High-Frequency Ventilation: Applications in Thoracic Anesthesia

Introduction

The term high-frequency ventilation (HFV) summarizes different techniques of ventilation with a ventilator frequency of more than 60 breaths per minute, tidal volumes (V _T ) of equal or less than dead space, as well as lower peak airway pressure, lower transpulmonary pressure, and increased functional reserve capacity when compared with conventional mechanical ventilation. There are four types of HFV: high-frequency jet ventilation (HFJV), high-­frequency oscillatory ventilation (HFOV), high-frequency percussive ventilation (HFPV), and high-frequency positive pressure ventilation (HFPPV), ( Table 14.1 ). This chapter will describe the different modes of HFV and their application in thoracic anesthesia procedures.

HFV shows several particularities because of the high respiratory rate and the small V _T . Importantly, the mechanisms of gas exchange differ from those during conventional mechanical ventilation. During HFV, gas exchange is determined by so-called Taylor dispersion, convective dispersion, cardiogenic mixing, molecular diffusion, pendelluft, and bulk flow. A detailed description of the mechanisms of gas exchange during HFV is beyond the scope of this chapter and can be found elsewhere.

In thoracic surgery, there are different indications for HFV, including rigid bronchoscopy, airway stenosis, imaging and stereotactic and catheter ablation, treatment of bronchopleural fistula, and respiratory therapy after cardiac surgery. Possible contraindications for HFV are extreme adiposity, severe obstructive pulmonary disease, and an increased risk of aspiration whenever an open ventilation system is used, such as during HFJV. For successful HFV application, careful patient selection, close monitoring, knowledge of the technique, and a well-trained team are essential. The particular HFV technique to be used should be chosen according to local expertise and equipment availability.

Types, Application, and Equipment of High-Frequency Ventilation

High-Frequency Jet Ventilation

HFJV delivers gas from a high-pressure source through a small-bore cannula or catheter in the airway, an airway device, or a bronchoscope, followed by a passive expiration. HFJV maintains oxygenation and ventilation by delivering V _T of 1 to 3 mL/kg of predicted body weight (pbw), equal to or less than anatomic dead space at a respiratory rate of 60 to 300 breaths per minute. A diagrammatic representation of airway pressure versus time recording is illustrated in Fig. 14.1 . Because of the high pressure, a characteristic noise appears. To deliver the jet of gas from the ventilator, a small cannula (usually 14–16 gauge) or catheter is placed in the endotracheal tube or the trachea, leaving the airways open to the atmosphere. If the catheter is long enough, it can be positioned at distant sites within the bronchial tree, allowing proximal surgical procedures. HFJV is contraindicated in patients who are not fasted because of the open system and lack of aspiration protection. After passing a pressure regulator, the gas mixture reaches a manifold of four electromagnetic solenoid valves, which portion the applied gas volume. The high flow and high pressure at the tip of the cannula result in air entrainment from the surrounding tissue. Therefore the set jet inspiratory oxygen fraction does not equal the tracheal inspiratory oxygen fraction. During initiation of HFJV, the driving pressure is set at around 25 to 30 pounds per square inch (psi). Thereby a V _T of 2 to 3 mL/kg pbw can be reached depending on the compliance of the respiratory system. Furthermore, the inspiratory fraction (defined as inspiratory time divided by the sum of the inspiratory and expiratory times) can be chosen and is mostly set initially at 30%. Usually, a respiratory rate of 100 to 150 breaths per minute is chosen. In the United States, HFJV is only approved up to 150 breaths per minute. Increasing the respiratory rate results in lower V _T because of the reduced inspiratory time. A positive end-expiratory pressure (PEEP) can be added if deemed necessary. Thereby, the diaphragmatic excursion is reduced, and the lungs remain nearly static. Expiration is a passive process, with a continuous wall-mounted air outflow, which can protect the airways from contamination. However, this outflow does not confer protection against aspiration.

Superimposed High-Frequency Jet Ventilation

Superimposed HFJV combines a high- with low-frequency jet ventilation using a double or twin-jet technique. A diagrammatic representation of airway pressure versus time recording is shown in Fig. 14.2 . Thereby, minute ventilation is increased and carbon dioxide (CO ₂ ) removal improved. Although the low frequency is responsible for CO ₂ removal, the high frequency is important for oxygenation. The technique allows achieving an adequate gas exchange with lower airway pressures as during conventional ventilation. Ventilation is usually delivered via two different lumens, whereby both ventilation modes can be adjusted separately.

Sanders Jet Ventilation (Manual Jet Ventilation)

The Sanders jet ventilation introduced in 1967, uses a high-pressure gas source that is applied to an open airway in short bursts via a small-bore catheter. The technique uses a hand-operated valve (manual jet ventilation), as illustrated in Fig. 14.3 , connected to 100% oxygen, and a pressure-limiting device to deliver gas to the patient at 50 psi or less with a respiratory rate usually in the range of 10 to 14 breaths per minute. Respiratory rate and duration of breath are determined by the anesthesiologist, who monitors chest rise and oxygen (O ₂ ) saturations to determine adequacy of oxygenation and ventilation during the procedure.

