Jakob Wittenstein, Marcelo Gama de Abreu The term high-frequency ventilation (HFV) is used to describe different techniques of mechanical ventilation that use respiratory rates of 60 or more breaths per minute, tidal volumes equal to or less than the dead space, as well as lower peak airway pressures, lower transpulmonary pressure, and increased functional reserve capacity compared with conventional mechanical ventilation. There is a wide range of indications for HFV in thoracic surgery, such as rigid bronchoscopy, airway stenosis, imaging, stereotactic procedures and catheter ablation, treatment of bronchopleural fistula, and respiratory therapy after cardiac surgery. Contraindications for HFV are extreme adiposity, severe obstructive pulmonary disease, and an increased risk of aspiration whenever an open ventilation system is used as during high-frequency jet ventilation. HFV offers many benefits for thoracic procedures, such as increased surgical visibility, decreased conflict of space within the airways, decreased gas leak because of lower airway pressure when a major bronchopleural fistula is present and minimizing movements associated with normal tidal volume for imaging and radiation purposes, and a quasi-static surgical field. The complication rates of HFV are relatively low when patients are selected carefully, contraindications are excluded, the team is well trained, and backup strategies are available. This chapter will describe the different types and the use of HFV in thoracic procedures. OLV; HFV; HFJV; HFOV; HFPV; thoracic surgery; thoracic anesthesia The term high-frequency ventilation (HFV) summarizes different techniques of ventilation with a ventilator frequency of more than 60 breaths per minute, tidal volumes (VT) 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.1 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. Table 14.1 – Combination of high- with low-frequency jet ventilation – VT = depending on peak airway pressure – Oscillatory pump or membrane ventilator – VT = depending on peak airway pressure pbw, Predicted body weight; psi, pressure per square inch; RR, respiratory rate; VT, tidal volume. HFV shows several particularities because of the high respiratory rate and the small VT. 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.2 A detailed description of the mechanisms of gas exchange during HFV is beyond the scope of this chapter and can be found elsewhere.3 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. 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 VT 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.4 HFJV is contraindicated in patients who are not fasted because of the open system and lack of aspiration protection.5 After passing a pressure regulator, the gas mixture reaches a manifold of four electromagnetic solenoid valves, which portion the applied gas volume.6 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 VT 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 VT 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 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 (CO2) removal improved. Although the low frequency is responsible for CO2 removal, the high frequency is important for oxygenation. The technique allows achieving an adequate gas exchange with lower airway pressures as during conventional ventilation.7 Ventilation is usually delivered via two different lumens, whereby both ventilation modes can be adjusted separately.7 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.8 Respiratory rate and duration of breath are determined by the anesthesiologist, who monitors chest rise and oxygen (O2) saturations to determine adequacy of oxygenation and ventilation during the procedure.9 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.2 The deformation of the membrane is determined by the set working pressure. The generated VT is usually less than anatomic dead space (≤2 mL/kg pbw). VT is influenced by the respiratory rate with a higher rate allowing less inspiratory time and resulting in lower VT. VT size furthermore depends on the size of the endotracheal tube, and respiratory system resistance and compliance. The pressure amplitude or VT size determines mainly CO2 elimination.10 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. HFPV is an exceptionally pulsatile form of HFV that delivers small VT at rates up to 900 breaths per minute. VT 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, CO2 elimination can be a problem, during HFPV, CO2 removal is enhanced because of the increased minute ventilation. Further benefits include an increased clearance of secretions compared with other types of HFV.1 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. 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.11 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).11 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.4 The self-centering, laser-safe Hunsaker Mon-Jet tube, as illustrated in Fig. 14.7, was introduced in 1994 to provide subglottic automated jet ventilation.12 The tube is approximately 35.5 cm long and has two lumens and is laser resistant. Through the smaller lumen, airway pressure can be measured continuously and end-tidal CO2 intermittently, representing a safety feature of the Hunsaker tube. Therefore the risk of barotrauma can be minimized. The end of the tube has a green plastic cage that keeps the jet centered in the trachea to minimize any chance of the jet being directed against the wall of the trachea and dissecting through the mucosa causing a pneumomediastinum or pneumothorax. However, in case of laryngeal growths, especially vocal cord polyps, the cage of the Hunsaker tube might become trapped during intubation. Both the anesthetist and the operating surgeon should be aware of this possible complication.13 The tube itself is made of fluroplastic that is extremely resistant to combustion. Further details on use of laser and airway fire can be found in the package insert.
High-Frequency Ventilation: Applications in Thoracic Anesthesia
Abstract
Keywords
Introduction
Type
Technique
Settings
High-frequency jet ventilation
Superimposed high-frequency jet ventilation
High-frequency oscillatory ventilation
High-frequency percussive ventilation
Types, Application, and Equipment of High-Frequency Ventilation
High-Frequency Jet Ventilation
Superimposed High-Frequency Jet Ventilation
Sanders Jet Ventilation (Manual Jet Ventilation)
High-Frequency Oscillatory Ventilation
High-Frequency Percussive Ventilation
High-Frequency Positive Pressure Ventilation
Application
Equipment
Hunsaker Mon-Jet tube
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High-Frequency Ventilation: Applications in Thoracic Anesthesia
