Methods for Assessing Respiratory Mechanics

The methods employed to assess respiratory mechanics in ARDS patients can be classified into two categories: static, or quasi-static, and dynamic.

Static Methods

Static, or quasi-static, methods are based on specific maneuvers that require to discontinue the patient from the normal setting of mechanical ventilation and to provide an estimation of compliance, and eventually resistance. Traditional techniques like the super syringe or the constant flow for P-У curve assessment and end- inspiratory occlusion can be classified under this category. Because these methods are able to estimate only a single value of compliance and resistance, they need to be repeated periodically in order to have the information regarding variations occurring over time in a given ARDS patient.

In a sedated ventilated patient, the pressure developed by the respiratory muscles (Pmus) is equal to zero, and therefore PRS is only given by the pressure generated by the mechanical ventilator (Pvent) and Eq. 14.1 becomes

Therefore, bringing the respiratory system to a given volume by applying a given pressure, compliance can be determined by measuring the pressure that under static conditions (e.g., by interrupting the flow at end-expiration) equals P EL and by applying Eq. 14.4 in a situation where PRES = 0.

The resistance can be determined, in turn, by calculating the pressure (Pres) that has to be added to PEL to reach peak inspiratory pressure just before interrupting the flow.

The measurement of esophageal pressure (Pes), used as an estimate of pleural pressure (Ppl), is the only way to distinguish between the pulmonary system (comprising the airways and the lung) and the chest wall. When PES measurement is available during an end-inspiratory occlusion maneuver, its plateau pressure can be used to evaluate chest wall elastance (?cw) (Fig. 14.4), which can be significantly high in patients with ARDS as a result of intra-abdominal hypertension, pleural effusion, massive ascites, thoracic trauma, and edema of the intrathoracic and intraabdominal tissues as a result of fluid resuscitation [14. 15, 16, 17].

It must be noted that adjusting the ventilator settings only on the basis of PAO may be incorrect [18], being PTP the real “lung-distending” pressure promoting alveolar recruitment and lung inflation.

Moreover, in patients with ARDS, the reduced chest wall compliance, the presence of edema, or abdominal distension makes Pes very often elevated. Therefore, the calculated PTP can be negative at end-expiration, this indicating closed airways, or flooded or the atelectatic lung, and in this case, PEEP must be increased until PTP becomes positive to keep the airways open.

Super-Syringe Technique

The static reference method is the super-syringe method (Fig. 14.5a) [19]. Fixed increments of gas volume up to a total volume of 1.5-3 liters are applied to the

(a) Representative example of airway opening pressure

Fig. 14.4 (a) Representative example of airway opening pressure (Pao), flow, and volume tracings obtained during a standard breath (volume-controlled mode with a constant flow) followed by an occlusion maneuver. Rapid interruption of airflow at the airway opening is performed using a mechanical ventilator equipped with facilities for end-inspiratory occlusion. Total respiratory system compliance (Crs) is obtained as the ratio between tidal volume (ДV) and elastic pressure PEL (difference between plateau pressure Pplat, measured after a few seconds of occlusion, and PEEP). Total respiratory system resistance (Rrs) is obtained as the ratio between resistive pressure PRES (difference between peak inspiratory pressure, PIP, and Pplat) and inspiratory flow V’INSP The patients are sedated and paralyzed (Pmus=0, respiratory muscle generator switched-off in the figure). (b) Representative example of airway opening pressure (Pao), esophageal pressure (Pes), and flow and volume tracings obtained during an occlusion maneuver. Chest wall compliance (Ccw) is obtained as the ratio between tidal volume (Д V) and elastic pressure of the chest wall PEL, CW (difference between PES plateau pressure and PES at end-expiration. The patients are sedated and paralyzed (Pmus=0, respiratory muscle generator switched-off in the figure)

(a) The super-syringe technique is a static method consisting of inflating the lungs in volume steps

Fig. 14.5 (a) The super-syringe technique is a static method consisting of inflating the lungs in volume steps (Д V in the figure) of 50-100 ml up to 1.5-3 liters, starting from the functional residual capacity (FRC). The volume of gas administered (with fractional inspired oxygen of 1.0) is determined by the displacement of the piston. Airway opening pressure (PAO) is measured at the connection of the endotracheal tube by a pressure transducer, with zero referred to the atmospheric pressure. The patients are sedated and paralyzed (PMUS = 0, respiratory muscle generator switched-off in the figure). The pressures and the volumes are recorded simultaneously, and the pressure-volume curve is constructed from the different plateau pressures that correspond to the administered volumes. The entire procedure takes about 60 s. A similar maneuver can be performed during deflation in successive steps (not shown in the figure). (b) An alternative, simple technique to obtain a pressure volume curve from an ARDS patient without having to disconnect the patient from the ventilator is to inflate the respiratory system by a constant flow delivered by the ventilator [23, 24]. This quasi-static technique can be performed on any intensive care ventilator that is equipped with a constant flow generator. (c) Several studies have been performed to compare the quasi-static technique at constant flow with the static technique [25-27]; the results showed that the compliances obtained by the two methods are very similar. An important parameter to be defined is the value of delivered constant flow. High constant flows (between 20 and 60 L/min) reliably estimate only the slope of the P-V curve, while upper and lower inflection points are overestimated because of the resistive effect (Fig. 14.2) [23, 24]. While very low flow allows accurate estimates, long measurement periods are required to inflate the lungs, which may result in a loss of lung volume during the maneuver because of oxygen uptake by the lungs patient. After each increment, the static airway pressure is measured during a pause of a few seconds when there is no flow, and the pressure is the same in the entire system from the super syringe to the alveoli. The lung is then deflated in a similar way, and the inflation and deflation P-V curve is plotted. Oxygen consumption is constant during the whole procedure (i.e., inflation and deflation), which takes 45^120 seconds. During inflation carbon dioxide removal from the alveoli is null, and its partial pressure in the blood increases. During deflation, carbon dioxide removal is therefore lower than baseline, and this causes expired volume to be less than inspired volume and the P-V curve has a marked artifactual hysteresis. In ARDS patients, alveolar recruitment taking place during the stepwise inflation to a high pressure will also cause hysteresis [20, 21].

 
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