Chapter 36
- Respiratory Monitoring
- Richard E. Moon
- Enrico M. Camporesi
A monitor has been variously defined
as "something that reminds or gives warning"[1]
and "an instrument used to measure continuously or at intervals a condition that
must be kept within prescribed limits."[2]
Although
the former definition is applicable to self-contained instruments (e.g., pulse oximeters),
we have used the wider view incorporated in the latter definition, including, for
example, intermittent measurements and the relevant respiratory physiology as applied
to assessment of patient well-being.
Although outcome data are sparse, there is general agreement that
patient safety has been enhanced by the development of technologies that permit accurate
physiologic monitoring; basic studies to elucidate the causes of mishaps, including
incident monitoring; educational efforts by national patient safety organizations
such as the Anesthesia Patient Safety Foundation and the Australian Patient Safety
Foundation; and the promulgation of monitoring standards that have become widely
accepted. Eichhorn and colleagues[3]
outlined minimal
recommended standards for patient monitoring during anesthesia at hospitals within
the Harvard Medical School system, commonly referred to as the Harvard
Standards.
In addition to monitors of cardiovascular function and patient temperature, these
investigators suggested that mandatory respiratory monitors include methods of continuous
monitoring of the patient's ventilation (including a breathing system disconnection
monitor) and a monitor of oxygen (O2
) concentration within the patient's
breathing system. A strong recommendation was made for monitoring end-tidal carbon
dioxide (CO2
) concentrations. Since then, the American Society of Anesthesiologists
(ASA) has also promulgated basic monitoring standards.[4]
Included in the ASA Standards (www.asahq.org/publicationsServices.htm) are a requirement
for measurement of inspired O2
concentration and quantitative assessment
of blood oxygenation, such as pulse oximetry. Adequate illumination and exposure
of the patient are necessary to assess color. Adequacy of ventilation must be continually
evaluated, and quantitative monitoring of CO2
or volume of expired gas,
or both, is strongly encouraged. When an endotracheal tube is inserted, correct
placement of the tube must be verified by clinical assessment and by identification
of CO2
in the expired gas. Continual end-tidal CO2
analysis
using a quantitative method such as capnography should be performed from the time
of endotracheal tube placement until extubation or initiation of transport to a postoperative
care location. During mechanical ventilation, a device capable of detecting airway
disconnects must be in continuous use. Continual
is defined as repeated regularly and frequently in steady, rapid succession; continuous
is defined as prolonged without any interruption at any time.
The limited ability of human organ systems to function anaerobically
dictates a transport system that can maintain
Figure 36-1
Oxygen transport cascade. A schematic view of the steps
in oxygen transport from the atmosphere to the site of utilization in the mitochondrion
is shown here. Approximate PO2
values
are shown for each step in the cascade, and factors determining those partial pressures
are shown within the square brackets. There is a distribution of tissue PO2
values depending on local capillary blood flow, tissue oxygen consumption, and diffusion
distances. Mitochondrial PO2
values are
depicted as a range because reported levels vary widely. (Adapted from Nunn
JF: Nunn's Applied Respiratory Physiology, 4th ed. Boston, Butterworth-Heinemann,
1993.)
O2
delivery to peripheral tissues. The monitoring of one component of
this system, the cardiovascular system, is discussed in Chapter
32
. This chapter discusses the other component, the respiratory system,
and the appropriate monitoring necessary to detect malfunction in its gas exchange
function.
