximeters in GP may have a role in recognizing hypoxia which otherwise may go undetected
- leading to a difference in managment

11. Recognition of COPD Garcia-Pachon, Prim Care Respir J 2004
Early identification of COPD: Although SaO2 levels correlated with FEV1 pulse oximetry is not a useful test for the selection of patients for screening spirometry
With a cut-off value (SaO2<98%) they could detect 79% of COPD patients, with a specificity of 37%
23% with significant COPD (FEV1<50%) had normal SaO2 (SaO2>95%)

12. Screening patients with COPD for LTOT using pulse oximetry
LTOT for > 15 hours a day increase life expectancy and quality of life in COPD patients with servere hypoxia
Over a 12-month period 114 patients with COPD were screened with pulse oximetry in two practices with a combined list size of 15742
13 had SaO2<92% and went to hospital for arterial blood gas analysis
3 had PaO2<7,3 kPa and received LTOT
SaO2<92% is the cutt off point for selecting patients who require arterial blood gas analysis. Sensitivity 100% an specificity 69% in detection of PaO2<7,3 kPa.
Oximetry has a valuable role in the selection of patients who need definitive arterial blood gas analysis

13. Acute exacerbations of COPD
Oxygen saturation measured by pulse oximetry can not replace analysis of an arterial blood gas sample
But pulse oximetry can be an effective screening test for systemic hypoxia (PaO2<8 kPa)
Patients with SaO2<92% should have arterial blood gases preformed
DSAM clinical recommendation on COPD SaO2 <90% => consider hospitalizing

14. Patients with acute respiratory problems or dyspnoea of unknown cause
A supporting tool alongside history and examination when managing patients with acute dyspnoea or dyspnoea of unknown cause in general practice – can be used as a red flag
Routine use of pulse oximetry in patients suspected for pneumonia can detect clinically unrecognized hypoxaemia
A study found that 10% of the patients with a pneumonia where hypoxic
A study concluded that pulse oximetry was not a useful method of excluding pneumonia in infants and should not influence the decision to obtain a CXR

15. Case 3 60 year-old woman, history of astma, visit her GP
2 days with coughing, dyspnoea, pain in the left side of the thorax when coughing
Afebril, normal skin colour, normal heart & lung stethoscopy, puls 76, CRP<10, normal EKG
Diagnose?

16. Future uses of pulse oximetry in primary care
In patients with DM lower-extremity arterial disease is common and under diagnosed
Pulse oximetry of the toes was found to be as accurate as the ankel-brachial index to screen for LEAD in pt.’s with DM
Pulse oximetry of the toes was considered abnormal if the SaO2 was more than 2% lower from the finger
A combination of the two test was found to increase sensitivity from PO 77% (ABI 63%) to 86%

17. Conclusion
Non-invasive, simple, valide, low-costed method of monitoring SaO2
Oxymetry in primary care help in the assessment of hypoxia and in identifying unsuspected hypoxia
It does have numerous indications
Assessment for LTOT in patients with COPD (SaO2<92%)
Exacerbations of COPD (obs SaO2<92%)
Acute severe asthma in children and adults
Other acute respiratory problems e.g. pneumonia
Dyspnoea of unknown cause
Pulse oximetry has some limits
Cannot distinguish between different forms of haemoglobins e.g. carboxyhaemoglobin
Reduced perfusion e.g. cold, cardiac failure
Nail vanish and paint