Tissue Oxygenation Assessment

TYPE OF HYPOXIA

• Hypoxaemic – low oxygen tension
• Stagnant – reduced tissue blood flow
• Anaemic – low Hb
• Histiotoxic/cytopathic – abnormal cellular oxygen utilisation

OXYGEN TOXICITY

• cell dysfunction from high oxygen tension

OXYGEN CASCADE

OXYGEN DELIVERY MEASUREMENT

Inspired O2 (PiO2)

• PiO2 = FiO2 x (barometric pressure – saturated vapour pressure of H20)
• PiO2 = 0.21 x (760 – 47) – sea level
• PiO2 = 150mmHg
• gas supply pressures are continuously measured
• FiO2 is monitored within the inspiratory limb of ventilators
• Note that gas supply may vary in extent of humidification, however the inspired gas will be humidified (ideally close to 100%) during passage to the alveoli through the respiratory conduction system

PiO2 to Alveoli

• Open communication between inspired gas supply and the alveoli is indicated by:

1. absence of upper airway obstruction
2. expired tidal and minute volumes and airway pressures for the ventilated are within correctly set alarms
3. an appropriate end-tidal CO2 waveform

Alveolar Gas (PAO2)

• marked variation in PO2 between lung units makes measurement of ETO2 pointless
• clinical assessment chest movement, air entry and CXR
• electrical impedance tomography: non-invasive tracking of lung volume changes
• assessment of V/Q mismatch: Multiple Inert Gas Technique (MIGET) -> impractical, FiO2 to SpO2 is more practical
• The ‘short’ form of the alveolar gas equation is: PAO2 = PiO2 – PaCO2/0.8
  • where PiO2 = (Patm – PH2O) x FiO2
    • PiO2 is the partial pressure of oxygen inspired by the lung alveoli
  • the ‘long form’ is PAO2 = (Patm – PH2O) x FiO2 – PaCO2/RQ+ f
    • Patm is atmospheric pressure
    • PH20 is the water vapour pressure (47mmHg if 100% humidification at body temperature, 37C)
    • FiO2 is the fraction of inspired oxygen
    • PaCO2 is the arterial partial pressure of CO2
    • RQ is the respiratory quotient (usually 0.8 but can , and vary with diet and metabolic state)
    • F is a commonly ignored correction factor of 2-3 mmHg accounting for changes in the partial pressure of nitrogen.

Alveolar Gas to Arterial Blood (PAO2 -> PaO2)

• A-a gradient
• PaO2/FiO2 ratio
• venous admixture (Qs/Qt): use of the shunt equation, requires a PAC

Arterial Blood

• blood gas analysis and co-oximetry
• continuous intra-arterial blood gas monitoring: invasive, not validated yet, accurate
• transcutaneous PO2 and PCO2 monitoring: reliable CO2, not so accurate O2, heat skin -> burns, regular calibration required
• pulse oximetry
• haemoglobin-oxygen affinity: described by the oxy-Hb dissociation curve

OXYGEN CONSUMPTION MEASUREMENT

DO2/VO2 relationships

• oxygen delivery index (DO2I): CI x arterial oxygen content (CaO2) x 10, difficult to interpret because varies greatly in illness
• oxygen consumption index (VO2I): CI x (CaO2-CvO2) x 10 via the reverse Fick method or indirect calorimetry – many random error and inaccuracies

Mixed Venous Blood

• blood aspirated from an unwedged PA
• mixed venous PO2 (PvO2): low value -> intracellular hypoxia, high value -> doesn’t exclude histiotoxic hypoxia or tissue shunting.
• mixed venous oxygen saturation (SvO2): normal = 0.7-0.8, hypoxia + lactate acidosis = 0.3-0.5, > 0.8 = high flow states (sepsis, hyperthyroidism, severe liver disease)
• central venous saturation (SCVO2): normally 3% lower than SvO2, trends and response to management run in parallel.
• venoarterial PCO2 gradient (∆PCO2): normally about 6mmHg, markedly increases in low output states and cardiac arrest, lacks sensitivity and specificity as a global index of tissue hypoxia.

Plasma Lactate and Redox indices

• produced as a result of anaerobic metabolism in the context of inadequate tissue oxygenation.
• lactic acidosis: production exceeds metabolism or metabolism is decreased by organ dysfunction.
• normal = < 1mmol/L

Regional Oxygenation Indices

• regional PCO2: reflects the balance between PaCO2, tissue blood flow and tissue CO2 production, a rising gap between PaCO2 and tissue PCO2 signifies falling tissue blood flow (gastric tonometry and sublingual capnometry)
• cerebrovenous oxygen saturation monitoring: retrograde passing of catheter into jugular bulb to measure venous saturation (goal = 55-85%)
• direct tissue PO2 measurement: animal data, highly impractical
• othrogonal polarisation spectroscopy: microcirculation assessment
• in vivo MRI
• optical spectroscopy