Nellcor UK Training Programme

Nellcor is a division of Tyco Healthcare

Introduction

Pulse oximetry has provided a safe and simple method of assessing a patient's arterial blood oxygenation since the early 1980s. Around this time, pulse oximetry monitoring was initiated in the anaesthesia setting in an effort to optimize patient safety. The goal was to minimize unrecognized episodes of hypoxaemia associated with growing malpractice claims. Episodes of hypoxaemia diminished during this adoption period; and the use of pulse oximetry expanded to various inpatient care settings, such as post-anaesthesia care, critical care and the general care floor. Today, pulse oximetry reaches many healthcare arenas, including subacute care, long-term care, outpatient clinics, procedure areas, physician offices and the home.

Understanding the information that a pulse oximeter provides, and making appropriate assessments and decisions about patient oxygenation, improves patient care. This improved care is evident in two areas: First, by allowing early identification of hypoxaemic episodes that can impact patient safety; second, by facilitating clinical management of the patient. This training programme is intended to support the understanding of critical clinical concepts related to pulse oximetry, and support appropriate applications of this technology.

Objectives

By the end of this training programme, you will be able to:

• Describe how oxygen is carried in the blood.
• Define the relationship of oxygen saturation to total oxygen content.
• Review the clinical significance and incidence of hypoxemia.
• Discuss the basics of pulse oximetry technology.
• Review three factors that may cause arterial blood oxygen saturation (SpO₂) to differ from arterial hemoglobin oxygen saturation (Sa0₂).
• Analyze the utility of the SpO₂ value in patients with anemia, dysfunctional hemoglobin, venous pulsations and edema.
• Describe three considerations for sensor selection for patients being monitored with pulse oximetry.
• Identify the role of pulse oximetry in monitoring patients for hypoxemia.
• Identify three possible applications for the use of pulse oximetry in your clinical setting.

Review of Oxygen Transport Physiology

To ensure adequate oxygenation, several physiologic mechanisms must occur:

1. The blood must have adequate amounts of oxygen.
2. There must be adequate amounts of oxygen carriers, or hemoglobin molecules.
3. There must be adequate cardiac output to carry the oxygen to the tissues.
4. The cells must be able to adequately use the oxygen that is delivered.

This training programme focuses on the first two mechanisms of ensuring adequate oxygenation.

How Oxygen enters the blood

During inspiration, oxygen from the air around us enters the airways and is transported down to the alveoli (air sacs) in our lungs. Because the concentration, or partial pressure of oxygen in the alveoli, is higher than in the pulmonary capillaries, the oxygen moves across the alveoli. It then enters the pulmonary capillary bloodstream for transport to the tissues in our body. This movement of oxygen from an area of higher concentration to an area of lower concentration is called "diffusion."

How Oxygen is carried in the blood

Once oxygen enters the blood, it is carried in two forms. A small amount of oxygen is dissolved in the arterial plasma, and is measured and reported as PaO₂, which represents the partial pressure of oxygen in the arterial plasma. About 1% to 2% of all oxygen present in the blood is carried this way. However, because the oxygen concentration dissolved in the blood is so high, much of the oxygen moves from the plasma and is carried bound to hemoglobin molecules. Haemoglobins are proteins in the red blood cells.

This combined oxygen and haemoglobin is referred to as "oxyhaemoglobin." Haemoglobin not bound with anything is called "deoxyhaemoglobin" or "reduced haemoglobin." Normally, 98% to 99% of oxygen in the blood is carried as oxyhaemoglobin. Oxygen carried on the arterial haemoglobin is measured and reported as Sa0₂, which is the arterial haemoglobin oxygen saturation.

Total Oxygen Content

The amount of oxygen dissolved in plasma is an important determinant of the amount of oxygen that is bound with haemoglobin. When there are adequate amounts of oxygen dissolved in the plasma, normally there are greater amounts of oxygen bound with haemoglobin molecules. When inadequate amounts of oxygen are dissolved in the plasma, there may be less oxygen combining with haemoglobin molecules. Therefore, both PaO₂ and Sa0₂ are important indicators of blood oxygenation. However, most of the total arterial blood oxygen content is attributed to the oxygen combined with haemoglobin.

ARTERIAL OXYGEN SATURATION

Arterial haemoglobin oxygen saturation is often determined by a measurement of an arterial blood sample, and reported as Sa0₂. The saturation of haemoglobin is the ratio of the number of oxyhaemoglobin molecules to the total number of haemoglobin molecules available to bind with oxygen. This number is expressed as a percentage.

A pulse oximeter also measures arterial blood saturation. This measurement is often reported as SpO_2.

The normal patient range for any arterial haemoglobin saturation,whether Sa0₂ or SpO₂ is 95% to 99%

Anaemia

Haemoglobin values must be considered when assessing the adequacy of arterial oxygen content. The anaemic patient may have the same normal Sa0₂ or SpO₂ levels as a patient with a normal haemoglobin value.

Although all of the haemoglobin molecules are carrying oxygen, the anaemic patient has fewer haemoglobin molecules. The total arterial oxygen content in this patient's blood is therefore lower. The anaemic patient may be at greater risk whenever oxygen demand increases or oxygen supply decreases.

Hb content ~ 15 gm/dl SpO2 = 100%

Hb content ~ 8 gm/dl SpO2 = 100%

Hb content < 15 gm/dl SpO2 = ? %