High Frequency Percussive Ventilation in the Pediatric ICU

High Frequency Ventilation. What Is the Best Choice?

by Kenneth Miller, MEd, RRT-NPS, AE-C

High frequency ventilation is one strategy respiratory care practitioners can use to protect the lungs of their patients.

The goal of mechanical ventilation is to optimize gas exchange while minimizing the chance of causing ventilator-induced trauma.1 Over the past decade, these goals have reshaped the way clinicians utilize different ventilatory strategies to treat patients with refractory hypoxemia. Many different ventilatory strategies have been employed to meet these clinical objectives. Lung protective strategies such as delivering a low-tidal volume, pressure-limited ventilation, recruitment ventilatory options (eg, airway pressure release ventilation), and the external administration of other gases or pharmacological agents have all been utilized.2

High frequency ventilation (HFV) is a ventilatory strategy that utilizes a form of mechanical ventilation that combines very high respiratory rates (>60 breaths per minute) with tidal volumes that are smaller than the volume of anatomic dead space. The clinical rationale for this type of ventilation is that gas exchange is optimized by utilizing small tidal volumes with minimal alveolar stretch. There are three basic types of HFV: high frequency jet ventilation, high frequency oscillatory ventilation, and high frequency percussive ventilation. Their function, their role in ventilating different respiratory dysfunction etiologies, and the patient population each is employed for can be similar or unique (Table).

 

High Frequency Jet Ventilation

High frequency jet ventilation (HFJV) refers to HFV delivered using a jet of gas. It is initiated by inserting a catheter into the lumen of the endotracheal tube. A small (14 to 16 gauge) cannula is then connected to a specialized ventilator. An initial pressure of approximately 35 psi drives the jet of gas from the cannula with an initial respiratory rate of 100 to 150 breaths per minute and an inspiratory fraction less than 40%. The inspiratory fraction is the inspiratory time divided by the sum of the inspiratory and expiratory times. Applied positive end-expiratory pressure (PEEP) and/or sigh breaths are added if needed via conventional ventilator. HFJV employs an endotracheal tube adaptor in place of the normal 15 mm ET tube adaptor. A high pressure “jet” of gas flows out of the adaptor and into the airway. This duration of the jet is very brief—about 0.02 seconds—and at high frequency: 4 to 11 Hz. Tidal volumes ≤1 ml/kg are used during HFJV. This combination of small tidal volumes delivered for very short periods of time creates the lowest possible distal airway and alveolar pressures produced by a mechanical ventilator. Exhalation is passive. Jet ventilators utilize various I:E ratios—between 1:1.1 and 1:12—to help achieve optimal exhalation. Conventional mechanical breaths are often used to aid in reinflating the lung. Optimal PEEP is used to maintain alveolar inflation and promote ventilation-to-perfusion matching.3

 

Figure 1. Bunnell Life Pulse Jet.

HFJV is provided by the Bunnell Life Pulse Jet (Figure 1). It is a flow interrupter that uses a pinch valve to generate a stream of high frequency pulses. These rapid pulses of fresh gas generate the tidal volumes, which allow ventilation to occur primarily from flow streaming (Taylorian dispersion), which permits ventilation even with below-dead-space tidal volumes. The gas is squirted into the lungs at a very high velocity, which produces flow streaming, sending gas via laminar and transitional flow down the core of the bronchial tree, and minimizing the effect of dead space.

A conventional ventilator is always run in tandem with the jet to generate the PEEP and sigh breaths. Expiration on HFJV is passive from elastic recoil. A special ET adaptor is used during HFJV. This adaptor has a jet port through which the high frequency jet pulses are introduced and a pressure monitoring port for determining the delivered pressures. Currently, HFJV is utilized primarily in the neonatal population. Adult indications are reserved for oral-facial, laryngeal, and tracheal operating room procedures.4

High Frequency Oscillatory Ventilation

High frequency oscillatory ventilation (HFOV) uses a reciprocating diaphragm to deliver respiratory rates in the range of 3 to 15 Hz (up to 900 breaths per minute) through a standard endotracheal tube. This rate is so fast that the airway pressure merely oscillates around a constant mean airway pressure. The respiratory rate is set directly by the clinician. The mean airway pressure is set by adjusting the inspiratory flow rate and an expiratory back pressure valve (similar to applied PEEP). Some pumps allow the mean airway pressure to be set directly.

 

Figure 2. CareFusion 3100A.

The constant mean airway pressure maintains alveolar recruitment, avoids low end-expiratory pressures, and avoids high peak airway pressures. It also impacts oxygenation. Specifically, a higher mean airway pressure is associated with better oxygenation. The tidal volume (also called amplitude) is small during HFOV, usually less than or equal to the anatomic dead space. The amplitude depends on the endotracheal tube size and respiratory frequency: a smaller amplitude results when the endotracheal tube is small or the respiratory frequency is high. Currently, adult/pediatric and neonatal versions exist. HFOV is employed in all patient populations.

