Pressure Measurements

The respiratory system is an elastic structure that requires the generation of force (pressure) for its displacement (35). In many critically ill patients, positive pressure is delivered to the lungs through an endotracheal tube by a mechanical ventilator with the aim of improving arterial blood gases and unloading the respiratory muscles (36-38). The total respiratory system is composed of complex structures, namely the lungs, the upper and lower rib cage, and the diaphragm-abdominal compartments, each displaying different mechanical properties; in disease states, the situation is even more complicated due to time-constant inhomogeneity, expiratory flow limitation, etc. (see Assessment of the Function of the Active Chest Wall in Section 6 of this Statement). Nevertheless, assessment of respiratory system mechanics is commonly based on a relatively simple equation of motion (Equation 1) (35):

where Pappl is the pressure applied to the system by either the ventilator or the combined action of the ventilator and the patient's inspiratory muscles, V and V are the inflation volume and flow, respectively, C is the respiratory compliance, and R is the flow resistance. The endotracheal tubes and the ventilator circuits are rather stiff structures, such that a patient's compliance provides a good representation of the overall elastic properties of the patient-ventilator ensemble. In contrast, the endotracheal tube poses a substantial, highly flow-dependent resistance, which is important from the standpoint of a patient's respiratory muscle activity, because this resistance has to be overcome during lung inflation (39, 40) and it may retard expiratory flows, thus promoting dynamic hyperinflation (41, 42). Due to the flow characteristics of the endotra-cheal tubes, Equation 1 becomes Equation 2:

where Rt is the nonohmic component of total flow resistance, and k1 and k2 are constants related to laminar and turbulent flow, respectively. The additional work of breathing due to the endotracheal tube can be considerable at high levels of minute ventilation, and it may be negligible at low inspiratory flows.

Inspiratory effort is usually increased in critically ill patients because of abnormal respiratory mechanics, i.e., low compliance and high flow resistance (43-47). During mechanical ventilation, a variable portion of the ventilatory workload is decreased such that a patient's inspiratory muscles are un loaded by an amount that should be proportional to the degree of mechanical support (48-56). However, in some instances, a patient's inspiratory effort during ventilator-assisted breaths differs only slightly from that during unassisted breathing due to several factors: excessive ventilatory drive consequent to either metabolic factors (e.g., sepsis, fever, etc.) or psychologic (e.g., pain, anxiety, etc.) phenomena; substantial time lag between the onset of a patient's inspiratory effort and full machine support due to delayed opening of the ventilator circuit valves; ventilator-inspiratory flow that does not meet patient demands, especially at the onset of inspiration; intrinsic positive end-expiratory pressure (PEEP); and excessive Vt that requires a long expiratory time, during which ineffective patient's efforts may occur. If a patient's effort remains significant during mechanical ventilation, the respiratory muscles cannot recover from fatigue. Accordingly, progressive reduction and eventual discontinuation of mechanical ventilation can be very difficult (2, 3, 57).

Measurement of pressure at the airway opening is easy in ventilator-dependent patients because a stiff endotracheal tube bypasses the compliant upper airway, allowing the rapid transmission of changes in alveolar pressure to the airway opening even in presence of time-constant inhomogeneity (58, 59).

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