High-Frequency Oscillatory Ventilation

HFOV is based on an oscillatory pump or membrane which delivers a respiratory rate of 180 to 900 breaths per minute through the endotracheal tube. A diagrammatic representation of airway pressure versus time recording is shown in Fig. 14.4 . A piston pump or membrane driven by an ­electric motor is used to create a high-frequency sine wave. The deformation of the membrane is determined by the set working pressure. The generated V _T is usually less than anatomic dead space (≤2 mL/kg pbw). V _T is influenced by the respiratory rate with a higher rate allowing less inspiratory time and resulting in lower V _T . V _T size furthermore depends on the size of the endotracheal tube, and respiratory system resistance and compliance. The pressure amplitude or V _T size determines mainly CO ₂ elimination. Heated and humidified gas is continuously delivered at a flow rate of up to 60 L/min orthogonal to the membrane or pump, so-called bias flow. The airway pressure oscillates around a constant mean airway pressure determined by the expiratory valve and the bias flow. The oscillations are transmitted through the endotracheal tube and airways. However, oscillations are attenuated along the way with the smallest pressure swings in the alveoli. In contrast to HFJV and conventional ventilation, expiration is an active process. When the membrane/pump moves back, air is actively drawn out of the lungs. Thereby air trapping is less common during HFOV as compared with HFJV.

High-Frequency Percussive Ventilation

HFPV is an exceptionally pulsatile form of HFV that delivers small V _T at rates up to 900 breaths per minute. V _T is delivered by a pneumatically powered, time-cycled, pressure-limited ventilator with inspiratory and expiratory oscillation. A diagrammatic representation of airway pressure versus time recording is shown in Fig. 14.5 . Basically, it is HFOV oscillating around two different pressure levels, which are the inspiratory and expiratory airway pressures. At the end of the endotracheal tube sits a sliding Venturi so-called phasitron, which acts as an inspiratory and expiratory valve and is driven by a high-pressure gas supply. The high frequency oscillatory subtidal volume breaths are superimposed on conventional inspiratory-expiratory pressure-controlled cycles, which are usually set at a rate of 10 to 15 breaths per minute. In the expiratory phase, the lungs empty passively until the preset PEEP. Although, during HFOV and HFJV, CO ₂ elimination can be a problem, during HFPV, CO ₂ removal is enhanced because of the increased minute ventilation. Further benefits include an increased clearance of secretions compared with other types of HFV.

High-Frequency Positive Pressure Ventilation

HPPV is a rarely used form of HFV, which is delivered by a conventional ventilator with the respiratory rate set near the upper limits of the ventilator. A diagrammatic representation of airway pressure versus time recording is shown in Fig. 14.6 . For HPPV, no special equipment is needed. However, the upper respiratory rate is limited.

Application

HFV can be administered supra- or infraglottic through a catheter (HFJV) or an endotracheal tube (HFOV and HFPV) or directly through a special laryngoscope or bronchoscope (HFJV) with an appropriate connector. The supraglottic airway access is usually established by the surgeon. Ventilation quality depends on the ability of the surgeon to align the gas flow with the airway, which is influenced by surgical priorities and anatomic abnormalities.

The infraglottic approach is mostly used when direct laryngoscopy is difficult or an obscured surgical field is present. It may prevent contamination of the lungs from gastric ­contents and blood because of the continuous gas flow from the lungs protecting the distal airways.

The transtracheal airway access is used especially for patients with airway tumors that prevent the placement of an endotracheal tube, or condition after head and neck radiotherapy, with alterations of the trachea or as an emergency rescue maneuver. However, the transtracheal route is considered to be a higher risk strategy than supraglottic or subglottic HFV because establishing the airway is risky (e.g., tracheal trauma, pneumomediastinum).

Equipment

HFOV and HFPV devices are connected to the endotracheal tube. For HFJV, the ventilator can be connected to a small-bore catheter or cannula, a bronchial blocker, or to a special laryngo- or bronchoscope. HFJV is dependent on the passive gas outflow from the lungs, in contrast to HFOV, where exhalation is an active process.

For HFJV, HFOV, and HFPV, a special ventilator is needed. There are several different jet ventilators available. Newer models have an internal or external gas climatization (heating and humidification of the gas) and can measure the airway pressure. If the pause pressure (pressure at the tip of the cannula measured between delivery of jet ventilation) exceeds a preset maximum, a safety alarm is activated, and the ventilator stops ventilation to avoid barotrauma. Furthermore, a jet catheter or jet cannula is necessary.

For HFOV, the ventilator generates the high respiratory rate with an electromagnetically driven membrane or an oscillatory pump. Newer HFOV ventilator models have a volume guarantee mode included. HFPV is generated by a phasitron which sits at the end of the endotracheal tube.

For the induction of anesthesia and at the end of the procedure until the complete reverse of anesthesia, it can be helpful to insert a laryngeal mask.