ASSESSMENT OF TISSUE OXYGENATION
Arterial blood sampling is an essential part of the assessment of a patient’s oxygenation status, but can be very a hazardous task if you are a mosquito!
Introduction
When assessing the adequacy of tissue oxygenation it is essential to remember that the PaO2 is only one factor in the total oxygen delivery equation.
Oxygen Flux
The total oxygen delivery (or “oxygen flux”) to the tissues is determined by the following equation:
O2 flux = CO x O2 content of the blood.
= Cardiac output x (O2 carrying capacity of Hb + dissolved O2 )
= CO (mls /min) x (1.39 Hb gm / 100 mls x SAT % + 0.3 / 100mls at PaO2 of 100 mmHg)
O2 carrying capacity of Hb = 1.39 x 15gm / 100mls blood x 100 / 100 (assuming 100% saturation)
Assuming: 1 gm of Hb binds 1.39 mls of O2 and that the Hb level is 15gm/ 100mls. This comes to 20.8 mls O2 per 10 mls of blood.
The amount of dissolved O2= Solubility x PaO2 (Henry’s law: the amount of O2 dissolved in blood is to its partial pressure)
The solubility of O2 in blood = 0.003 mls / 100 mls / mm Hg of O2
Therefore at 100 mmHg of O2 = 0.3 mls O2 100 mls of blood
Therefore the oxygen content of blood = 20.8 mls / 100 mls blood + 0.3 mls / 100mls blood. This comes to 21.1 mls O2 / 100 mls blood.
Assuming the average adult at rest has a CO of 5 liters / min
X / 5000 = 21.1 / 100
X = approximately 1000 mls of O2 is delivered to the tissues per minute.
Therefore, in summary, adequate tissue oxygenation depends not only on the PaO2, but also on an adequate cardiac output, and hemoglobin level.
The Importance of an Adequate PaO2
O2 250
Consumption
By cells
mls/min
1000 mls
O2 delivery (flux)
In the normal situation at rest for an adult:
●O2 consumption by cells is 250 mls/min
●O2 delivery is 1000 mls/min
Therefore in the resting situation there is a good reserve between the oxygen needs of the cells and the total oxygen that is delivered to them.
In the normal patient, however, there is a critical level of oxygen delivery level, below which oxygen consumption of the cells fails, (and the “lights start to go out!”)
This critical level is known as the oxygen delivery index and is about 8-10 mls / kg / min,
or about 330 mls / M2 / min
The Role of Dissolved Oxygen
Under normal circumstances the amount of dissolved oxygen in the blood contributes only a minor degree to tissue oxygenation. Hemoglobin carries by far most of the oxygen in the blood. The goal of oxygen therapy is to ensure maximal hemoglobin saturation.
In disease processes where Hb function is severely limited such as anemia or carbon monoxide poisoning the component of dissolved oxygen can be life saving.
Dissolved oxygen content can be greatly enhanced by the use of hyperbaric pressurization.
In fact 100 % oxygen delivered at 3 atmospheres of pressure can provide about 6 mls of oxygen per 100 mls of blood, the body’s total oxygen requirement at rest.
100 % O2 at 3 ATM
1 ATM of 100 % O2 = 760- 47 (water vapor) = 713 mmHg
3 ATM = 3 x 713 = 2139 mmHg
Dissolved O2 = 0.003 mls / 100 mls / mmHg
= 0.003 x 2139
= 6 mls O2 / 100 mls blood.
The Causes of Tissue Hypoxia
As described above it is important to remember that tissue hypoxia is not only determined by a low PaO2.
There are 4 pathological causes of tissue hypoxia:
1.Hypoxic hypoxia(Here the PaO2 and O2 Saturations are LOW)
There are 5 causes of this type of hypoxia:
●Hypoventilation.
●V/Q mismatch.
●Shunt
●Diffusion impairment.
●Low inspired FiO2
2.Anemic hypoxia, (Here the PaO2 and O2 Saturations are NORMAL)
The amount of oxygen carried by the Hb is reduced.
Examples include:
Anemia
Methemoglobinemia
Carbon monoxide poisoning
3.Ischaemic Hypoxia, (Here the PaO2 and O2 Saturations are NORMAL)
The perfusion of the tissue is inadequate.
