Ventilatory Mechanics
38834Why do we care about lung mechanics during mechanical ventilation? We all should if we want to provide adequate ventilatory support with minimum adverse effects. It would be useful if we could easily measure lung compliance, airway resistance, functional residual capacity, and other pulmonary function parameters during mechanical ventilation. Unfortunately, it is difficult to conduct formal pulmonary function testing on critically ill patients who frequently require high airway pressures and flows and may be paralyzed or otherwise unable to cooperate with testing. We will explore a few simple bedside maneuvers that anyone can perform either with or without patient cooperation to assess lung mechanics.
Ventilatory mechanics can be assessed before intubation to determine the need for mechanical ventilation and prior to extubation to determine a patient’s readiness to wean from the ventilator. Ventilatory mechanics are used to assess a patient’s ability to protect their own airway, maintain adequate alveolar ventilation as well as determine respiratory muscle strength. There are many criteria used to assess muscle strength.
The Maximum inspiratory pressure, MIP also referred to as NIF is the most negative or lowest pressure that is generated against an occluded airway during a forceful inspiratory effort. MIP is measured using a pressure manometer and indicates the inspiratory muscle strength and reflects the strength of the diaphragm and other inspiratory muscles.
Tidal volume (VT) and vital capacity (VC) are measurements that can be obtained with bedside spirometers and provides an overall assessment of respiratory muscle function because it measures the patient’s ability to generate adequate volumes of air.
The MEP is a static expiratory pressure measured by exhaling into a closed system. MEP reflects the strength of the abdominal and other expiratory muscles and is used to assess cough effort. A cough is a mechanism that protects the airway. A cough must be sufficient to clear the airways of secretions and thus the inability to effectively cough may indicate the need for mechanical ventilation.
Determining a patient’s readiness to wean is best approached by a carefully supervised spontaneous breathing trial (SBT). During an SBT the patient’s ability to tolerate the trial can be assessed by calculating their rapid shallow breathing index (RSBI). RSBI is the ratio of respiratory rate to tidal volume and though there are many variables that indicate the patient’s tolerance of an SBT, RSBI is one of the best measured indicator’s of the patient’s success to wean. RSBI is a calculated measurement and is determined using the formula: RSBI = f/VT. A RSBI of 105 indicates that the patient either needs more ventilatory support or needs to be returned to their previous mode of ventilation.
Ventilatory Mechanics Normal Acceptable
Minute ventilation (l/min) 5 to 6 5
Vital Capacity (ml/kg) 65 to 75 > 15
MIP (cmH2O) -50 to -100 -20 or greater
Cough Effort MEP (cmH2O) >40
Readiness to Wean
RSBI < 105
Lung mechanics provide highly valuable information in the management of mechanically ventilated patients. Lung mechanics assist respiratory therapists in determining the need for mechanical ventilation, how well it is being tolerated by the patient, and when mechanical ventilation can be discontinued. In addition to lung mechanics that help us determine respiratory muscle strength we also reviewed lung mechanics such as airway resistance, dynamic and static compliance and their contribution to the normal physiology of our lungs.
If you recall, airway resistance can be estimated by dividing the difference between peak and plateau airway pressures by the mean inspiratory flow rate. Some ventilators have an inspiratory flow rate setting that you can read for an estimate of delivered flow rate while others give an inspiratory time setting where you have to divide the tidal volume by the inspiratory time to determine the inspiratory flow rate. An alternative way of determining airway resistance is to calculate a nonsense parameter known as the dynamic compliance. The dynamic compliance is the result of dividing the delivered tidal volume by the peak airway pressure. Since peak airway pressure is determined by a combination of the lung compliance, the airway resistance, inspiratory flow rate, and the tidal volume, this value does not really give a quantitative estimate of airway resistance itself but can be used to detect changes in the airway resistance if all other factors are held constant. This makes the value useful for comparing measurements on a single patient over a short period of time but it is too much to ask to expect that all of the other variables affecting peak airway pressure will stay the same from day to day or certainly from patient to patient. Because of the limitations of dynamic compliance measurements, it makes more sense to just follow the peak pressure to plateau pressure gradient since it requires less math and is just as useful as the dynamic compliance calculation.
Prompt
For this assignment, you will provide detailed responses to the following question and provide detailed responses to the case study.
1.A patient with fibrotic lung disease is on a volume control ventilator at a CMV rate of 8 breaths/min. The respiratory care practitioner notices it is taking more and more pressure to ventilate the patient. What would you recommend? Why?
2.Over several hours the RCP notices that Cstat and Cdyn are decreasing. What is the most likely nature of the problem (Restrictive or Obstructive)? Explain your answer.
3.Review the data below regarding your patient and interpret the data. Hint: Is the airway resistance and/or compliance increasing or decreasing? Why? or why not?
0800 hrs. 1200 hrs.
Delivered VT 800 ml 800 ml
PEEP 5 cmH2O 5 cmH2O
Peak Pressure 55 cmH2O 65 cmH2O
Plateau Pressure 45 cmH2O 55 cmH3O
Peak Flow 60 l/m 60 l/m
Submit your answers in at least 500 . You must cite at least three references in APA format to defend and support your posit
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