The amount of oxygen carried by blood - physically



According to Henry's law, the volume of gas that dissolves in certain volume of liquid at constant thermodynamic temperature is directly proportional to the pressures of the gas above the liquid:

• When gas and liquid come into contact, part of the gas dissolves in this liquid, until the partial pressures of the gas in the environment and in the liquid are equal
• The amount of oxygen carried by the blood physically is therefore directly proportional to the partial pressure of the oxygen in the blood
• It is given in mmHg or kPa and is indicated as PaO2 for arterial blood and PvO2 for mixed venous blood

The amount of oxygen carried by blood


Oxygen, as a component of atmospheric air, is inhaled into the lungs, where it diffuses in the alveoli into the alveolar capillaries, in the direction of the difference in its partial pressures.

http://www.doping-prevention.de/cs/human-body/respiratory-system/respiratory-system.html

The amount of oxygen carried by blood


Physically and chemically:

• Approximately 0.26 ml of oxygen is dissolved (physically) in 100 ml of blood
• One gram of hemoglobin can bind about 1.34 ml O2, while 1 l of blood of a healthy adult contains about 156 g of hemoglobin
• In this way (chemically) over 20 ml of oxygen can be transferred through 100 ml of blood
• Thus, about 70 times more oxygen is transferred by chemical bonding than by physical dissolution – almost all oxygen

The amount of oxygen carried by blood


The amount of oxygen transported by chemical bonds is expressed using blood oxygen saturation, denoted SaO2 in arterial blood or SvO2  for mixed venous blood:

• It is given as a percentage and can be calculated from the known concentrations of oxyhemoglobin cHbO2 and deoxyhemoglobin cRHb:

Oxygen saturation of hemoglobin


Blood contains:

• Oxyhemoglobin (HbO2)
• Deoxyhemoglobin (DeoxyHb)
• Methemoglobin (MetHb)
• Carboxylhemoglobin (COHb)

  

Oxygen saturation of hemoglobin


Blood contains:

• Oxyhemoglobin (HbO2)
• Deoxyhemoglobin (DeoxyHb)
• Methemoglobin (MetHb)
• Carboxylhemoglobin (COHb)

  

The amount of oxygen carried by blood


The amount of oxygen released from hemoglobin depends on the drop in pressure and is given by the so-called dissociation curve of hemoglobin for oxygen: 

In addition, it also depends to a lesser extent on temperature and pH value and hemoglobin type.

Partial pressure

The highest partial pressure of oxygen and oxygen saturation are in the arterial blood, while the lowest are in mixed venous blood:

Saturation monitoring 

• Non-invasive monitoring of peripheral tissue oxygen saturation (SpO2) is made possible by pulse oximetry
• Invasively, arterial and venous blood oxygen saturation can be measured directly in the bloodstream using intravascular oximetry
• Through transcutaneous oximetry, the partial pressure of oxygen PaO2 can be measured using a sensor placed on the surface of skin

Optical measurement of blood oxygen saturation


Absorbance:

• Absorbance is a dimensionless quantity indicating how much light radiation has been absorbed by the solution 
• It is the negative logarithm of the ratio of the radiation intensity from the existing solution Io to the radiation intensity Ii entering the solution: 

• The molar extinction coefficient ε is generally dependent on the wavelength of the radiation, the Lambert-Beer law for constant concentration c and distance l then becomes:

Optical measurement of blood oxygen saturation


Dependence of molar extinction coefficient ε and absorbance A on wavelength for RHb and HbO2:

• If blood is considered as a solution of deoxyhemoglobin RHb and oxyhemoglobin HbO2, for the absorbance of such a solution the relation will apply:
  

  

With the use of the LED diode, the integral of absorbance can be determined in a certain range of wavelengths:

• Blood oxygen saturation is determined based on the ratio between the integral absorbance values measured using a red and infrared LED diodes
• Diodes with a maximum emitted power at a wavelength of 660 nm for red and 950 nm for infrared are used
• At and around these wavelengths, εRHb and εHbO2 are sufficiently different from each other, and at the same time, these wavelengths are quite far from each other

  

As the transilluminated tissue is inhomogeneous, there is considerable scattering 
 of light radiation:

