Page 427 - Clinical Small Animal Internal Medicine

40 Mechanical Ventilation 395

There are two widely available options for provision of kg) may be targeted in animals with severe lung disease
VetBooks.ir continuous spontaneous ventilation: continuous positive to avoid overdistension of the less severely diseased lung
tissue. When using volume control ventilation, the clini-
airway pressure (CPAP) and pressure support ventilation
(PSV). In CPAP, the ventilator does not deliver breaths;
rather, all breaths are spontaneous breaths and are com- cian presets the desired tidal volume. As overdistension
of the lung can lead to significant alveolar injury, it is rec-
pletely patient generated. The respiratory rate, inspira- ommended to start with no more than 10 mL/kg initially.
tory time, and tidal volume are all determined by the The tidal volume can subsequently be increased if it is
patient. CPAP provides a constant level of positive air- determined to be inadequate once an initial steady state
way pressure (the amount is preset by the operator) is achieved (typically 15 minutes after initiating IPPV or
throughout the respiratory cycle. Airway pressure is thus following a settings adjustment). If pressure control ven-
maintained at supraatmospheric pressures throughout tilation is used then the operator presets a desired peak
the respiratory cycle. CPAP has the potential benefits of airway pressure and once the animal is connected to the
decreasing respiratory system resistance and increasing ventilator, the tidal volume achieved with the preset
pulmonary compliance. These effects as well as the pressure is evaluated and peak inspiratory pressure
increase in mean airway pressure may serve to improve adjusted as needed. A tidal volume of around 10 mL/kg
gas exchange and oxygenation relative to what the patient would be an acceptable result in most cases.
could achieve without this support. Increased pulmo- The clinician must bear in mind that whole‐body car-
nary compliance effects of CPAP can best be explained bon dioxide stores are enormous relative to the relatively
by considering the shape of a typical lung pressure–vol- minimal whole‐body oxygen stores (excluding pinniped
ume relationship (aka compliance curve). Pulmonary mammals). Step changes in ventilation will result in
compliance curves are typically nonlinear with a flat abrupt, modest changes in PaCO 2 , but a new steady‐state
slope at volumes just above functional residual capacity value can take quite a long time to be reached. Only the
(FRC). This relatively flat slope indicates that changes in CO 2 in large, well‐perfused organs can truly be consid-
pressure generated by early inspiratory effort are accom- ered rapidly exchangeable and this represents a small
panied by only slight changes in lung volume (low com- fraction of whole‐body stores. Allowing 15 minutes to
pliance). CPAP can reduce the work of breathing by pass prior to reassessing steady‐state PaCO 2 values is a
shifting the starting point of breath initiation to a por- clinically reasonable approach.
tion of the curve with a steeper slope (higher compli-
ance). Further, maintaining the lung at a higher volume Airway Pressure Settings
increases mean airway diameter of noncartilaginous air-
ways and reduces airway resistance. With modern venti- Patients with healthy lungs such as hypoventilating anes-
lator models, the machine will alarm if the animal does thetized patients, patients with reduced respiratory
not generate adequate breaths or develops apnea. This drive, or those with reduced ventilatory capacity typi-
makes it a useful “monitoring” mode for weaning patients cally only require peak airway pressures in the range of
or monitoring intubated patients. 8–15 cmH 2 O. Avoiding peak inspiratory pressures above
As with CPAP, all PSV breaths are spontaneous breaths; 20 cmH 2 O is preferable whenever feasible. Animals with
the ventilator does not trigger, deliver, or initiate any significant lung disease often have relatively stiff lungs
breaths. In PSV, the tidal volume is generated by the and typically require higher airway pressures to achieve
patient, but also augmented by the machine. The amount adequate tidal volumes. Peak airway pressures as high as
of pressure support provided during the inspiratory period 30–35 cm H 2 O may be required in such animals. When
depends on what level of PSV support the clinician has using pressure control ventilation, the peak airway pres-
prescribed. This mode reduces the effort required to sure target is set by the clinician. Once the animal is con-
maintain adequate alveolar ventilation and gas exchange nected to the ventilator, the tidal volume achieved with
in patients with adequate respiratory drive but reduced that airway pressure should be determined. Alternatively,
ventilatory capacity. Pressure support ventilation can be in volume control, the tidal volume is preset and the
used independently, in conjunction with CPAP, or to aug- associated airway pressure needs to be monitored.
ment the spontaneous breaths during SIMV. Initially, airway pressures of 10–15 cmH 2 O should be
targeted; higher airway pressures can be utilized if indi-
cated by unacceptable blood gas values.
Tidal Volume Settings
The normal tidal volume reported for dogs and cats is Trigger Settings
generally in the range of 10–15 mL/kg although many
reports cite a tidal volume for healthy sedated cats of Most modern ventilators will allow the patient to trig-
approximately 20 mL/kg. Lower tidal volumes (6–8 mL/ ger machine breaths, which allows breath delivery to be