High-Frequency Ventilation Management of Thoracic
Surgery
Conventional intermittent positive-pressure ventilation delivers
relatively large tidal volumes (10 to 15 mL/kg) at respiratory rates usually less
than 30 breaths/min. In contrast, high-frequency ventilation (HFV) delivers very
small tidal volumes (<2 mL/kg) at rates between 60 and 2400 breaths/min. Because
HFV uses much smaller tidal volumes (and much lower peak inspiratory airway pressure),
which can be delivered through very small airway catheters, it may be uniquely useful
in facilitating the performance of thoracic surgery in the following three ways.
HFV is an umbrella term encompassing different delivery systems
and respiratory rate ranges. Current methods used to provide HFV are quite diverse,
but by using the criteria of ventilation rate and type of gas delivery mechanism,
it is possible to separate HFV into three general categories. With regard to oxygenation,
all forms of HFV should be regarded as a method of achieving PEEP (or above ambient
airway pressure) and thereby minimizing shunt without the necessity of imposing a
large volume (and pressure) excursion on top of this to eliminate CO2
.
It is conceptually similar to the approach of holding the lung slightly to moderately
distended with increased mean airway pressure and thereby minimizing shunt while
removing carbon dioxide by an extracorporeal membrane.[398]
[399]
Use in Major Conducting Airway Surgery
The most important advantage of HFV in thoracic surgery is that
small rapid tidal volumes may be delivered through small airway tubes; thus, if a
major conducting airway (trachea, carinal area, main stem bronchus) has to be divided,
the transit of a small airway tube through the surgical field causes much less interference
with surgery than does the passage of a large standard endotracheal tube or DLT.
The small airway catheters present the surgeon with a relatively unobstructed, accessible
circumference of trachea and bronchus so that the ends of a divided airway can be
properly aligned for the construction of an unstressed and airtight anastomosis.
Both high-frequency positive-pressure ventilation (HFPPV) and high-frequency jet
ventilation (HFJV) have been used successfully with small airway catheters for several
different types of airway surgery.[400]
In contradistinction
to these findings with HFPPV, high-frequency oscillatory ventilation delivered through
a standard single-lumen tube has been found to cause changes in the diameter of large
airways and produce a large "mediastinal bounce" with each oscillation, thus making
surgery on the major airways and around the hilum and mediastinal structures "almost
impossible."[401]
With all types of HFV for airway
surgery, the logistics of small catheter placement and securement are formidable,
and suctioning the airway and distal part of the lung may be problematic.
Use in Bronchopleural Fistula
Bronchopleural fistulas and tracheobronchial disruptions represent
the second category of thoracic surgical procedure where HFV has been proposed to
be beneficial by minimizing air leaks and mediastinal emphysema (as a result of the
lower inspiratory pressures and tidal volumes, which theoretically cause less gas
leak through the pathologic low-resistance pathways).[400]
In most of the bronchopleural fistula cases, PaCO2
was unacceptably high despite high minute ventilation on volume-controlled intermittent
positive-pressure ventilation, whereas HFJV restored normocapnia. However, these
theoretical advantages have seldom been realized clinically. Indeed, in a report
by Albelda and coauthors[402]
of seven consecutive
patients with an average bronchopleural fistula leak greater than 5 L/min, HFJV (125
to 150/min) caused only two patients to have clinically important decreases in airway
pressure and air leak fistula flow, and none had significant improvement in gas exchange.
The authors recommend measurement of tracheal pressures to predict what is happening
to air leak fistula flow.[402]
Use in Minimizing Movement of the Operative Field
The third proposed advantage of HFV for thoracic surgery is to
minimize the tidal movement of the operative field; theoretically, the lower peak
inspiratory pressures and tidal volumes of HFV should result in much smaller inflation
and deflation movements of the lung. Therefore, HFV of the nondependent lung might
provide a relatively "quiet" operative lung field, and HFV of the dependent lung
could contribute to the quiet lung field by providing a minimally moving mediastinum.
[400]
Unfortunately, in no report was the use of
HFV compared with conventional one-lung ventilation, with or without nondependent
lung CPAP, in terms of operating conditions and efficiency of gas exchange.
The studies of selective nondependent lung CPAP indicate that
delivery of any amount of oxygen to the nondependent lung alveolar space will cause
some oxygen uptake from the nondependent lung and decrease the right-to-left shunt
in this lung. Consequently, it is not surprising that selective HFV of the nondependent
lung, while the dependent lung is ventilated with conventional intermittent positive-pressure
breathing, increases PaO2
when compared
with simple collapse of the nondependent lung and conventional mechanical ventilation
of the dependent lung.[403]
However, because the
same increase in arterial oxygenation and operative conditions may be obtained by
selective nondependent lung CPAP with much simpler equipment (and less risk of complications),
it is far more logical to use selective nondependent lung CPAP than HFV to improve
arterial oxygenation and operative conditions during one-lung ventilation.
In one report the dependent lung was selectively ventilated with
HFV while the operated lung was completely collapsed.[404]
When compared with selective dependent lung intermittent positive-pressure ventilation,
selective dependent lung HFV significantly improved arterial
oxygenation, presumably because of lower dependent lung PVR (as a result of either
lower dependent lung airway pressure[405]
or some
other unexplained phenomenon[406]
), and enhanced
operating conditions (because of minimal mediastinal movements). However, the use
of dependent lung HFV to minimize mediastinal and hilar movements must remain controversial
in view of a 1986 report describing a mediastinal bounce with each oscillation, which
made surgery around these structures nearly impossible.[401]
In summary, in view of the limitations of HFV, its use as a routine procedure for
any of the three possible thoracic surgery situations (airway surgery, bronchopleural
fistulas, minimizing operative field movement) cannot be recommended.