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

1. Pulse oximetry in general practiceBy 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