pulse oximetry in general practice by s ren brorson md gp l.
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Pulse oximetry in general practice By Søren Brorson, MD, GP. Introduction to pulse oximetry. Introduced in the early 1980s Measures the percentage of haemoglobin which is saturated with oxygen (SaO2)

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introduction to pulse oximetry
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
how does an oximeter work
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
limitations of pulse oximetry
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%)
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?
limitations of pulse oximetry7
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
does pulse oximetry influence the patient managment
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
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.
recognition of hypoxia
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

recognition of copd
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%)
screening patients with copd for ltot using pulse oximetry
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

acute exacerbations of copd
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
patients with acute respiratory problems or dyspnoea of unknown cause
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
case 3
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?
future uses of pulse oximetry in primary care
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%
  • 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