Learning Objectives Covered:
1. Explain the importance of monitoring plateau pressures and its use in calculating static compliance
2. Explain the use of volume-controlled ventilation and pressure-controlled ventilation
3. List and describe ventilatory support treatment plans for patient’s based on their clinical diagnosis
Compliance is a measurement of the distensibility of the lung or the ability of the lung to distend. It is expressed as a change in volume divided by a change in pressure using the standard units of Liters/cmH20. The normal lung + thorax compliance of an adult is around 0.1 L/cmH20. When the compliance is low, more pressure will be needed to deliver a given volume of gas to a patient. Diseases that cause low lung compliance are classified as restrictive diseases and include Adult Respiratory Distress Syndrome (ARDS), pulmonary edema, pneumonectomy, pleural effusion, pulmonary fibrosis, and pneumonia among others. Emphysema is a typical cause of increased lung compliance.
When measuring lung compliance one must know the delivered tidal volume and must also know the change in alveolar pressure that results from the addition of that known tidal volume. Alveolar pressure is the pressure in the distensible parts of the respiratory tract and is determined by the tidal volume and the lung/chest compliance. Airway pressure is the pressure measured at the patient’s airway during mechanical ventilation. Airway pressure is equal to alveolar pressure when there is no occurrence of airflow. At the end of a mechanical inspiration, flow to the distal parts of the lungs continues even after inspiratory flow from the ventilator stops, as time is required for gas to reach the periphery of the lung. To measure alveolar pressure, one must measure the airway pressure at a time when both pressures are equal, i.e. when there is no flow.
We normally assume that alveolar and airway pressure starts out at atmospheric (our zero reference) before an inspiration starts. To equalize airway and alveolar pressures, we only have to prevent exhalation after inspiration has ceased by utilizing an inspiratory hold maneuver. The actual calculation is to divide the delivered tidal volume by the plateau pressure where the plateau pressure is the steady-state pressure measured during an inspiratory hold maneuver. Since approximate values are adequate for clinical use, clinicians use the plateau pressure minus the end expiratory pressure that is then divided into the exhaled tidal volume as measured by the ventilator. This compliance measurement is referred to as static compliance since it is measured after an inspiratory hold and there is no gas flow during its measurement.
Cstatic = exhaled VT (ml) Pplat (cmH2O) – PEEP (cmH2O)
VT – Tidal Volume
Pplat = Plateau Pressure
A spontaneously breathing person has a normal compliance of approximately 100mL/cmH2O. In intubated patients, normal compliance is approximately 50mL/cmH2O.
Volume Control Ventilation is a type of ventilation in which a clinician sets a constant preset volume that is delivered to the patient’s lungs. In order for volume to remain constant with each breath, if compliance or airway resistance is changed then the ventilator changes the amount of pressure needed to deliver the breath. In other words, pressure will adjust to ensure that the preset tidal volume is delivered. For example, a patient receiving mechanical ventilation has developed congestive heart failure. Congestive heart failure is a restrictive disorder that results in pulmonary edema filling the interstitial spaces of the lungs. The edema makes inflating the lungs difficult. Since the ventilator is set to deliver a specific tidal volume, the pressure needed to deliver the tidal volume will be increased because the pressure needed to overcome elastic compliance is increased. Using excessive pressures to deliver ventilatory support increases the risk of injury to the lungs. This type of injury an is referred to as barotrauma. Barotrauma is injury to the lungs as a result of pressure changes. A specific type of injury that commonly occurs during delivery of mechanical ventilation is a pneumothorax, which is a rupture of one or both lungs.
The pressure used to overcome both elastic compliance (of the lungs and chest wall) and airflow resistance of the airways is referred to as the Peak Inspiratory Pressure (PIP or Ppeak). Peak inspiratory pressure is the maximum pressure in the circuit reached during delivery of a mandatory breath from a ventilator. Therefore, if volume remains constant then pressure must be adjusted to ensure that the set tidal volume is delivered despite any changes that occur in the lungs. The advantage of volume control ventilation is that alveolar ventilation remains constant so PaCO2 is not affected. During volume control ventilation, a minimum minute ventilation can be guaranteed which is useful when stabilizing ventilation.
Pressure Control Ventilation is a type of ventilation in which the ventilator delivers an inspiration until a preset pressure is reached. During pressure control ventilation, pressure is limited and if the compliance or airway resistance is changed then the volume of air delivered is changed. In other words, the preset pressure will not be exceeded but the tidal volume will change depending on changes that occur in the lungs. Take for example a patient who has developed secretions in the airway. Secretions accumulate in the airways and cause airflow resistance. The more airflow resistance that is encountered on inspiration the more pressure that is needed to overcome the obstruction. Think of pressure as a driving force. Inspiratory pressure overcomes the resistance and compliance of the lungs to inflate the lungs so the lungs can be filled with air. However, in pressure control ventilation the delivered pressure is limited. Once the set pressure is reached inspiration is terminated. This may result in the lungs may not being fully inflated which means less air delivered to the lungs with smaller tidal volumes. The more airway resistance affecting the lungs the less volume of air will be delivered. The same works for compliance. The lower the compliance (stiffer lungs) the less volume of air will be delivered.
For this assignment, you will provide detailed responses to the following questions.
Be sure to review the link below regarding Calculations Commonly Performed in Respiratory Care
1. Describe the difference between dynamic compliance and static compliance. What useful information do we receive by monitoring dynamic compliance? What useful information do we receive by monitoring static compliance?
2. Calculate compliance given Vt = 500 ml, Peak airway pressure = 30 cmH2O, Plateau pressure= 25 cmH20, PEEP = 10 cmH2O.
3. Calculate static and dynamic compliance given Vt = 760 ml, Peak airway pressure = 38 cmH2O, Plateau pressure= 33 cmH20, PEEP = 7 cmH20.
4. Calculate static and dynamic compliance on a patient who is on a volume ventilator and has the following measurements: Tidal Volume = 780 ml, Peak Airway Pressure = 45 cmH20, Plateau pressure 40 cmH2O, PEEP = 10 cmH2O.
5. Calculate static and dynamic compliance: Tidal Volume 800 ml, Peak Airway Pressure 20 cmH2O, Peak Inspiratory Pressure 30 cmH2O, Plateau pressure 35 cmH2O, Peep 10 cmH2O.
6. Explain permissive hypercapnia and why this strategy is used for ventilating COPD patients in acute respiratory failure.
7. What is the recommended strategy for ventilating patients with ARDS?
8. What is the recommended strategy for ventilating patients with traumatic brain injury?
Submit your answers in at least 500 words on a Word document. You must cite at least three references to defend and support your position.