Acute Resuscitation and Crisis Management: Acute Critical Events Simulation (ACES)

Chapter 13 Respiratory Mechanics

C H A P T E R 13 RESPIRATORY MECHANICS You are asked to assess a patient who is on the ventilator because of acute respiratory failure due to an asthma exacerbation. The nurses areconcerned because the high pressurealarms are sounding off as are the low volume alarms,A respiratory therapist is making adjustments but they are questioning why repeated changes are necessary. Lois CHAMPION, PIERRE CARDINAL, AND RICHARD HODDER Troubleshooting a mechanical ventilator requires an understanding of respiratory mechanics. The compliance of the lung, both static and dynamic, is a crucial element that influences respiratory mechanics. We will discuss this, along with other factors that contribute to resistance to air flow. Our goal is to illustrate how these components of the equation of motion for the respiratory system can influence patient care. Work is exerted with every breath, whether spontaneous or mechanical, to overcome elastic and resistive forces. An increased work ofbreathing will therefore occur with decreased compliance or distension of the respiratory system, or an increased resistance to airflow. Ultimately, the goal is to reduce the work ofbreathing for the patient thereby allowing the patient to be removed from the ventilator. This chapter will review the pertinent aspects of respiratory mechanics to allow physicians to better manage patients with altered pulmonary function. • Compliance and Resistance Compliance isthe volume change per unit of pressure change (Table 13.1). Compliance is a measure of the stiffness of the respiratory system, and is inversely proportional to elastic recoil (i.e.,elastance).Clinically, one cannot differentiate between lung and chest wall compliance (Figure 13.1). In order to achieve this distinction, it would be necessary to measure the pleural pressure. The insertion of an esophageal 123 Table 13,1 - Respiratory Mechanics Equations Equation 13.1 compliance = change in volume / change in pressure Equation 13.2 dynamic compliance = tidal volume n.b. PAWP is peak airway pressure PEEPtotgJ = applied PEEP * auto-PEEP Equation 13.3 static compliance = tidal volume Equation 13.4 resistance = pressure / flow Equation 13.5 resistance = PAWPinspiratory flow n.b. PAWP is peak airway pressure ^pteteau *s pteteau airway pressure Equation 13.6 work = pressure * volume Equation 13.T Ventilation =* Resistive + Elastic Pressure Pressure Pressure Equation 13.8 Time constant = compliance x resistance (ml/cm H2O) (cm H2O/rnl sec -1) manometer could estimate the pleural pressures, but this is invasive and can cause the patient discomfort and thus isnot routinely performed. Theconsequence of this is that the airway pressures generated with mechanical ventilation depend not only on the stiffness ofthe lungs, but alsoupon the stiffness of the chest wall and abdomen. Compliance also depends on lung volume. Post pneumonectomy patients, for example, will have a lower measured compliance but an unchanged lung dispensability, or specific compliance, of the remaining lung. Specific compliance refers to compliancethat has been correctedfor actual lung volume. Dynamic compliance is the tidal volume divided by the peak airway pressure minus the amount of positive end expiratory pressure (PEEP) in the system (Table 13.1). For example, a tidal volume of 600ml with a PEEP of5 cm H2O that generates a peak airway pressure of 35 cm H2O represents a dynamic compliance of 20 ml/cm H2O (or .02 I/cm H2O). The dynamic compliance is not considered to be a true measure of compliance since peak airway pressure reflects the pressure required to overcome not only the elastic forces of the lung but also the resistive forces to airflow. If the tidal volume of 600 ml was delivered with a high flow rate over a fraction of a second, the peak airway pressure would be much higher than if the same tidal volume was delivered over several seconds. The higher peak airway pressure reflects a change in resistance with the high flow rate, rather than a change in respiratory system compliance. 124 ACUTE CRITICAL EVENTS SIMULATION (ACES) COURSE SYLLABUS PAWP-PEEPtotal Pplateau- PEEPtotal n.b. Pplateau is plateau airway pressure PEEPtotal = applied PEEP+ auto-PEEP Pplateau Figure 13.1 - Compliance Formulasand Normal Measurements Figure 13.1 presents the formulas for the different types of compliance and the reported normal values.1 Note that the lungs and chest wall are in series so that compliance of the lungs and chest wall add as reciprocals. StaHc compliance is considered to be a better reflection of the compliance since the changes due to airflow resistance are no longer a confounder (Table 13.1). Thus, in order...