Assessment of Hypoxaemia Dr. Immaculate Kariuki Consultant Paediatrician Nairobi, Kenya

Intended Learning Outcomes To understand what hypoxaemia is To be familiar with assessment of severity of hypoxaemia To gain an understanding of how to interpret arterial blood gas analysis results

What is hypoxaemia? Normal oxygen saturations 94-100% on room air Hypoxia is defined as saturations below 92 % Hypoxaemia is defined as a PaO2 <80mmHg calculated from an arterial blood gas (especially necessary for critically ill patients) A common complication of LRTI present in up to 2/3 of patients admitted with pneumonia

Why assess for hypoxaemia? Uncorrected hypoxaemia and acidosis set the stage for respiratory failure and death Clinical signs of pneumonia identify hypoxia poorly and cyanosis is a late (terminal) sign It guides decisions on oxygen therapy before irreversible changes occur and decide on addition of steroids for PCP Degree of hypoxaemia used to classify PCP severity WHO recommends use of pulse oximetry to identify hypoxia in patients with pneumonia and manage with oxygen accordingly Sources: 2. Subhi et al.The prevalence of hypoxaemia among ill children in developing countries: a systematic review Lancet Infect Dis, 9 (2009), pp. 219–227 3.WHO Pocket book of hospital care for children: guidelines for the management of common illnesses with limited resources (2nd edn.), World Health Organisation, Geneva (201

Clinical assessment only: poor predictor of hypoxia Does this infant have PCP? (WHO advice) Mwaniki et al .Bull World Health Organ 2009;87:263–270

How to assess for hypoxia: Pulse oximetry http://www.who.int/patientsafety/safesurgery/pulse_oximetry/who_ps_pulse_oxymetry_tutorial2_advanced_en.pdf

Is anything wrong with these scenarios?

Where is the problem?

Pulse oximetry reduces mortality Children . Oxygen therapy determined using clinical signs only. 703 children. Oxygen therapy determined using pulse oximetry Mortality 46 (6.5%) Mortality 26 (10%) MORTALITY RISK RATIO 0.65 ( 0.41-1.02) p=0.07 Duke Tet al Int. J. Tuberc. Lung Dis ;5: 511–519.

Blood gas analysis NORMAL ARTERIAL BLOOD VALUES Pa O2 : 75-100mmHg : 10.5-13.5 kPa Pa CO2: 35-45mmHg : 4.5-10.5kPa pH : 7.38 - 7.42 O2 sats: 94 - 100% HCO3: 22 - 28 mEq/L

A-a gradient Calculation Difference between a calculated alveolar PO2 (PAO2) and arterial oxygen levels (PaO2) read from the blood gas analysis. The calculated PAO2 is derived from: PAO2    = (Patm - Pwater) FiO2 - PaCO2/.8 where      Patm = the atmospheric pressure (760 mm Hg)* Pwater = the vapor pressure of water at body temp (47 mm Hg)* FiO2 = the fraction of O2 in the inspired gas (21% on room air) PaCO2  = the partial pressure of CO2 in arterial blood (from ABG) O.8    = respiratory quotient * at sea level

Assessment of severity using blood gas analysis Normal A-a gradient of 5-7mmHg Mild to moderate PCP PaO2 above 70mmHg on room air A-a gradient ≤ 35mmHg Moderate to severe PCP PaO2 below 70mmhg on room air A-a gradient ≥ 35mmHg

Summary Hypoxia is a common and serious complication of PCP in children Clinical signs alone predict hypoxia poorly Pulse oximetry helps identify hypoxia and need for oxygen Where available, blood gas analysis may be used to classify PCP severity