The oxygen cascade
describes the process of declining oxygen tension from atmosphere to
The amount of oxygen
in the blood is calculated using the formula: [1.34 x Hb x (SaO2/100)] +
0.003 x PO2 = 20.8ml
The delivery of oxygen to the tissues per minute is calculated from: DO2
= [1.39 x Hb x SaO2 + (0.003 x PaO2)] x Q
extraction is calculated by subtracting mixed venous oxygen content from
arterial oxygen content.
dissociation curve describes the non-linear tendency for oxygen to bind
to hemoglobin: below a SaO2 of 90%, small differences in hemoglobin
saturation reflect large changes in PaO2 Right to left shunting causes
hypoxemia resistant to oxygen therapy.
Vasoconstriction is a physiologic protective mechanism which prevents
right to left shunting of blood.
Right to left shunt
causes hypoxemia unresponsive to oxygen therapy Diffusion defects and
ventilation perfusion mismatches cause hypoxemia, responsive to
exogenous oxygen and positive pressure ventilation. Absorption
atelectasis refers to the tendency for airways to collapse if proximally
obstructed, gases are reabsorbed, this process is accelerated by
nitrogen washout techniques.
The objective of
oxygen therapy is to give the patient as much oxygen as is required to
return the PaO2 to what is normal for the particular patient.
Oxygen is given thru
fixed and variable performance devices.
devices deliver a flow of oxygen equal to or in excess of peak
devices use the deadspace of the nasopharynx or face masks as a
reservoir of oxygen. They cannot deliver high inspired concentrations of
therapy is used to increase the amount of oxygen dissolved in the
plasma, by increasing the ambient pressure Carbon monoxide causes tissue
ischemia by avidly binding to hemoglobin and displacing oxygen. The
treatment is 100% oxygen, and possibly HBO therapy.
High inspired oxygen
concentrations cause toxicity by causing formation of oxygen free
radicals (which damage tissues), and by causing absorption atelectasis
and V/Q mismatch.
oximeters measure the absorption of red and infrared light by pulsatile
blood. They are inexpensive, continuous and portable. Accuracy declines
below a SpO2 of 90%.