Respiratory mechanics

Two independent patient-related forces have to be overcome to achieve lung inflation by positive-pressure MV (Figure 223-1): (1) resistance to the flow of gases within the airway and (2) compliance and elastance of the lung and chest wall, respectively. The latter concept can be difficult for the anesthesia provider to understand. In an inflated lung, the elastin within the lung is stretched. If you then remove a lung from the thoracic cavity and if the airway is open, the lung will collapse. This does not happen within the thoracic cavity because the parietal pleura is adherent to the thoracic cavity. In an intubated paralyzed patient at end expiration, or when the lungs are at their functional residual capacity, positive pressure must be applied to expand the lungs to deliver a breath. The amount of pressure (plateau pressure [Pplat]) applied to expand the lungs to a known volume is a measure of the compliance of the lung (Compliance = ΔV/ΔP). For example, 5 cm of H₂O pressure in a patient with a VT of 500 would equal a compliance of 100. In a patient with a diseased lung (e.g., with severe acute respiratory distress syndrome), the compliance would typically be less than 20.

The reciprocal of compliance, elastance, is defined as the ΔP/ΔV and is more commonly used to describe the pressure-volume relationships within the thoracic cavity. Unlike the lung, which collapses if opened, the chest wall or cavity would expand if opened.

Taken together the resistive and elastic properties (also referred to as compliance) of the lung and chest wall make up the patient-related impedance during any mode of positive-pressure inspiration. During each MV inflation, a dynamic interaction occurs between these patient-related variables (airway, lung, and chest wall) and MV settings (VT, airway pressure, and inspiratory flow). Airway pressure is equal to the sum of the pressure required to overcome the resistive component of the respiratory system (influenced by airway resistance and inspiratory flow) plus the pressure required to overcome the elastic component of the respiratory system (influenced by lung compliance, chest wall elastance, and VT):

< ?xml:namespace prefix = "mml" />Airway pressure = (Resistance × Flow) + (Stiffness × VT)

Mode of mechanical ventilation

In contemporary mechanical ventilators, microprocessor-controlled valves rapidly adjust airflow and pressure during inspiration to achieve the desired lung inflation. Conventional intraoperative MV usually provides complete ventilatory support (control ventilation) when the patient is expected to have no spontaneous ventilatory effort. The common modes of intraoperative ventilatory support include volume control or vv control at respiratory rates less than 60 breaths/min.

Volume control mechanical ventilation

During volume control mode, VT and TI (or respiratory rate and inspiratory: expiratory [I:E] ratio) are set by the clinician and inspiratory flow remains constant (Figure 223-2). In volume control MV, the resistive impedance (transmission of the VT through the airway) and the elastic impedance (compliance of the lung or chest wall) both contribute to the peak airway pressure (Ppeak).

A short end-inspiratory pause may be included in the respiratory cycle in volume control MV so that, after the VT has been fully delivered and just prior to exhalation, airflow is reduced to zero for a fixed proportion of TI. The airway pressure recorded during this end-inspiratory interval is called the plateau (Pplat), pause, or end-inspiratory hold pressure (see Figure 223-2). In many anesthesia ventilators, the default setting in volume control mode may include no end-inspiratory pause so that only Ppeak is recorded. However, it is always possible (and desirable) to include an end-inspiratory pause when using volume control MV so that both Ppeak and Pplat are recorded.

In contrast to Ppeak, which is influenced by both resistive and elastic components, Pplat is influenced only by compliance factors, as there is no resistive contribution during zero-flow conditions. When we use volume control MV and include an end-inspiratory pause, Pplat provides a more accurate estimate of the maximal distending pressure inside the alveolar sac than does Ppeak.