It must be clearly understood that, because of the sigmoid shape of the haemoglobin dissociation curve, measurement errors (particularly in the venous sample) can lead to major errors in the calculation of the derived measurements. For example, a pH measurement error of 0.05 pH units or a PO2 error of 2 mm Hg in the venous sample will result in a 10% error in the calculation of oxygen consumption in a 70 kg adult with a normal metabolic rate. Refer to the section entitled 'Simulation Exercises' for the details of this calculation.

It should also be borne in mind that specimens of blood drawn from the pulmonary artery will not reflect the true mixed venous blood under certain conditions:

1. If the catheter is wedged or partially wedged at the time of sampling. - Under these conditions the blood will reflect the composition of left atrial blood if it is in a West zone ³ III arterial branch or 'Alveolar Capillary' blood if in West zone I or II branch (Figure 1).

2. If there is a left to right intracardiac shunt (such as occurs with a ventricular septal defect) the pulmonary arterial blood will again be 'arterialised' and there will be a step up in the saturation of the mixed venous blood as it passes through the right ventricular cavity.

Mixed venous saturation measurements can also be obtained oximetrically if appropriate catheters and monitors are used. These permit continuous, real-time display of mixed venous saturation (SVO2) ⁴. Several studies have suggested that SVO2 monitoring may constitute a useful early warning of an impending low cardiac output state, but it is worth remembering that there are other causes of a falling SVO2 apart from a reduction in cardiac output. The determinants of SVO2 can be approximately defined by a modified Fick equation (Table 4):

SVO2 = SaO2 - VO2 / CO * 1.306 * Hb.

Where 'S' is the fractional saturation, 1.306 is the oxygen combining power of haemoglobin ⁵ and Hb is the haemoglobin in gms/L. (This modified equation does not allow for the presence of dissolved oxygen). Inspection of this equation shows that SVO2 can not only be affected by cardiac output, but also by changes in arterial saturation, oxygen utilisation and haemoglobin concentration. Possible changes in any of these parameters should always be considered when evaluating the causes of a change in SVO2.

(Students of the obscure will know that Adolf Fick was also the inventor of the contact lens ('Kontact Brille') and fitted such lenses to both short-sighted animals and humans!!)

Mixed venous blood in a normal patient at rest is about 75% saturated. As a general rule, any condition which leads to a sustained mixed venous saturation of less than 50% will be poorly tolerated and a mixed venous saturation of less than 30% should be viewed as a medical emergency.

The oxygen content equations ^{6, 7, 8} used by the 'Calculators' and 'Haemoglobin' pages of the simulator are more accurate than the equation shown above and include the dissolved oxygen in their content calculation.

Superior vena caval (or innominate vein) blood gas analyses are not generally very useful, although jugular bulb measurements may be used in the assessment of the adequacy of cerebral perfusion. In the resting patient, SVC blood has a lower saturation than IVC blood.

A paediatric PAC can be inserted into the jugular bulb for the continuous monitoring of jugular venous oxygen saturation ⁸.

1. Edwards JD; Mayall RM Importance of the sampling site for measurement of mixed venous oxygen saturation in shock. Crit Care Med, 26:1356-60, 1998 Aug

2. Dongre SS; McAslan TC; Shin B Selection of the source of mixed venous blood samples in severely traumatized patients. Anesth Analg, 56:527-32, 1977 Jul-Aug

3. West JB, Dollery CT, Naimark A. Distribution of blood flow in isolated lung: relation to vascular and alveolar pressures. J. Appl. Physiol. 19: 713 (1964)

4. Burchell SA, Yu M, Takiguchi SA, Ohta RM, Myers SA Evaluation of a continuous cardiac output and mixed venous oxygen saturation catheter in critically ill surgical patients. Crit Care Med 1997 Mar;25(3):388-91

5. Gregory IC The oxygen and carbon monoxide capacities of foetal and adult blood. J. Physiol. 236:625 (1974)

6. Kelman GR. Digital computer subroutine for the conversion of oxygen tension into saturation. J Appl Physiol 1966; 21:1375-1376.

7. Kelman GR, Nunn JF. Nomograms for correction of blood pO2, pCO2, pH and base excess for time and temperature. J Appl Physiol 1966; 21:1484-1490.

8. Thomas LJ. Algorithms for selected blood acid-base and blood gas calculations. J Appl Physiol 1972; 33:154-158.

8. Howard L, Gopinath SP, Uzura M, Valadka A, Robertson CS Evaluation of a new fiberoptic catheter for monitoring jugular venous oxygen saturation. Neurosurgery 1999 Jun;44(6):1280-5

Last edited on: 13/11/2000