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The ventilator as a diagnostic instrument.

Mechanical ventilation is virtually always initiated with direct therapeutic intent, except when needed temporarily to assure safe performance of a hazardous procedure (such as endoscopy, bronchoscopy, trans-esophageal echocardiography or supine positioning of an orthopneic patient). Whatever the therapeutic objective, however, airway intubation simultaneously offers an opportunity to evaluate important aspects of pulmonary function that otherwise remain uncertain or unsuspected. In these days of VILI awareness, most caregivers pay attention to peak cycling and plateau airway pressures, but the time constraints of daily practice discourage more thoughtful analysis of the useful information streaming by. Relatively few physicians, nurses, or RCPs take full advantage of this rich opportunity to inform and guide care decisions.


What diagnostic information does intubation make possible? First, a few basics: From two primary measurements--airway pressure and flow--come numerous descriptors of respiratory mechanics derived from them. Pressure is force per unit area, while flow is volume per unit time. Volume can also be considered as area x length. Putting it all together, the product of airway pressure and flow has units of force-length per unit time. As might be recalled from high school physics, force-length units are work (or energy) units, and work units per unit time quantify power. Therefore, by measuring airway pressure and flow we have what we need to determine how much energy the ventilator expends during inflation to deliver a tidal breath with the set characteristics. The stiffer the respiratory system or the more resistive the conducting airway, the more is the energy required.

When one considers that energy can neither be created nor destroyed, it becomes clearer why pressure and flow tracings form the backbone for all mechanics measurements. In fact, the tracings of airway pressure and flow provide unique and reliable insights into the patient's work of breathing, the mechanical properties of the respiratory system, and the responses to our interventions. Under passive conditions the ventilator performs all the work of inflation; during triggered ventilation, the patient and ventilator work together to accomplish the same task. In theory, an identical amount of work is performed to provide the same flow and tidal volume to the respiratory system, whether the patient contributes or not. Under passive conditions we learn about the ease or difficulty of ventilation; when the patient triggers ventilation, the airway pressure tracing provides clues as to patient effort. In fact, the difference between the airway pressure areas during constant flow ventilation delivered passively and during triggered breathing precisely quantifies the patient's work of breathing. It is wise to pay close attention to the airway pressure profile during flow regulated assist-control breathing--whether ventilation occurs passively or not.

The characteristics of the chest set the need for the patient-ventilator system to generate elastic and flow resistive pressures. A large, flexible airspace opposes inflation rather feebly, stored elastic pressure is low relative to delivered volume, and compliance is said to be relatively high ("good"). Larger tidal volumes obligate higher pressures, whatever the chest size and elastic properties might be. In parallel, if the airways conducting flow are of normal dimension and number, resistance is relatively low ("good"). Higher pressures are needed to initiate higher flows, whatever the airway structure might be. When the aerated lung compartment is relatively small, calculated values for compliance will be low and those for the resistance will be high. This occurs independently of airway and lung tissue properties because we express resistance and compliance in absolute units (ml/cmH2O and cmH2O/liter/second). Note that structurally the tissues and airways can be perfectly normal, but if the container receiving flow is small, the lungs will appear to be diseased. Interestingly, the time constants for inflation or deflation--the products of compliance and resistance--are relatively unaffected by aerated lung volume.

Until very recently, we have been missing two vital components of the mechanics evaluation--the size of the gas-accessible compartment and the contributions of chest wall and muscle effort to the recorded airway pressure, the latter reflected by the pleural pressure. Despite the focus of the recent literature on airway pressure and flow based indicators (such as plateau pressure, mean airway pressure, stress index, inflection/deflection points and tidal compliance) for setting tidal volume and PEEP, it is hazardous to rely on any of these without knowing the intrapleural pressure and absolute volume of the lungs. As I have pointed out before in this column, none of the clinical trials that draw conclusions regarding safe or dangerous airway pressure thresholds have assured that the chest wall was normally flexible, that muscle tone was relaxed, or even that the breath was passively delivered. The importance of intrapleural pressure to the determination of transpulmonary pressure and therefore actual lung stretch has been recently emphasized in work by Talmor and colleagues (NEJM 2008), who set PEEP in ARDS effectively using transpulmonary pressure, determined as the difference between airway and esophageal balloon ("pleural") pressures.

In the same year, a different approach to assessing vital information needed to inflate the lung safely was provided by Chiumello and colleagues (AJRCCM 2008), who demonstrated the value of relating the tidal volume to the resting lung volume, or FRC. Setting tidal volume as 6 ml/kg of lean body weight and keeping plateau pressure < 30 cmH2O is a good first approximation to what is best, but these guidelines do not assure safe ventilation.

Newer technology enables the caregiver to measure aerated FRC effortlessly and repeatedly at the bedside by tracking gas composition responses to small, automated variations in the inspired oxygen fraction. Knowing FRC has several important benefits. Apart from any utility it might have in VILI avoidance, an ability to monitor FRC presents other useful possibilities. Variations in the size of the aerated compartment for a given airway pressure may reflect resolution or worsening of the underlying disease process. Alterations of body position may be accompanied by important changes in measured FRC--information of great benefit for nursing interventions and avoidance of needless over-pressuring the airway. (Why raise PEEP if a simple change of body position is as effective?) In the setting of airflow obstruction, say asthma or decompensated COPD, the measured FRC may correlate inversely with the number of open airways. Loss of aerated space could reflect mucus plugging or bronchospasm. Even here, response to PEEP would be of interest, as a dramatic change in FRC could signal the need for more or less applied end expiratory pressure. The FRC comprises a missing link that keeps the ventilator from providing an assessment of lung mechanics similar to that obtained in the PFT lab in the outpatient setting.

A very important component of the complete PFT evaluation remains unaddressed at the bedside of the critically ill patient. That component is the diffusing capacity--a measure of gas exchanging efficiency. The diffusing capacity for carbon monoxide (DLCO) is invaluable when trying to determine the presence of vascular obstruction or emphysema. DLCO can be decreased because of intrinsic lung disease, occlusion of blood vessels by vasculitis or clot, or the simple reduction of functional lung tissue due to infiltration, atelectasis, airway plugging or external compression. When DLCO is referenced to absolute lung volume, it affords an opportunity to assess the health of the aerated tissues, helping to cone down on the cause for dyspnea or impaired gas exchange. In this context, it is noteworthy that congestive heart failure and atelectasis tend to improve the DLCO normalized to aerated lung size. Experimental studies have been conducted with diffusing capacity, but to my knowledge, this technology has not been commercially developed or even well characterized for the intubated patient. In my opinion, it would make a very useful complement to the FRC measuring technology already available.

Physics and physiology remain the cornerstones of mechanical ventilation practice. Mastery of the tools already available and those just introduced for routine care will serve the conceptually well-prepared and thoughtful caregiver extremely well in the effort to rescue those in cardiorespiratory crises. With the need for practice rooted in applied physiology in mind, we have developed a state-of-the art, "hands-on", two and one half day course dedicated to the ventilated patient, which will be held this year in St. Paul, Minnesota, October 8-10, 2010. For more information: Please consider attending!

Dr. Marini, Professor of Medicine at the Univ. of Minnesota, is a clinician-scientist whose investigative work has concentrated in the cardiopulmonary physiology and management of acute respiratory failure.

by John Marini MD
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Author:Marini, John
Publication:FOCUS: Journal for Respiratory Care & Sleep Medicine
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2010
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