The Oxygen Delivery Problem

For those working in acute or cardiopulmonary sections of physical therapy you may have considered this:

“If a patient has a low oxygen saturation and they respond to supplemental oxygen why don’t we just put them on a non rebreather mask non-stop? It would surely provide them with enough oxygen that they would never desaturate”.

A non-rebreather mask (NRB)

A non-rebreather mask (NRB)

For starters a non rebreather mask (NRB) is an oxygen delivery device that provides patients with a fraction of inspired oxygen (FiO2) of 100% and is used on patients in critical conditions such as ARDS . Normally the air we inspire is a mixture of gases, mainly nitrogen (78%), and the FiO2 is 21%. The amount of both gases in this mixture is important physiologically for a number of reasons. Due to the increased affinity of hemoglobin for oxygen at the alveolar level due to the Haldane effect (also see Bohr effect transport of O2 to working tissue) oxygen is preferentially absorbed over other gases and nitrogen remains in the lungs which help maintain the inflation of the alveolar sacs. If one were to increase the percentage of inspired O2, over a period of time there would be less nitrogen available to maintain the patency of alveoli. Due to the physiological principles described above this would eventually result in alveolar collapse or the technical term “absorption atelectasis

Secondly, increased blood levels of O2 can suppress the ventillatory drive, especially in patients with Chronic Obstructive Pulmonary Disease (COPD) who demonstrate CO2 retention(1-2). CO2 retention, defined as increased blood gas values of CO2, can occur in patients with severe COPD (1). The mechanisms for this physiological process are still not completely understood. Carbon dioxide values, in a normal functioning system, regulates the drive to breath, via central and peripheral chemoreceptors (3). In patients with CO2 retention this mechanism is altered and their body responds to circulating levels of oxygen; lower levels of O2 facilitates breathing and higher amounts suppress (1-3). Therefore increasing the amount of delivered oxygen to a patient with this condition could possibly result in apnea.

Hyperoxia (higher than normal levels of oxygen) has also been shown have other systemic effects on the body (4-7). In the peripheral vasculature, hyperoxia causes vasoconstriction. The amounts of vasoconstriction and blood flow reduction varies in body area as the coronary arteries and brachial arteries demonstrate markedly reduced blood flow when exposed to hyperoxic states, the reduction in the cerebral arteries appears to be less (5-7). In addition to the vasoactive effects, hyperoxia can also lead to an increase in reactive oxygen species which can lead to oxidative stress and damage tissue (7).

Rarely does one chemical, tissue or system act completely in isolation. Your body is not a petri dish and we do not operate in a vacuum. The effects from something seemingly innocuous to one organ system may result in deleterious effects to another. Just because the reaction in a cell to a given amount of substance is beneficial is does not always mean that more of that chemical is always good. Human physiology is a story, with many subplots and characters with an exer-expanding number of volumes as we learn more about the body.

1 Kim S et al, Oxygen Therapy in Chronic Obstructive Pulmonary Disease Proc Am Thorac Soc. May 1, 2008; 5(4): 513–518. source 

2 Gorini M et al, Breathing pattern and carbon dioxide retention in severe chronic obstructive pulmonary disease Thorax 1996;51:677-683 source 

3 Jones and Barlett Learning LLC 2014, Regulation of Ventilation pgs 4-14, source 

4 Dean J et al, Hyperoxia, reactive oxygen species, and hyperventilation: oxygen sensitivity of brain stem neurons, J Appl Physiol 96:784-791, 2004 source 

5 Farguhar H et al, Systematic review of studies of the effect of hyperoxia on coronary blood flow, Am Heart J. 2009 Sep;158(3):371-7 source 

6 Xu F et al, Effect of hypoxia and hyperoxia on cerebral blood flow, blood oxygenation, and oxidative metabolism. J Cereb Blood Flow Metab. 2012 Oct;32(10):1909-18. source 

7 Rossi P and Boussuges A, Hyperoxia-induced arterial compliance decrease in healthy man, Clin Physiol Funct Imaging. 2005 Jan;25(1):10-5 source.

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