• As it passes through the tissues, the intensity decreases by one to two orders of magnitude
• Tissue absorbance is different for each subject
• The tissue undergoes periodic changes in blood volume due to the pulsatile nature of the blood flow
• This creates a variable component of absorbance, the value of which changes from minimum to maximum during one heartbeat, i.e. in a period of approximately one second
• From this point of view, the absorbance of the tissue and the blood contained in it are constant at its minimum volume

  

Sensors for pulse oximetry are produced in different designs:

• Finger sensors are intended for adults and older children
• If unrestricted movement is required, sensors placed on the earlobe are used
• In newborns, the palm, wrist or foot is sometimes used for scanning

  

Operation of the pulse oximeter 

• The radiation source must always be opposite to the detector

• Possible artifacts from patient movement but also due to external light (phototherapy)
• Invalid values in case of poor blood supply to the peripheral parts of the body (e.g. due to low temperature)
• It is necessary to check the pulse oximeter min. 1x/year
• Electrical safety measurement – leakage currents
• Do not use damaged  SpO2 sensors

Transcutaneous oximetry


The measurement of the partial pressure of oxygen PaO2 is possible using the so-called Clark type polarographic electrode:

• In this electrochemical method, we measure the current at a constant external voltage
• Oxygen dissolved in the sample diffuses to the cathode through a hydrophobic, gas-permeable membrane
• Here, under constant external voltage, it is reduced to water
• The cathode is usually platinum or gold

An Ag/AgCl electrode serves as the anode, which is also the reference electrode

A graph of dependence of the current on the polarization voltage at different oxygen concentrations for a typical PaO2 electrode, also called a polarogram:

• For measurement a polarization voltage of 0.67 V is used, belonging to the flat point of dependence, which is sufficient for managing the reaction
• It is so small that other electrochemical events are not involved in the reaction
• The resulting current is then almost linearly dependent on the number of O2 molecules in the solution - the oxygen concentration

  

PaO2 measurement is based on the following chemical reaction:

• Which means that the hydroxyl ions in the reaction are protected by the electrolyte
• Oxidation takes place at the anode, which is also the reference electrode
• The four electrons required for the reaction are obtained via the Ag/AgCl electrode:

  

Graph of the corresponding linearized dependence:

• The current coming through the meter is approximately 75 nA·kPa−1 PaO2, i.e. about 10 nA·mmHg−1 PaO2, at a temperature of 37℃
• The chemical reactions taking place in the probe are strongly temperature dependent
• To maintain a linear dependence between PaO2 and current, the electrode temperature must be maintained within ±0.1 ℃

  

These electrodes are separated from the analyzed environment by a semipermeable membrane, which prevents the access of mainly adsorptive substances:

• Between the membrane and the electrodes is a thin layer of electrolyte (e.g. KCl solution)
• It contains a small amount of water, which prevents the enclosed sensor from drying out
• At the same time, the electrolyte is the medium in which the chemical reaction takes place

Under normal physiological conditions, the PaO2 at the skin surface is nearly atmospheric regardless of the PaO2 in the subcutaneous tissue:

• Hyperemia of skin causes PaO2 to approach arterial PaO2
• Hyperemia can be induced by the administration of a drug, heating or scraped skin
• A heating element and a temperature sensor are located inside the sensor

Heating of the skin has two beneficial effects:

• Diffusion of oxygen through the stratum corneum of the skin increases
• Dilation of skin capillaries increases blood flow under the sensor

For a quick response to a change in the oxygen concentration in the analyzed medium, the thinnest possible membrane and the smallest possible volume of the electrolyte solution between the electrodes and the membrane are advantageous:

• An extremely thin membrane will cause less reproducibility of the result and greater dependence of the current on changes in transport of oxygen
• The sensitivity of the analysis increases with decreasing the thickness of the membrane and size of the measuring electrode
• The larger surface of the electrode, the higher the residual current, which must be subtracted from the measured current

Older types of devices allowed fast measurement only in newborns, whose skin diffuses oxygen much better than in adults:

• The advent of new materials for the realization of permeable membranes (Teflon, polyethylene) made it possible to shorten the measurement time to several tens of seconds
• Current transcutaneous oximeters are applicable to both newborns and adults