The s/t is an OCI used to assess the degree to which venous blood is not fully oxygenated during its transit through the lung.1,2 Fung disease, atelectasis, and high Fio2s (> 0.7) can increase s/t.3 Calculation of s/t is useful in the evaluation of disease progress and treatment efficacy and in the assessment of gas exchange during the intraoperative period.4
The calculation of s/t requires determination of the Co2, which is measured in a blood sample obtained via a pulmonary arterial catheter.2 Alternatively less invasive surrogate measures have been proposed as indicators of s/t.1,4–9 In humans, 1 approach is to assume a value for Co2 on the basis of normal C(a – )o2 (3.5 mL/dL)5,10,11 with an OCI commonly referred to as the Fshunt.4 The Fshunt has been reported to provide reasonable estimates of s/t in human patients.4
Many of the other proposed indicators of s/t are OTIs, which are popular because they are easy to calculate. These include Pao2:Fio2, Pao2 – Pao2, (Pao2 – Pao2):Fio2, (Pao2 – Pao2):Pao2, and Pao2:Pao2.8 There is ongoing debate regarding the applicability and validity of these OTIs as substitutes of s/t, particularly when various Fio2s are administered.12
The purpose of the study reported here was to evaluate use of the OCI, Fshunt, as an indicator of s/t at various Fio2s in anesthetized sheep, in which the magnitude of s/t was varied by use of 1-lung and 2-lung mechanical ventilation. We hypothesized that, compared with OTIs, the Fshunt would provide a more reliable estimation of s/t under these conditions.
|C(a – )o2|
Arterial-to-mixed venous oxygen content difference
Arterial oxygen content
Pulmonary end-capillary oxygen content
Mixed venous oxygen content
Inspired fraction of oxygen
Oxygen content–based index
Oxygen tension–based index
Partial pressure of oxygen at which hemoglobin becomes 50% saturated with oxygen
Alveolar partial pressure of oxygen
|Pao2 – Pao2|
Alveolar-arterial difference in partial pressure of oxygen
Mixed venous partial pressure of oxygen
Arterial hemoglobin oxygen saturation
Mixed venous hemoglobin oxygen saturation
Delta Vet, Delta Med Srl, Viadana, MN, Italy.
Midazolam, Italian Biochemical Institute, Aprilia, Italy.
Temgesic, Reckitt Benckiser Healthcare, Schering-Plough, Walton, Milton Keynes, Buckinghamshire, England.
Lidocaine 2%, Fort Dodge Animal Health, Fort Dodge, Iowa.
Propovet, Hospira Inc, Chicago, Ill.
SurgiVet, Smiths Medical Inc, Waukesha, Wis.
Maquet Servo-i, Maquet Critical Care, Solna, Sweden.
SC 6002XL, Siemens, Boston, Mass.
Norcuron, Organon Laboratories, Hoddesdon, Hertfordshire, England.
Ringer Lattato, Panpharma, Italy.
Swan-Ganz, Edwards Lifesciences, Irvine, Calif.
Arrow International Inc, Reading, Pa.
Spacelabs Medical, Redmond, Wash.
GE ProSpeed SX, General Electric, Schenectady, NY.
IDEXX VetStat, IDEXX Laboratories Inc, Westbrook, Me.
Potassium chloride, Sigma-Aldrich Chemie, Buchs, Switzerland.
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