ABSTRACT
CONTENTS 7.1 Introduction ............................................................................................... 199 7.2 Scope and Methodology .......................................................................... 200 7.3 Lung Ventilation Model .......................................................................... 201 7.4 Determining Lung Compliance (Ca) and Airflow Resistance (Ra) ... 205 7.5 Formulating a Lung Ventilatory Index (LVI) Incorporating
Ra and Ca .................................................................................................... 207 7.6 Evaluating Lung Ventilatory Index ....................................................... 208
7.6.1 LVI Characteristics ....................................................................... 208 7.6.2 Comparing the Efficacies of Ra and Ca with LVI .................... 210 7.6.3 LVI as a Reliable Predictor of Ventilator Discontinuation .... 212
7.7 Assessing Lung Improvement Index (LII) and Rate of Lung Improvement (m) ....................................................................... 213
7.8 Conclusion ................................................................................................. 215 References ........................................................................................................... 215
This chapter is about modeling of lung ventilation response of COPD patients onmechanical ventilation, and howwe can develop a lung ventilatory index to enable us to assess their lung status and decide when they are ready to be weaned off themechanical ventilator. Inmechanically ventilated patients with chronic obstructive pulmonary disease (COPD), elevated airway resistance and decreased lung compliance (i.e., stiffer lung) are observed with rapid breathing. The need for accurate predictive indicators of lung status improvement is essential for ventilator discontinuation (or extubation) through stepwisereductioninmechanical support, as andwhen patients are increasingly
able to support their own breathing, followed by trials of unassisted breathing preceding extubation and ending with extubation. For determining if a patient is ready to be discontinued from mechanical
ventilation after the clinician has chosen an appropriate indicator to assess lung status he or she will incorporate this indicator into three general approaches for ventilator discontinuation which are (1) synchronized intermittentmandatoryventilation (SIMV)where thenumber of breaths is supplied by the ventilator and lowering the ventilator breaths will initiate more spontaneous breaths in the patient; (2) pressure support ventilation (PSV) which provides inspiratory pressure assistance based on spontaneous efforts; and (3) spontaneous breathing trial (SBT). The intent of the ventilatorydiscontinuation process is to decrease the level of support provided by the ventilator, requiring the patient to assume a greater proportion of the ventilatory workload. For stepwise reduction in mechanical ventilatory support, the most useful
clinically employed indicators have been rapid shallow breathing index (RSBI) <65 breaths=min=L (measured using ventilatory settings) and respiratory rate or frequency (RF) <38 breaths=min. However, these are extrinsic empirical indices; currently, there is no known easy-to-use, reliable indicator that incorporates the intrinsic parameters governing the respiratory system mechanics for indicating lung status improvement or deterioration and eventual ventilator discontinuation. For this reason, we have developed an easy-to-employ lung ventilatory index (LVI), involving the intrinsic parameters of a lung ventilatory model, represented by a first-order differential equation in lung volume response to ventilator driving pressure. The LVI is then employed for evaluating lung status of COPD patients requiring mechanical ventilation because of acute respiratory failure.
