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

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