During HFOV, the pressure oscillates around the constant distending pressure—equivalent to mean airway pressure (MAP), which in effect is the same as PEEP. Thus, gas is pushed into the lung during inspiration, and then pulled out during expiration. HFOV generates very low tidal volumes that are generally less than the dead space of the lung. Tidal volume is dependent on endotracheal tube size, power, and frequency. Different mechanisms (direct bulk flow-convective, Taylorian dispersion, Pendelluft effect, asymmetrical velocity profiles, cardiogenic mixing, and molecular diffusion) of gas transfer are believed to come into play in HFOV compared to normal mechanical ventilation. It is often used in patients who have refractory hypoxemia that cannot be corrected by normal mechanical ventilation, such as is the case of ARDS, ALI, or other oxygenation diffusion issues.5 In some neonatal patients, HFOV may be used as the first-line ventilator due to the high susceptibility of the premature infant to lung injury from conventional ventilation.6

Currently, HFOV is provided to the neonatal population via the CareFusion 3100A (Figure 2). It is a true high-frequency oscillator with a diaphragmatically sealed piston driver. It is theoretically capable of ventilating patients up to 30 kg. Tidal volume typically delivers ˜1.5-3.0 cc/kg (<dead space). The 3100A is an extremely efficient ventilator secondary to an active expiratory phase, but it is not capable of delivering sigh breaths for alveolar recruitment. Viasys 3100B, able to achieve a higher mean airway pressure, is designed for patients over 30 kg.

High Frequency Percussive Ventilation

High frequency percussive ventilation (HFPV) provides subtidal volumes in conjunction with cycled, pressure-limited controlled mechanical ventilation (ie, pressure control ventilation, PCV). It can be conceptualized as HFOV oscillating around two different pressure levels, the inspiratory and expiratory airway pressures. HFPV improves oxygenation, improves ventilation, and lowers airway pressures (peak, mean, and end-expiratory), compared to other modes of mechanical ventilation.7

HFPV is delivered via the volumeric diffusive ventilator (VDR). The VDR (Figure 3) is the brainchild of Forrest M. Bird, MD, PhD, ScD, FAARC. The VDR is classified as pneumatic-driven, time-cycle, pressure-limited, biphasic oscillatory breaths, and exhalation occurs passively. Mean airway pressure is a product of the peak airway pressure, inspiratory time length, pulse frequency rate, and PEEP setting. Currently, the VDR-4 is manufactured by Percussionaire Medical Devices Corp.

HFPV is possible because of a device called a Phasitron. The Phasitron is an inspiratory and expiratory valve located at the end of the endotracheal tube. High-pressure gas drives the Phasitron to deliver small tidal volumes at a high frequency (200 to 900 beats per minute), superimposed on the inspiratory and expiratory airway pressures of PCV. The PCV is typically delivered at a respiratory rate of 10 to 15 breaths per minute.

HFPV does not always require pharmacologic paralysis. In addition, it can clear secretions very effectively secondary to an internal mucokinesis. Currently, HFPV is utilized in all patient populations and is standard of care in regional burn centers for ventilatory support of patients with inhalation injury.8

Indications

There are no universally accepted indications for HFV. Its utilization has been described in a variety of clinical situations, including ALI/ARDS, bronchopleural fistula, inhalational injury, blunt trauma-induced ARDS, and head injuries complicated by high intracranial pressure.

 

Figure 3. Percussionaire VDR.

  • ALI/ARDS. The theoretical benefit of using HFV in      patients with ALI/ARDS relates to the small tidal volumes. A strategy of      low tidal volume ventilation has been proven in randomized trials to      improve mortality, possibly due to decreased alveolar distension and      ventilator-associated lung injury.9 Although the trials did not      use HFV, many clinicians suspect that HFV confers a similar benefit. Until      this is proven, HFV should not be considered routine care for patients      with ALI/ARDS. HFV is used by some clinicians when there is persistent      hypoxemia during the first 3 days of mechanical ventilation despite      maximal conventional therapy, although the data to support this are      limited.
  • Bronchopleural fistula. HFJV is approved by the US Food      and Drug Administration for ventilating patients in whom a large and      persistent bronchopleural fistula exists. However, the likelihood that      HFJV will allow the bronchopleural fistula to close is unclear. While HFJV      may promote fistula closure by limiting alveolar distension, this may be      outweighed in some patients by increased plateau airway pressure (alveolar      pressure), decreased oxygenation, or worse hypercapnia.
  • HFJV is also used in the neonatal environment with      infants who have persistent interstitial air leak. The lower tidal volumes      help reduce the air leak and promote lung repair.10 It has been      used as a ventilatory strategy during surfactant replacement and for the      support of patients with bronchopulmonary dysphasia.
  • HFPV has demonstrated some positive outcomes in      patients with inhalation injury and in patients with cerebral injury who      develop acute lung injury.11
  • HFV should be used with caution in patients with      obstructive lung disease. The high respiratory rate used shortens the      expiratory time, which can cause auto-PEEP and related sequelae.
Conclusion

HFV has demonstrated positive outcomes in specific patient populations. Large controlled randomized studies are lacking to adequately assess its role in reducing mortality and morbidity in patients with respiratory dysfunction. However, its impact in specific patient populations is well documented. HFV is a ventilatory strategy that respiratory care practitioners should add to their arsenal of ventilatory weaponry to optimize patient outcomes.


Kenneth Miller, MEd, RRT-NPS, AE-C, is clinical educator and department dean of wellness, respiratory care services, Lehigh Valley Health Network, Allentown, Pa. For further information, contact .

References
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