Examples include:
●Cardiogenic shock
●Hypovolemia
●Occluded vessel.
4.Histotoxic hypoxia, (Here the PaO2 and O2 Saturations are NORMAL)
In these cases tissue oxygenation and perfusion may both be adequate but the cells are unable to utilize oxygen at the cellular level.
Examples include:
●Cyanide poisoning.
●H2S poisoning
Assessing the severity and cause of hypoxic hypoxia
The usual PaO2 level in a young adult is about 95 mmHg, (with a range of 85 – 100)
This normal value decreases with age so that by age 60 the average value is about 85 mmHg. 3
The alveolar gas equation can be used to assess the severity and sometimes the cause of hypoxic hypoxia
The alveolar gas equation gives the approximate value of the partial pressure of oxygen in the alveoli, when the atmospheric partial pressure of oxygen is known.
PAO2 = PiO2 - PACO2 / R + F
PAO2 = the alveolar partial pressure of oxygen.
PiO2 = the partial pressure of the inspired oxygen
PACO2 = the alveolar partial pressure of carbon dioxide
Example:
Atmospheric air is 21% oxygen.
The PiO2 therefore = 21/100 x (760-47)/1 (taking into account water vapour pressure of 47 mmHg)
= 149 mmHg
A useful approximation to this calculation is 7 x % of inspired oxygen
7 x 21 = 147 mmHg.
For 30% inspire oxygen
7 x 30% 210 mmHg
PACO2 is approximately equal to the Pa CO2 = 40 mmHg under resting conditions.
R is the respiratory quotient = CO2 production / O2 consumption.
Its value is influenced by dietary factors involving the relative ratios of carbohydrate, fat and protein metabolism.
Under resting conditions its value is:
200 mls CO2 production / 250 mls O2 consumption (in mls / min)
= 0.8
F is a minor correction factor = 2 mmHg, which for practical purposes can be ignored.
Therefore under normal resting conditions:
PAO2 = 149 – 40 / 0.8
= 149 – 50
= 99 mmHg
This value is then compared with the measured PaO2 level and the A – a gradient calculated.
The normal A-a gradient is:
●Up to 15 mmHg
●< 10 % of the PiO2
The A- a gradient is useful for:
●Determining the degree of impairment of oxygen transfer, either on room air or on oxygen therapy.
●Determining whether the lungs themselves are the problem, (shunt, diffusion, V/Q mismatch) or whether the problem is primarily one of hypoventilation.
The alveolar gas equation describes the fall in oxygen and the rise in carbon dioxide that occurs in hypoventilation, when the A-a gradient is within normal limits.
If a patient is hypoxic, but the A-a gradient is normal, then the problem is not with the lungs (gas exchange), but rather one of hypoventilation, as demonstrated in the following example.
pH 7.33
PaO260 mmHg(on room air.)
PaCO272
Bicarbonate38 mmol / L
If PACO2 approximately = Pa CO2
Then, PAO2 = 150 – 72 / 0.8
= 150 - 90
= 60 mmHg
Therefore the A-a gradient is (60 – 60) zero.
Therefore the lungs are not the problem and it is one of hypoventilation.
In the example, it is probably a chronic problem, in view of the minor degree of acidosis (compensated) and the elevated bicarbonate level.
An acute hypoventilation problem would be represented with values such as the following:
pH6.9
PaO260 mmHg
PaCO272 mmHg
Bicarbonate10 mmol / L
Clinical Effects of Low Hemoglobin Levels
The clinical effects seen with anemia will depend on the following factors:
- The Hb level
- The rapidity with which the Hb level falls.
- The ability and degree with which an individual is able to compensate for the reduced Hb level.
In general the following may be used as a guide: 4
●Hb level 7.5 gm/100 mls Exertional dyspnea
●Hb level 6.0 gm/100 mlsGeneral weakness and lethargy
●Hb level 3.0 gm/100 mlsDyspnea at rest
●Hb level 2.0 gm/100 mlsHeart failure
References:
1.Oh TE Oxygen Therapy in Intensive Care manual 4th Ed 1997 p.209
2.West J B Respiratory Physiology 5th Ed 1995
3.West Pulmonary Pathophysiology 5th Ed p.18
4.Ganong 16th Ed Physiology p.578
Dr J Hayes
Reviewed, 14 April 2002