Page 279 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 245

hypoventilation or hypoxia. In normal individuals, maximum response to hypoxia
hypoventilation: Below normal
level of alveolar ventilation char- occurs below a PaO of 50 mm Hg (West, 2011). If the frequency exceeds 20 breaths
2
acterized by an elevated PaCO 2 . per minute and is rising, the patient should be evaluated for the cause of tachypnea.
Tachypnea may precede the development of respiratory failure and use of me-
chanical ventilation (Krieger & Ershowshy, 1994). During mechanical ventilation,
tachypnea is indicative of respiratory dysfunction (Gravelyn & Weg, 1980). When
tachypnea and low tidal volume are observed in a patient, successful weaning from
mechanical ventilation is not likely (Tobin et al., 1986).
Routine monitoring of a patient’s spontaneous respiratory frequency is a simple
and very useful method to assess the pulmonary status of a ventilator patient. This
is especially true during the weaning process. A sudden increase in spontaneous
respiratory frequency during the weaning attempt is indicative of moderate or severe
respiratory insufficiency or hypoxia.
A patient’s respiratory frequency can be monitored noninvasively and continuously
Rapid, shallow breathing by an easy-to-use monitoring system. Masimo Rainbow Acoustic Monitoring™
is a reliable sign of ventilatory
insufficiency. measures the respiratory frequency using an adhesive sensor with an integrated
acoustic transducer applied to the patient’s neck. A respiratory frequency monitor
may be indicated for patients in the postanesthesia setting and for those receiving
partial ventilatory support.

Temperature

In the intensive care unit, a patient’s temperature may be measured routinely at
regular intervals or monitored continuously via a rectal, esophageal, or pulmonary
artery catheter probe.
Hyperthermia. Hyperthermia can occur as a result of infection, tissue necrosis,
Hyperthermia causes a leukemia, or other conditions that increase a patient’s metabolic rate and oxygen
lower oxygen saturation at
any PaO 2 . utilization. Hyperthermia also shifts the oxyhemoglobin dissociation curve to the
right, causing a lower oxygen saturation level at any PaO . This oxygen desaturation
2
occurs because increased temperature promotes unloading of oxygen from hemo-
globin to the tissues.

Hypothermia. Hypothermia, though seen less commonly in the critically ill pa-
Hypothermia lowers a tient, can occur as a result of central nervous system (CNS) problems, metabolic
person’s basal metabolic rate.
disorders, and from certain drugs or toxins. Hypothermia is sometimes induced in
head trauma patients as a means of decreasing the patient’s basal metabolic rate.
Hypothermia is also induced in patients undergoing coronary artery bypass (CAB)
surgery. At this extreme low temperature, the hypothermic condition must be taken into
account during management of the ventilator and patient. For example, the measured
PaO and PaCO values are higher than the actual values when the sample is collected
2
2
under hypothermic conditions, but is analyzed at body temperature. In order to have
blood gas values that accurately reflect a patient’s true ventilatory and oxygenation status,
corrections to the patient’s core temperature should be done during blood gas analysis.
In other nonextreme hypothermic conditions (62°C), temperature corrections are
not necessary as long as the uncorrected PaO is above 60 mm Hg (Malley, 1990).
2

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