The physiological underpinnings of life-saving respiratory support

Abstract

Treatment of respiratory failure has improved dramatically since the polio epidemic in the 1950s with the use of invasive techniques for respiratory support: mechanical ventilation and extracorporeal respiratory support. However, respiratory support is only a supportive therapy, designed to "buy time" while the disease causing respiratory failure abates. It ensures viable gas exchange and prevents cardiorespiratory collapse in the context of excessive loads. Because the use of invasive modalities of respiratory support is also associated with substantial harm, it remains the responsibility of the clinician to minimize such hazards. Direct iatrogenic consequences of mechanical ventilation include the risk to the lung (ventilator-induced lung injury) and the diaphragm (ventilator-induced diaphragm dysfunction and other forms of myotrauma). Adverse consequences on hemodynamics can also be significant. Indirect consequences (e.g., immobilization, sleep disruption) can have devastating long-term effects. Increasing awareness and understanding of these mechanisms of injury has led to a change in the philosophy of care with a shift from aiming to normalize gases toward minimizing harm. Lung (and more recently also diaphragm) protective ventilation strategies include the use of extracorporeal respiratory support when the risk of ventilation becomes excessive. This review provides an overview of the historical background of respiratory support, pathophysiology of respiratory failure and rationale for respiratory support, iatrogenic consequences from mechanical ventilation, specifics of the implementation of mechanical ventilation, and role of extracorporeal respiratory support. It highlights the need for appropriate monitoring to estimate risks and to individualize ventilation and sedation to provide safe respiratory support to each patient.

Keywords: Extracorporeal membrane oxygenation; Mechanical ventilation; Myotrauma; Respiratory failure; Ventilator-induced lung injury.

Fig. 1

Conceptual framework of the interaction between respiratory support techniques and physiological background. As shown, the same target requires different techniques depending on the physiological status of the lung. Representative computed tomography (CT) scans are shown. The consolidated/not aerated tissue is, from left to right, represents 5–12–34–63%. The triggers for escalating respiratory support are also indicated. The addition of positive pressure (CPAP) is intended to reduce atelectasis and therefore venous admixture and limit the risk of oxygen toxicity and further reabsorption atelectasis if the FiO₂ is close to 1. As the gas volume of the lung decreases (the lung becomes heavier and the compliance lower) the effort to move the lung increases and additional ventilatory support is necessary either non-invasively or invasively. Finally, with a very large venous admixture and refractory hypoxemia, extracorporeal support is the only viable option. PaO₂ arterial partial pressure of oxygen, PaCO₂ arterial partial pressure of carbon dioxide, FiO₂ inspiratory fraction of oxygen, CPAP continuous positive airway pressure, NIV non-invasive ventilation, MV mechanical ventilation, ECCO₂R extracorporeal CO₂ removal, ECMO extracorporeal membrane lung oxygenation, P-SILI patient self-inflicted lung injury

Fig. 2

Basic pathophysiology of acute respiratory failure, risks, and benefits of invasive respiratory support. Acute respiratory failure in critically ill patients basically occurs as a combination of deranged oxygenation due to various degrees of ventilation perfusion mismatch and deranged ventilation secondary to an imbalance between the excessive metabolic and mechanical loads and a decreased cardiorespiratory capacity (coordinated activity of the brain, respiratory muscles, and cardiovascular function) as shown in the center of the image. Both mechanical ventilation and extracorporeal respiratory support aim at restoring the imbalance between load and capacity improving ventilation. The former acts through positive pressure ventilation assisting the respiratory muscles and improving overall capacity and the latter through CO₂ removal decreasing metabolic load. They also improve oxygenation through various mechanisms. In the top and bottom of the image, some of the benefits and costs of each technique are listed. CO₂ carbon dioxide, V/Q ventilation perfusion, ECLS extracorporeal life support, VILI ventilator-induced lung injury

Fig. 3

The equation of motion during passive, assisted and un-assisted breathing. General elements of the equation of motion are displayed: A, the total pressure applied to the respiratory system (left) equals the opposing forces that operate during inflation (right), i.e. resistive pressure (green), elastic pressure (blue), and initial pressure (yellow). B On the left side of the equation, it can be seen that the only source of energy inflating the respiratory system during passive mechanical ventilation is the ventilator. On the right, the airway pressure tracing for a single mechanical breath on volume-controlled ventilation with a set inspiratory pause is displayed with the partitioning into its opposing forces. C highlights that the total pressure applied during assisted ventilation is the sum of the ventilator’s positive pressure and patient’s muscular pressure (left side of the equation). On the right, representative airway pressure and esophageal pressure tracings of a single breath in pressure support are displayed. Muscular pressure is the difference between the chest wall recoil pressure (passive increase in pleural pressure during lung inflation) and the negative deflection in pleural pressure (measured with esophageal pressure). We do not display a partitioning of the total pressure applied into its opposing forces during assisted ventilation which is more complex and beyond the scope of this review. D shows that the only source of pressure for inflation during un-assisted ventilation is the patient’s muscular pressure (left). On the right esophageal pressure tracing of a single un-assisted breath is displayed. Partitioning of the total pressure generated by the respiratory muscles can also be performed and is displayed. P_res resistive pressure, P_el elastic pressure, P_initial initial pressure, R_rs resistance of the respiratory system, E_rs elastance of the respiratory system, Vol volume, PEEP positive end-expiratory pressure, P_vent positive pressure applied by the ventilator, P_aw airway pressure, P_mus muscular pressure, P_cw chest wall recoil pressure, P_EEPi intrinsic positive-end expiratory pressure

Fig. 4

Different forms of patient–ventilator interaction. During assisted mechanical ventilation, the total work to inflate the lungs is a combination of the work done by the ventilator and the patient. Various clinical conditions are shown where varying proportion of work is done by the patient or the ventilator as schematically illustrated by the scale on the side of each panel (clockwise A–D). A Shows a patient with synchronous assisted ventilation and equal work performed by the patient and the ventilator. D and B show excessive or insufficient work done by the ventilator as compared to patient’s needs (over- and under-assistance) resulting in low and excessive respiratory effort respectively. These conditions are associated with potential adverse consequences. Diaphragm disuse atrophy and sleep disruption can occur with over-assistance (in the context of apnea events as shown in the ventilator screen leading to frequent arousals and awakenings). Conversely, under-assistance with excessive effort can lead to patient self-inflicted lung injury and diaphragm load-induced injury. C Shows a unique condition, the occurrence of reverse triggering with strong efforts. During this condition, passive breaths (where the ventilator performs all the work) alternate with machine trigger breaths followed by patient’s effort that can lead to breath-stacking (as seen on the ventilator’s screen). Potential adverse consequences are also displayed including the occurrence of potentially injurious eccentric contractions (diaphragm contraction during lengthening—exhalation). Vent ventilator, P-SILI patient self-inflicted lung injury