•In case there is a need for precise oxygen delivery, it is better to use a conventional ventilator with an integrated oxygen blender.

C.O2 supplementation through conventional mechanical ventilation

•Most of the currently available mechanical ventilators are capable of delivering tightly controlled and high levels of FiO2 to intubated patients.

•The integrated microprocessor-controlled gas blenders allow blending of

compressed oxygen and air and precise delivery of a preset FiO2 for continued and long periods.

•It is also possible to deliver high FiO2 and even 100% oxygen through such systems.

•This is the reason that mechanical ventilation remains the cornerstone of management of severe hypoxia to improve oxygenation to a satisfactory level in most severe forms of respiratory failure.

Step 3: Monitor adequacy of oxygenation

Tissue oxygenation is a better method of assessment for the need of oxygen therapy. This can be determined through the following:

•Mixed venous oxygen saturation (SvO2) can be measured by taking blood sample from the proximal pulmonary artery. This reflects the amount of oxygen “leftover” in the venous blood after body tissues have removed (used) whatever oxygen they needed.

•The pulmonary artery catheter is required for measuring SvO2, which is associated with many complications.

•ScvO2, the central venous oxygen saturation measured by the placement of a catheter in the superior vena cava, can be taken as a surrogate for SvO2.

•A low SvO2/ScvO2 suggests that the cardiac output and the level of oxygenation are insufficient to meet the metabolic demands of body tissues.

Step 4: Understand risks of oxygen therapy in ICUs

Oxygen administration is not without risks and toxicities.

•Oxygen toxicity is likely in an ICU patient when high concentration (>0.5 FiO2) is needed for longer periods (³36 h).

•Oxygen toxicity in such a situation causes symptoms of restlessness, nausea, dyspnea, retrosternal discomfort, and paresthesia; later, lung injury and fibrosis may occur.

•The following precautions are therefore important:

•Use the lowest possible FiO2 but maintain the adequate and required oxygenation.

•Use judicious levels of positive end-expiratory pressure to reduce the overall FiO2 requirement.

•Regularly monitor manifestations of oxygen toxicity.

Suggested Reading

1.Jindal SK, Agarwal R. Oxygen therapy. 2nd ed; 2008. New Delhi: Jay Pee Brothers.

This book gives a detailed description of different aspects of oxygen therapy in all clinical conditions.

2.Huang YC. Monitoring oxygen delivery in the critically ill. Chest. 2005;128(5 Suppl. 2):554S–60S.

This article reviews the basic principles of DO(2) and the abnormal oxygen supply–demand relationship seen in patients with shock. It also discusses approaches for monitoring DO(2), including clinical symptoms/signs, acid–base status, and gas exchange, which provide global assessment, as well as gastric tonometry, which may reflect regional DO(2).

3.Kallstrom TJ. AARC clinical practice guideline: oxygen therapy for adults in the acute care facility—2002 revision and update. Respir Care. 2002;47:717–20.

4.Leach RM, Treacher DF. The pulmonary physician in critical care. 2. Oxygen delivery and consumption in the critically ill. Thorax. 2002;57:170–7.

Early detection and correction of tissue hypoxia is essential if progressive organ dysfunction and death are to be avoided. However, hypoxia in individual tissues or organs caused by disordered regional distribution of supplemental oxygen may be lifesaving in some situations but cannot correct inadequate oxygen delivery caused by a low cardiac output or impaired ventilation.

5.Schwartz AR, Kacmarek RM, Hess DR. Factors affecting oxygen delivery with bi-level positive airway pressure. Respir Care. 2004;49:270–275.

Delivered oxygen concentration during BiPAP is a complex interaction between the leak port type, the site of oxygen injection, the ventilator settings, and the oxygen flow.