Introduction

Mechanical ventilation is commonly initiated because the work of breathing is increased and patients have a decreased capacity to perform this work. To wean a patient from the ventilator successfully it is important to correct the imbalance between the imposed respiratory load and the ability of the patient's respiratory muscles to perform the required work of breathing. Therefore measurements of work of breathing can provide important information about the pathophysiological nature of respiratory compromise in a ventilator-dependent patient. However, accurate measurements of respiratory work are difficult to obtain in ventilator-dependent patients

Mechanical work is performed when a force moves its point of application through a distance. In mechanically ventilated patients, the mechanical work of breathing can be calculated by measuring the generation of intrathoracic pressure ( P) due to contraction of the respiratory muscles (or a ventilator substituting for them) and the displacement of gas volume (V):

The changes in pressure and volume can be analyzed graphically, and the area enclosed within a volume-pressure loop has the units of mechanical work. The various trans-structural pressures involved in overcoming different aspects of respiratory work include pressure difference across the lung (transpulmonary pressure) (PL) and pressure difference across the respiratory system (Prs):

' rs 'aw ' bs where Paw is airway pressure, Ppl is pleural pressure, and Pbs is body surface pressure.

The respiratory muscles perform work against several different forces (Tobin..Jnd y§n...„d® Graa.ff.199.4), including elastic forces developed in the tissue of the lungs and chest wall when a volume change occurs, flow-resistive forces resulting from flow of gas through the airways, viscoelastic forces resulting from stress adaptation within the lung and chest wall, and plastoelastic forces within thoracic tissues which are reflected in the difference in static elastic recoil of the lung and chest wall during inflation and deflation. Work required to overcome inertial forces, which depend on the mass of tissues and gases, is minimal and is usually ignored in calculating the work of breathing. The work required to overcome gravitational forces, to compress intrathoracic gas, and to distort the chest wall from its relaxed (passive) configuration makes a small contribution to the overall work of breathing. In addition, in ventilator-dependent patients work is performed to overcome the external applied impedances of many types, including resistive, elastic, and threshold loads.

Units of work

The most popular units of work are kilogram meters (kg m) and joules (J) (T.o.b.i.,n a.n.d^^ In general, 0.1 kg m approximates 1J, and this can be visualized as the energy that is needed to move 1 liter through a pressure gradient of 10 cmH 2O.

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