ReviewMechanical Ventilation: State of the Art
Section snippets
Basic Physiology
Understanding of the basic physiology of respiratory mechanics is necessary to optimally apply MV. Much of our progress in understanding and managing acute respiratory diseases comes from this understanding. The physiologic measurements obtained in the ventilated patient can be considered to be detailed pulmonary function testing and are available on a breath-to-breath basis.12
The forces at play during ventilation at any point in time are described by the equation of motion of the respiratory
Phase Variables of a Breathing Cycle
The modes of MV are commonly defined by 4 elements determining the phases of the respiratory cycle (Table 1). The trigger phase initiates a breath. When the ventilation is fully controlled, the trigger variable is time, ie, a breath is initiated at fixed intervals. When the ventilator synchronizes the breath delivery with a signal related to the patient's effort, inspiration is initiated when a given flow or pressure decrease is detected by the ventilator. The target (or controlled) phase is
Complications of MV
Mechanical ventilation is often lifesaving but is associated with serious complications, in part because it is delivered to patients at high risk of lung or cardiac compromise. These complications may be related to the direct mechanical effects of the intrathoracic pressures generated by the ventilator, to alveolar and systemic inflammation, or to neural stimulation. There is evidence of cross-talk between the lung and the brain and between the lung and the kidneys, all influenced by MV.33, 34
Main Ventilator Settings
Assist-control ventilation using volume or pressure as the target and PSV are currently the 3 main modes of ventilation used worldwide.85, 86 These modes allow the clinician to set FIO2, PEEP, and a target variable (pressure or volume). There is, however, a wide variety of pressure-controlled modes, including airway pressure release ventilation or dual modes, which has been addressed elsewhere.13, 87
Acute Respiratory Distress Syndrome
No other ICU syndrome has been studied as much as ARDS. Understanding the impact of MV on patients with ARDS has resulted in major changes in ventilator management over the past 25 years.
A consensus definition of ARDS was released in 1994, more than 25 years after its initial description.104 The most recent Berlin definition tried to overcome some of the limitations of previous definitions.11, 105 ARDS is currently defined by a new onset or worsening of respiratory symptoms with bilateral
Protective Ventilation for Patients With Relatively Normal Lungs
There is accumulating evidence for the beneficial effects of lung protective ventilation in patients without ARDS,136 including those undergoing major surgical procedures, patients without ARDS at presentation, and in brain-dead patients who are potential lung donors.
For surgical patients with previously healthy lungs, the conventional strategy has previously been to combine high Vts (∼10-15 mL/kg) with high FIO2 using low or no PEEP. The goal with this strategy was to prevent atelectasis.137,
Ventilation in Patients With COPD
Exacerbations of COPD are characterized by a marked worsening of respiratory mechanics secondary to increased airway resistance, expiratory collapse of small airways limiting expiratory flow, development of auto-PEEP and hyperinflation, and increased work of breathing. The development of auto-PEEP has important consequences including increased work of breathing (inspiratory threshold loading), decreased respiratory muscle efficiency (flattened diaphragms), and hemodynamic compromise. Patients
Weaning
The weaning process can compose as much as 40% of the total duration of MV.152 However, many uncertainties exist when one tries to describe this phase of the MV journey because various aspects are ill-defined. For example, when does the weaning start? As soon as the patient is intubated, or when the sedation decreased, or when the ventilator is switched to a mode allowing spontaneous breathing? A common framework is important to enable comparison of weaning duration among groups of patients.
Avenues for Improvement
Our understanding of the pathophysiology of acute respiratory diseases, the impact of ventilator settings on dyssynchronies, and the complications of MV have all markedly improved during the past few decades. Nevertheless, many unanswered questions remain. Given the potential iatrogenic consequences of inadequate delivery of MV, one might assume that avoiding invasive MV at any cost would benefit the patient. However, recent data suggest that spontaneous ventilation can also lead to lung injury
Conclusion
Decades of research, progress, and clinical monitoring has led to an increased understanding of the physiology of MV. A conceptual revolution occurred when the goal of MV moved from normalizing blood gas levels to minimizing VILI while maintaining adequate (albeit not necessarily normal) gas exchange. We now know that management during the acute phase has a strong impact on long-term outcome and disabilities, and this focus on long-term outcomes will be a focus for future research. The MV
Acknowledgments
We thank Dr Lu Chen for providing the tracings used in Figures 1 and 2.
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Potential Competing Interests: Dr Slutsky is a consultant for Baxter, Novalung/XENIOS AG, and MAQUET Holding B.V. & Co. Dr Brochard’s laboratory has received grants or equipment from Covidien (research on PAV), Maquet (NAVA), Fisher Paykel (high flow), Philips (sleep), Air Liquide (Helium, CPR), General Electric (lung volume, ultrasound).