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Subject 101: Oxygenation

Oxygenation

Medical doctors on general wards are frequently confronted with patients requiring oxygen suppletion. But which oxygen delivery system should be chosen and why?

This 101 focusses on the topic of oxygenation and its distinction from ventilation. Furthermore, several types of oxygen delivery systems are discussed.

Oxygenation is the process of adding oxygen to a system, in this case the human body. With the exception of administrating oxygen via a heart lung machine, oxygenation is a process confined to the lungs. It is dependent on the inspired oxygen fraction (FiO2) and the capacity of the lung tissue to allow diffusion of O2 molecules into the capillaries and thus the bloodstream. Oxygenation is not necessarily the same as ventilation.1 The latter refers to the exchange of air between the lungs and room air or, for example, ventilator and inherently also concerns the elimination of carbon dioxide from the bloodstream. In this subject 101 we focus on oxygenation.

When to administer oxygen?

Hypoxemia leads to dyspnea, pallor, or even cyanoses and subsequently organ failure. To prevent this, treatment providers have to be informed about the patient’s oxygenation status.3 A relatively cheap and easy to use monitoring device is the pulse oximeter, which uses the technique of photoplethymography to assess the saturation of the hemoglobin with oxygen. Using two wave lengths (660 nm and 940 nm) the absorption of the hemoglobin with oxygen can be used to assess the patient’s oxygen status. However, this measure of “saturation” of hemoglobin, does not always correlate to the actual amount of oxygen in the blood, which is described as the arterial oxygen saturation (SaO2).3 Differences between SaO2 and SatO2 occur when saturation is <70% and in case of carboxyhemoglobinemia (COHb) and methemoglobinemia (metHb).

Oxygen therapy is usually started when a patient shows signs of hypoxemia (generally defined as an arterial O2 saturation of < 90% or arterial PO2 of <60mmHg) or when the patient is clinically in need of oxygen suppletion. But might be preventive, for instance, in the case of trauma.

Oxygen delivery

To the healthcare providers’ disposal are several forms of oxygen delivery. Dependent upon the possibilities provided by the type of ward and the type of respiratory insufficiency of the patient, the following delivery systems can be chosen.4

Low flow nasal cannulas can supply the patient with a variable FiO2 of 24-40%. The oxygen flow rate, i.e. the amount of oxygen liters from the oxygen supply to the device, should be maximized and usually the range is 1-6 liters/min. A further increase in liters/min will not increase the FiO2 given to the patient, but with dry out the upper respiratory tract.4

Furthermore, a standard facemask can be given, requiring flow rates of 5-10L/min, resulting in a variable FiO2 range of 35-50%. Subsequently, a nonrebreather mask provides the patient of a (variable) higher FiO2 of 60-80%, as it is equipped with a reservoir bag. This system requires an oxygen flow rate of >10L/min. Further oxygenation therapy can be achieved using high flow nasal O2 therapy, but when at this stage the patient is usually admitted to an intensive care unit allowing the use of an artificial ventilator, to enable accurate administration of even higher oxygen inspiratory fractions.4

Side effects

Although oxygen therapy is usually safe and effective, when using above mentioned methods, there are side effects by inadequate or overuse of O2 therapy. Apart from nasal irritation which can occur due to nasal O2 therapy, O2 therapy may be detrimental in patients who suffer from chronic obstructive pulmonary disease. In these patients the blood oxygen level is the primary stimulator of the respiratory rate, opposed to blood carbon dioxide levels in normal patients, a phenomenon known as hypoxic respiratory drive.5   In these patients, oxygen therapy may lead to depressed breathing. Starting patients with hypoxic respiratory drive on a low concentration of supplemental oxygen typically helps avoid this potentially serious side effect of oxygen therapy.5

Another side effect, which can occur in every patient, is called pulmonary oxygen toxicity. This is a situation which results from increased oxygen tension in the alveoli, blood, or at the cellular level. Hyperoxia appears to produce cellular injury through increased production of reactive oxygen intermediates such as the hydrogen peroxide. Together with oxygen free radicals this accounts for retinal damage in prematures, as well as pulmonary damage in adults; which can cause mucous plugging, atelectasis, and secondary infection.5

This 101 was created in order explain the method of assessing oxygenation via blood saturation and subsequently the different methods of oxygen support a physician has at his/her disposal provided with a step-up plan. In general practice, oxygen therapy is given with a low threshold, therefore it is advisable to reevaluate the need of oxygen therapy regularly in order to prevent (long-term) adverse events.

S. Bossers & D. van Vemde

References

  • Galvagno SM.Emergency pathophysiology. SM Emergency Medicine and Critical Care.: Teton NewMedia, 2004.
  • Martini, Nath, Bartholomew. Fundamentals of anatomy & physiology, In: The respiratory system. Pearson 2012, p 840-850
  • Galvagno SM. Understanding Ventilation Vs. Oxygenation is Key in Airway Management. Journal of emergency medical services, 2012
  • Marino P.L. The ICU Book. In: Acute Respiratory Failure. Wolters Kluwer Health, 2014.
  • Malhotra A, Schwartz DR, Schwartzstein RM. Oxygen toxicity. Up To Date. c2007 [updated 2015 Nov 15; cited 2017 Jan 13]. Available here

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