High-Frequency Ventilation for Respiratory Immobilization

Uses of High-Frequency Ventilation for Respiratory Immobilization

Respiration imparts motion to numerous structures that proceduralists wish to maintain in their crosshairs. One of the earliest examples is extracorporeal shockwave lithotripsy. The initial reports were presented as American Society of Anesthesiologists abstracts, but subsequent reports provided greater detail and demonstrated significantly lower stone movement and shock requirements in contrast to conventional mechanical ventilation and spontaneous ventilation. Canty and Dhara described successful use of HFJV by a laryngeal mask airway (LMA). The applicability of HFJV with narrow blast path lithotripters was described by our group. Although it is clear from multiple reports that stone movement is markedly reduced and permits stone fragmentation with fewer shocks, the full implications of this have not been studied. It is known that renal oxidative stress is correlated with shock wave number, but no studies have addressed whether HFJV results in less renal injury.

Another important application of respiratory immobilization is in pulmonary vein isolation for atrial fibrillation. The technique was first described by Goode et al. We adopted the technique at the University of Pennsylvania in 2007 and have expanded our practice to over 500 cases annually. In an analysis of the impact of measures to improve catheter stability, 100 consecutive patients were identified in three groups—our baseline practice, the addition of steerable sheaths and electroanatomical mapping, and the addition of HFJV. The addition of all three measures improved freedom from atrial fibrillation at 1 year from 52% to 74%, as depicted in Figure 13-1 . This improvement was seen despite significant increases in body mass index, atrial dimension, the proportion of persistent versus paroxysmal atrial fibrillation, all of which are independent predictors of failure of atrial fibrillation ablation.

HFJV was also associated with a significant decrease in fluoroscopy time. HFJV has been embraced by the electrophysiology community. More than half of the Monsoon ventilators sold in the United States since 2010 have been placed in electrophysiology laboratories (Travis Schaztberger, Susquehana Micro, personal communication, 2011), suggesting that the technique is becoming more prevalent.

Imaging of the coronary arteries and pulmonary circulation can be affected by respiratory motion. We reported a case in which HFJV was used to eliminate respiratory motion in a patient whose respiration was too chaotic to permit imaging of the coronary arteries. This technique may hold promise for computed tomography (CT) and magnetic resonance angiography; although not approved for use in the United States, the Hayek MRI-RTX system (United Hayek Medical Industries) is available in Europe.

Respiratory motion may affect the cerebral circulation as well. Chen et al described a case in which respiratory motion made coiling of a cerebral aneurysm problematic and suggested that HFJV may have been beneficial.

Liver movement also can be problematic during ablative procedures. Yin et al demonstrated a 75% reduction in movement of implanted clips in the livers of dogs with HFJV in contrast to conventional ventilation. Fritz et al implanted gold markers in proximity to liver tumors in 10 patients and demonstrated that motion was restricted to 3 mm during HFJV. Biro et al reported use of HFJV to minimize tumor movement during CT-guided radiofrequency ablation of hepatic metastases and also found a 75% reduction in movement from 1.2 cm to 3 mm, in contrast to positive-pressure ventilation. This group also reported a significant reduction in fluoroscopic exposure during radiofrequency ablation of hepatic and renal masses using HFJV rather than conventional ventilation. HFJV is not as commonly employed during ablation of liver masses, although it is clearly an area poised for growth.

Finally, the lung is the organ most affected by respiratory motion. Lara-Guerra et al, in an ex vivo porcine lung model, demonstrated equivalent detection of artificial lesions averaging 6 mm with HFJV in contrast to inflation to 20 cm H ₂ O or total deflation. Fritz et al found a 25% reduction in planned treatment volume for stereotactic irradiation of small lung lesions with HFJV in contrast to spontaneous ventilation. With advances in technologies such as stereotactic body radiation therapy and proton beam therapy, reduction of respiratory motion in radiation oncology is another promising area.

Types of Ventilators

Respiratory rates above 60 breaths per minute are considered to constitute HFV. Although conventional anesthesia ventilators can deliver rates of 30 breaths per minute, corrugated anesthesia circuits are not capable of transmitting higher rates, because most of the tidal volume simply expands the circuit rather than being delivered to the patient. Small variations in airway resistance and lung compliance will alter the fraction of delivered gas that reaches the patient, making consistent ventilation at high respiratory rates with conventional systems problematic. The following types of HFV were developed to address this issue:

• 1.

  High-frequency jet ventilators. These ventilators use a high-pressure source, typically the oxygen supply line, to generate a brief pulse of high-velocity gas during inspiration with expiration typically being passive. The Acutronic Monsoon Universal Ventilator (Susquehanna Micro, Windsor, Pa.) ( Figure 13-2 ) is an example of this class of ventilator.

  

• 2.

  Dual-frequency jet ventilators. This HFJV and low-frequency jet modality provides more efficient carbon dioxide elimination but causes larger fluctuations in lung volume. The Carl Reiner TwinStream ventilator (Wien, Austria) ( Figure 13-3 ) is an example of this class of ventilator. It has never been marketed in the United States.

  

• 3.

  High-frequency oscillators. Oscillators use a piston or loudspeaker to create a bidirectional flow in the airway. The Sensormedics 3100B (CareFusion, San Diego, Calif.) ( Figure 13-4 ) is an example of this class of ventilator. Although this ventilator has been employed in NORA settings, the higher cost of consumables may be a barrier to wide acceptance.

