Does high-pressure, high-frequency oscillation shake the foundations of lung protection?

Abstract

There is a strong temptation to take a reductionist approach that would permit us to assign a single mechanistic cause for ventilation-induced lung injury (VILI) and thereby identify a simple bedside intervention to avoid its consequences. However, convincing investigations have uncovered at least three ways in which adverse tidal cycling pressures may injure the lung: (1) over distension of already inflated alveoli, with cellular distortion, epithelial wounding, and capillary stress fracture [1]; (2) tidal opening and closure with attendant (or causal) surfactant dysfunction, liquid bridge disruption, and high interfacial surface forces [2]; and (3) stress focusing/shear at points of micromechanical heterogeneity [3]. Over the years each has had a spokesperson for its primacy, backed by an impressive body of supportive scientific literature.Whatever the emphasized viewpoint, however, consensus exists that attaining high peak transpulmonary cycling pressures is an essential precondition for VILI. Excessive transpulmonary forces reflect the reduced capacity of the 'baby lung' to accept its distending and tidal volumes [4]. But with such critical elements respected and most unstable units held open, it seems reasonable that high-frequency oscillation (HFO), a strategy that targets reduced tidal volume and open lung, should be nearly ideal as a lungprotective methodology. It was therefore upsetting that a rigorously designed and executed clinical trial demonstrated that HFO could increase mortality risk [5].In this issue of Intensive Care Medicine, pioneering contributors to VILI research (Didier Dreyfuss and colleagues) present a cogent, detailed, and deliberately provocative argument that the cause of this unexpected result might be violation of the basic directive to avoid high airway pressures, with the likely mechanism being sustained and excessive tissue stretch [6]. The implication is that opening/closure and stress focusing, though acknowledged contributors to VILI at conventional ventilation frequencies, have received disproportionate attention. To evaluate the plausibility and vulnerability of this argument, the challenging complexity of VILI must be appreciated.VILI is a multifaceted process influenced by mechanical and non-mechanical factors. From the mechanical side, two factors are keys: maximal alveolar pressure and excursion of alveolar pressure. This duo is estimated clinically as the plateau and driving pressures (DP = VT/C or plateau minus PEEP), and both are important. There appears to be a fuzzy threshold of maximal applied transpulmonary pressure below which generation of extensive tissue damage is unlikely and above which the risks for cellular distortion and wounding, increased vascular permeability, and inflammation rise in nonlinear fashion [1,7]. Once above threshold, frequency of breath delivery becomes increasingly important, in part because native repair processes have inadequate time to mend cell membranes between cycles [8]. A persuasive case has been made for the primacy of surfacta...

How ARDS should be treated

“…Therefore, whatever maneuver reduces the applied mechanical power (such as reducing tidal volume), driving pressure or respiratory rate will reduce the probability of VILI. The disappointing results of high-frequency oscillation studies [ 36 , 37 ] can be considered under the aspect of power: even small tidal excursions, multiplied by the driving pressure and by the hundreds of cycles per minute, may generate an intolerable mechanical load. For a given mechanical load, the risk of VILI decreases if the lung is made more homogeneous, thereby reducing the presence of stress risers [ 31 , 32 ].…”

Imposed Work of Breathing During High-Frequency Oscillatory Ventilation in Spontaneously Breathing Neonatal and Pediatric Models

“…The approach of using low frequencies in patients with restrictive respiratory-system conditions may increase the risk of barotrauma and/or rheotrauma, due to poor downstream oscillation pressure-wave damping. 26,27 In the more compliant lung (eg, during weaning from HFOV, when more effective spontaneous breathing efforts by the patient can be allowed), oscillation pressure-wave damping becomes much more important because it reduces the risk of barotrauma. Therefore, lower frequencies can be used, and oscillation pressure amplitudes at the airway opening can be further reduced while maintaining the same ventilation efficiency.…”

Transpulmonary pressure during high-frequency oscillation ventilation: Is it the culprit?

“…High-frequency oscillation ventilation (HFOV) seems the perfect embodiment of the “open lung theory” as it suggests extremely low tidal volumes combined with very high mean airway pressures. With disappointment clinical trials showed that the application of HFOV was associated with a decreased survival [ 1 ]. The culprit has not been identified, and a possible cause is the effect of increased intrathoracic pressures on the right ventricle and very high intrathoracic pressures which act as an obstacle to blood flow [ 1 ].…”

“…With disappointment clinical trials showed that the application of HFOV was associated with a decreased survival [ 1 ]. The culprit has not been identified, and a possible cause is the effect of increased intrathoracic pressures on the right ventricle and very high intrathoracic pressures which act as an obstacle to blood flow [ 1 ]. When a positive pressure is applied to the respiratory system, it is spent in part to inflate the lung and in part to inflate the chest wall.…”

“…As the mechanical ventilator performs a mechanical work on the lung parenchyma, all the ventilator-related variables can be unified if the mechanical work is measured: at each breath energy is transmitted by the ventilator to the lungs and, if excessive, may disrupt chemical bounds and trigger inflammation [ 6 ]. During HFOV, very small tidal volumes are delivered with a very high-frequency summing to a possible elevated mechanical power [ 1 , 6 ]. In physics, the definition of mechanical work is pressure (absolute value) times delta volume and consequently includes the effect of both the tidal volume and the elevated TP.…”