Canonical Question
Resp – Oximetry
Master answer
Physiology
Oxygen Delivery
= Cardiac output x Arterial oxygen content
\[ DO_2 \; = \; CO \; \times C_aO_2\]
Oxygen Demand
= Cardiac output x Arteriovenous O2 content difference
\[ VO_2 \; = \; CO \; \times (C_aO_2 \; – \; C_vO_2)\]
Oxygen flux
\[ = DO_2 \; – \; VO_2 \]
\[ = \; CO . C_aO_2 \; – \; [CO . (C_aO_2-C_vO_2)] \]
\[ = \; CO \; . \; (C_vO_2) \]
\[= CO \; . \; [(Hb*1.34* \underline{S_vO_2}) + (p_vO_2*0.003)] \]
Hence SvO2 is a surrogate marker for Oxygen flux
| Decrease in ScvO 2 or SvO 2 | Increase in ScvO 2 or SvO 2 |
|---|---|
| O2 consumption ↑ Stress, Pain Hyperthermia Shivering | O2 delivery ↑ CaO2 ↑ Cardiac output ↑ |
| O2 delivery ↑ CaO2 ↓ (anemia, hypoxia) Cardiac output ↓ | O2 consumption ↓ Analgesia, Sedation Mechanical ventilation Hypothermia |
Measurement: Co-oximetry
Co-oximetry:
- A co-oximeter is a blood gas analyzer that, in addition to the status of gas tensions provided by traditional blood gas measurements, measures concentrations of oxygenated hemoglobin (oxyHb), deoxygenated hemoglobin (deoxyHb or reduced Hb), carboxyhemoglobin (COHb), and methemoglobin (MetHb) as a percentage of the total hemoglobin concentration in the blood sample
- Different haemoglobin species have different absorption spectra.
Mechanism / Measurement
- Uses Beer-Lambert law to detect different Hb species.
- “Incidence of light is inversely proportional to the path distance and concentration of light absorbing particles within the path”
- multi-wavelength spectrophotometry (measures the absorption of light passing through blood from several dozens of wavelengths)
- Complex, but straightforward internal computations
- enables the instrument to distinguish between oxy-Hb, deoxy-Hb, and carboxyhemoglobin,-COHb, methemoglobin -metHb, other hemoglobin moieties and ‘background’ light-absorbing species
- This is reported as the fractional oxyhaemoglobin content (FO2Hb), which is defined below
- The FO2Hb measure provides a more accurate picture of the availability of oxygen to the tissues in the presence of haemoglobin variants.
- If the original blood sample contained no carboxy-Hb or met-Hb, then the values for FO2Hb and SaO2 would be identical.
\[ FO_2Hb \; = \; {{oxy-Hb} \over {oxy-Hb \; + \; deoxy-Hb \; + \; carboxy-Hb \; + \; met-Hb}} \]
Mixed venous vs Central venous O2 Sats
| Mixed Venous Oxygen Sats SvO2 or SmvO2 | Central Venous oxygen sats ScvO2 | |
|---|---|---|
| Measure of Oxygen flux [matching between delivery (DO2), and consumption (VO2)] | Surrogate for SvO2 | |
| Blood drawn from | Proximal Pulmonary Artery More invasive Intermittent only | SVC Less Invasive Can be continuous or intermittent |
| Clinical picture | Average venous oxygenation Balance between systemic delivery and consumption | Venous oxygen saturations of blood from upper body |
| Physiology | SvO2 > ScvO2 as it contains blood from both SVC and IVC Normally 65-70% <65% – impaired tissue oxygenation >80% – hyperoxic or cytotoxic states, shunting | ScvO2 < SvO2 as it contains predominantly SVC blood (which has lower sats than IVC) Normally ~65% <50% tissue hypoxia >75% high flow states |
| Advantages | Systemic picture | No PA catheter required Easily performed in ICU through CVC |
| Disadvantages | Arrhythmias Catheter knotting Perforation of PA Infection | Unreliable correlation with systemic oxygenation in advance sepsis/septic shock Complications of CVC |
| Situations where this value is higher than the other | Normal states | a) Anaesthesia – because of increase in CBF & depression of metabolism b) Patients with head injury where cerebral metab is depressed c) Shock: because of diversion of blood from splanchnic circulation, there is increased O2 extraction and therefore IVC saturation decreases. |
| Other Data Generated | Qt, PA pressures, derived indices and body temperature measurements | CVP |
| Evidence | Study by Gattinoni – only RCT as far as SvO2 is concerned showed no benefit from SVO2 monitoring Study by Jones – showed trending lactate clearance was non-inferior | Study by Rivers- early goal directed therapy improved outcome in septic shock. Newer studies – ProMISE, ProCESS, ARISE have shown no benefit (or harm) from EGDT. |
Sources/Further Reading:
Bloos, F., Reinhart, K. Venous oximetry. Intensive Care Med 31, 911–913 (2005). https://doi.org/10.1007/s00134-005-2670-9
CICM Examiner’s Report Fellowship Paper 2 2006
JC 2020
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2019A Q08 | Compare and contrast the measurement (40% of marks) and interpretation (60% of marks) of both central venous and mixed venous oxygen saturations. | historical_member | — |