The Genius of Adolf Eugen Fick

Fick was a 19th century German physiologist. In 1870, he worked out that cardiac output could be measured from a subject's oxygen consumption, if the concentration of oxygen entering and leaving a given organ were known.

Thus, cardiac output= Oxygen consumption/Arterial O2 content-Venous O2 content

It is easy to calculate the arterial or venous oxygen content if you know the O2 saturation (SaO2) in arterial or venous blood, as almost all the O2 is carried by haemoglobin. Each gram of Hb carries 1.34 ml of O2.

Thus, Arterial oxygen content= 1.34*Hb concentration*SaO2

As cardiac output is expressed in l/min and Hb in g/dl, to make matters uniform, you have to multiply the denominator by 10.

Thus, cardiac output (l/min)= O2 consumption (l/min)/[10*1.34*Hb(g/dl)*SaO2]-[10*1.34*Hb(g/dl)*SvO2]

In practice, it is rarely necessary to determine the absolute cardiac output, but Fick's principle has been put to good use to determine the degree of left to right shunt by calculating the respective blood flow to pulmonary (Qp) and systemic circulation (Qs)in conditions such as ASD & VSD. In significant left to right shunt, Qp>Qs. If Qp is more than twice Qs, most authorities would recommend percutaneous closure of ASD or VSD with an Amplatzer device or a surgical closure if the shunt is very large, as with some VSDs.

It is easy to determine O2 saturation during cardiac catheterisation consecutively in the superior venna cava, right atrium, inferior vena cava, right ventricle and pulmonary artery. Pulmonary venous oxygen content is considered to be the same as the oxygen saturation of arterial blood measured during pulse oximetry, provided there is no right to left shunt. The latter can be detected by a drop off in O2 saturation while moving into the putative chamber, e.g decline in SO2 while moving from right atrium to right ventricle, suggests a right to left shunt from a VSD, while the same thing happening while moving down from the SVC to the RA indicates a right to left shunt via an ASD.

As the SVC normally has 3 times the flow as in the IVC, the systemic mixed venous O2 content is calculated as (3*SVC SO2+IVC SO2)/4.

Oxygen consumption can be be calculated by using a special spirometer with the subject rebreathing air and using a CO2 absorber.

Using the Fick principle, Qp/Qs can be written thus:

Qp/Qs= [Oxygen consumption/ 10*1.34*Hb*(Pulmonary venous SO2-Pulmonary arterial SO2)] divided by,
[Oxygen consumption/10*1.34*Hb*(Systemic arterial SO2-Systemic mixed venous SO2)]

or simply, Qp/Qs= Systemic arterial SO2-Systemic mixed venous SO2/Pulmonary venous SO2-Pulmonary arterial SO2

In practice, an alternative and non-invasive way to measure the pulmonary:systemic flow ratio in left to right shunts is to use Doppler Echo to find the area of the aortic and pulmonary outflow tracts and multiply them with their respective velocity time integral.