  

• 4.

  High-frequency percussive ventilators. The Percussionaire line of ventilators (Sandpoint Idaho) ( Figure 13-5 ) uses a valve termed the Phasitron to create high-frequency oscillations that are superimposed on conventional volume control, principally for lung recruitment.

  

• 5.

  Negative-pressure high-frequency oscillators. These ventilators use an external shell with an oscillating pressure imposed into the space surrounding the patient’s chest, as was done with the “iron lung” but at high frequencies. The Hayek RTX oscillator is an example of this class of ventilator.

The initial focus of HFV was in care of patients with acute respiratory distress syndrome (ARDS), and devices such as the Percussionaire that are well suited to alveolar recruitment may be ill-suited to respiratory immobilization because they impose slow phasic changes in lung volume. Another early application was in tubeless ventilation during laryngotracheal surgery. Devices employing modalities such as superimposed HFJV to improve the efficiency of carbon dioxide elimination produce tidal oscillations in lung volume. Respiratory immobilization exists to minimize changes in lung volume, thus discussion of HFV will focus on the most commonly employed modality for lung immobilization—HFJV.

The following types of jet ventilators have been developed:

• 1.

  Manually operated pneumatic devices. The Sanders injector is a manually operated valve with pressure reduction that is connected to a high-pressure source such as wall oxygen or an oxygen cylinder. VBM Medical manufactures the Manuject III (Noblesville, Ind.) ( Figure 13-6 ), which is approved for “transtracheal ventilation in specific emergency situations of upper airway obstruction, used in conjunction with a transtracheal catheter, or a cricothyrotomy needle, which is inserted through the cricothyroid membrane.” ^∗

  ∗ See http://www.accessdata.fda.gov/cdrh_docs/pdf11/K112783.pdf . Accessed April 21, 2013.

  Although it is possible to use a Sanders injector as a high-frequency device, use beyond a few minutes is challenging, and producing stable lung volumes is difficult.

  

• 2.

  Fluidic valve devices. Fluidic valves were used in early ventilators such as the Bird Mark II, now produced by JD Medical (Phoenix, Arizona), shown in Figure 13-7 . All that is necessary is a source of compressed gas. The ventilator consists of two parallel channels, with one output channel of each valve being connected to the switching channel of the other. This creates a bistable oscillator. Tuning is achieved by adjusting three valves that control overall flow and the flow through the two switching channels. Achieving consistent results with fluidic valved devices can be challenging, but the portability and low cost of the device may be a compelling factor.

  

• 3.

  Electronically controlled devices. Electronically controlled devices, such as the Acutronic Monsoon Universal Ventilator (see Figure 13-2 ), use a microprocessor-controlled solenoid valve. The device has built-in safety features to prevent barotrauma and has the capacity to add humidity to the inspired gas.

Physics of High-Frequency Jet Ventilation

The attraction of HFJV is its ability to eliminate carbon dioxide with lung excursions that are close to the anatomical dead space. Carbon dioxide is eliminated by the following combination of effects :

• 1.

  Convection. Convection is simple bulk flow of gas back and forth down the large airways. It is responsible for the majority of carbon dioxide elimination in frequencies employed clinically.

• 2.

  Axial dispersion. Axial dispersion is the result of different flow profiles during inspiration and expiration that permit net flow to occur in opposite directions within the airway, as illustrated in Figure 13-8 . These effects make the actual dead space smaller than the anatomical dead space.

  

• 3.

  Pendelluft. Pendelluft is the to-and-fro movement of gas between adjacent lung segments of different time constants. It brings carbon dioxide closer to the large airways, where convection can remove it, as illustrated in Figure 13-9 .

  

• 4.

  Diffusion. Diffusion is the result of Brownian motion of gas and will also bring carbon dioxide closer to the large airways where convection can remove it.

These effects operate in different areas of the lung, with convection dominating the large airways and diffusion most prominent in the alveoli. Carbon dioxide is eluted from the lung by the combination of these effects, albeit less efficiently than with conventional ventilation.

Entrainment

Entrainment of side stream gas is commonly seen during HFJV. This phenomenon has been erroneously labeled the Venturi effect, which is depicted in Figure 13-10 . Entrainment is simply the effect of friction of the fast-moving jet stream acting to accelerate adjacent gas. It requires no special geometry and thus is not correctly described as the Venturi effect. When entrainment occurs in a tube, this increase in velocity down the tube will result in a decrease in transmural pressure because of the same laws of physics responsible for the Venturi effect. The amount of gas entrained is highly dependent on the site of entrainment and is significantly greater at the entrance to the tube than within the tube ( Figure 13-11 ).

Entrainment is also affected by airway resistance and lung compliance. Although entrainment of volatile agents into the jet stream has been described, the efficiency of agent delivery is low, can be expected to vary as conditions change, and thus has not been pursued. Entrainment can be from ambient air, an open blow-by system, or a semi-closed circle system. The latter permits conservation of inspiratory humidity, as will be discussed later, and offers the ability to add positive end-expiratory pressure (PEEP) and rapidly convert to conventional ventilation. These features are useful despite the increased dead space such systems impose.