65

Continuous Blood Gas

Monitoring

10

M. Kabir Abubakar

Blood gas monitoring is essential for the management of

critically ill patients. Arterial blood gas measurement is the

gold standard for assessing gas exchange and acid base status

in critical care, but is invasive and provides only intermittent

data. Noninvasive methods, including pulse oximetry, transcutaneous blood gas measurement, and capnometry, have

been developed to supplement arterial blood gas measurement and provide real-time continuous monitoring in critically ill neonates.

Pulse Oximetry

Pulse oximetry is the most common method of continuous

oxygen monitoring in clinical care. It is noninvasive, easy to

use, readily available, and able to provide continuous monitoring of arterial oxygen saturations and heart rate.

A. Definitions

1. Arterial oxyhemoglobin saturation measured by arterial

blood gas analysis is referred to as SaO2.

2. Arterial oxyhemoglobin saturation measured noninvasively by pulse oximetry is referred to as SpO2.

B. Background

1. Principles of oxygen transport

a. Approximately 98% of the oxygen in the blood is

bound to hemoglobin.

 The amount of oxygen content in the blood is

related directly to the amount of hemoglobin in

the blood, amount of oxygen bound to the hemoglobin and to the partial pressure of unbound, dissolved oxygen in the blood (PaO2) (1). The relationship of arterial PO2, in near-term infants, to

percent saturation measured by pulse oximeter is

shown in Fig. 10.1.

b. The relationship between blood PaO2 and the

amount of oxygen bound to hemoglobin is presented graphically as an oxygen–hemoglobin affinity

curve (Fig. 10.2). Percent oxygen saturation is calculated using the formula

Oxyhemoglobin

Oxhyhemoglobin + Deoxyhemoglobin

× 100

2. Principles of pulse oximetry

a. Based on the principles of spectrophotometric oximetry and plethysmography (2)

b. Arterial saturation and pulse rate are determined by

measuring the absorption of selected wavelengths of

light

 Oxygenated hemoglobin (oxyhemoglobin) and

reduced hemoglobin (deoxyhemoglobin) absorb

light as known functions of wavelengths. By measuring the absorption levels at different wavelengths of

light, the relative percentages of these two constituents and SpO2 are calculated.

c. A sensor composed of two light-emitting diodes

(LEDs) as light sources and one photodetector as a

light receiver is employed. The photodetector is an

electronic device that produces a current proportional to the incident light intensity (2,3).

 There are two methods of sending light through

the measuring site: transmission and reflectance.

In the transmission method, the emitter and photodetector are opposite of each other with the measuring

site in between. In the reflectance method, the emitter and photodetector are next to each other on the

measuring site. The light bounces from the emitter to

the detector across the site. The transmission method

is the most common type used and is discussed here.

(1) One LED emits red light with an approximate

wavelength of 660 nm.

 Red light is absorbed selectively by deoxyhemoglobin.

(2) The other LED emits infrared light with an

approximate wavelength of 925 nm.

 Infrared light is absorbed selectively by oxyhemoglobin.