02 Oct Static vs. Dynamic Mechanics

388191.Identify equations used to calculate static lung compliance, airway resistance, dynamic lung compliance
2.Explain the clinical significance of static compliance, dynamic compliance, airway resistance and mean airway pressure
3.Identify pathological conditions that alter lung compliance and airway resistance

Background

Airways-for-ventilator-posts.jpgRespiratory mechanics refers to principles that express the physiology and function of the lungs. These changes are measured and closely monitored during mechanical ventilation. Respiratory mechanics include principles such as compliance, resistance, elastance, flow, and work of breathing. There are two forces that affect how much pressure is needed to obtain adequate tidal volume during inspiration. These two forces include the friction encountered in the airways as air flows through (airway resistance) and the increase in the elastic recoil of the alveoli as it inflates to a larger volume (compliance). The tendency of the lungs to recoil during inspiration becomes greater as the lungs inflate to accommodate larger volumes of air. In other words, think of the alveoli as balloons. The more air used to inflate a balloon the greater the tendency of the balloon to recoil. There are two major forces in the lungs that cause an inflated lung to recoil inward: the elastic forces of the lungs and the surface tension that lines the alveoli. Elastic forces of the lungs are important for the removal of carbon dioxide. Our lungs must exhale a significant amount of air with each breath so they can accommodate and be filled with oxygenated air. Adding air with each breath without first removing enough of the previous gasses is referred to as breath stacking. Breath stacking increases work of breathing making it difficult to breathe.

Respiratory mechanics can be assessed under static or dynamic conditions. Static conditions refer to respiratory mechanics that are assessed in the absence of airflow (no airflow). Dynamic conditions refer to respiratory mechanics that are assessed during airflow.

Compliance

Compliance is a measurement of the distensibility of the lung. It is the ease with which lungs can be distended or stretched and is expressed as a change in volume divided by a change in pressure. The standard unit of compliance is Liters/cm H20. The normal lung + thorax compliance of an adult is around 0.1 L/cm H20. When the compliance is low, more pressure will be needed to deliver a given volume of gas to a patient. Disease states resulting in low compliance 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.

Dynamic compliance

Dynamic compliance is measured as gas moves in and out of the lungs and refers to the ratio of the change in volume (tidal volume) to the change in distending pressure. Therefore, dynamic compliance describes the pressure needed to overcome the frictional forces of the airways (airway resistance) as well as the inflation pressure needed to overcome the elastic forces of the alveoli (compliance). The pressure needed to overcome these forces is referred to as peak inspiratory pressure or PIP. Alterations in airway resistance as well as in lung and chest wall compliance will influence dynamic compliance. The more resistance that is encountered in the airways or the harder it is to inflate the alveoli, the more pressure required to move air into and out of the lungs. The relationship between airway resistance and dynamic compliance is inversely proportional meaning that as airway resistance increases, dynamic compliance decreases. Dynamic lung compliance is measured by the ventilator using the algorithm Cdyn = VT / PIP – PEEP.

Static compliance

Static compliance is measured at the end of inspiration, during an inspiratory pause, when the lungs are fully inflated. Since static compliance is obtained while performing an end-inspiratory hold, this measurement directly measures the elastic properties of the alveoli and only accounts for the ability of the alveoli to inflate (expand). In the absence of airflow an end-inspiratory hold measures the pressure required to oppose recoil forces and maintain alveolar inflation. This pressure is referred to as plateau pressure. It is important to note that as lung compliance decreases (becomes more stiff) plateau pressure increases. Furthermore, it is important to note that any forces that cause an increase in plateau pressure will also cause an increase in peak inspiratory pressure. As lung compliance increases (improves) plateau pressure decreases. Static lung compliance is measured by the ventilator using the algorithm Cstat = VT / Pplat – PEEP.

Airway Resistance

Air moves through the bronchial airways either as laminar flow or as turbulent flow. Laminar flow is a streamlined gas flow in which the gas molecules move through the airways in a parallel pattern to the sides of the bronchial tubes. Laminar flow occurs at low flow rates and exerts low-pressure gradients. Turbulent flow is random gas flow in which the gas molecules encounter resistance from the airways and from collision by other molecules. Turbulent gas flow occurs due to high flow rates and exerts high- pressure gradients. Keep in mind that a high flow rate causes turbulent gas flows and therefore causes more pressure to be exerted in the airways resulting in higher PIPs. Airway resistance is the amount of pressure required to deliver a given flow of gas and is expressed in terms of a change in pressure divided by flow. The standard units of resistance are cm H20/L/second and the normal value for an adult is around 0.5 – 1.5 cm H20/L/sec while in states of a disease this value may be 100.0 cm H20/L/sec or higher. There really aren’t any diseases characterized by decreased airway resistance since normal values are so low but there are many disease states that result in increased airway resistance including the use of artificial airways, asthma, emphysema with airway collapse, mucus plugging, vocal cord paralysis, and endobronchial obstruction either from tumors or foreign bodies.

The table below summarizes the effects of changing lung mechanics during mechanical ventilation

Condition

Peak Inspiratory Pressure

Dynamic Compliance

Plateau Pressure

Static Compliance

PIP during VCV (VT remains constant)

VT during PCV (PIP remains constant)

Decreased lung compliance

Decrease

Decrease

Decrease

Decrease

Increase

Decrease

Increased lung compliance

Increase

Increase

Increase

Increase

Decrease

Increase

Increased Raw

Increase

Decrease

Unaffected

Unaffected

Increase

Decrease

Decreased Raw

Decrease

Increase

Unaffected

Unaffected

Decrease

Increase

Now let’s take a look at a pathological condition that alters lung mechanics. Patients with obstructive airway disease, such as COPD, have a diminished airway patency due to chronic inflammation (which causes thickening of the mucosa), bronchoconstriction and increased mucus production. The consequence of these conditions is an increase in airway resistance that results in premature airway closure of the small to medium airways during exhalation. Premature airway collapse results in air trapping and also requires much greater inspiratory effort to open them with the next breath. During an exacerbation of their condition, a patient with COPD can be observed using various maneuvers that indicate they are experiencing respiratory distress. These patients can be observed with the following symptoms: rapid, shallow breathing; hunched over in a tripod position; accessory muscle usage; and breathing with pursed lips (pursed lip breathing).

Due to an increase in airway friction (airway resistance) these patients breathe using the techniques mentioned above in an effort to make exhalation less difficult. Pursed-lip breathing is a technique that improves the mechanics of breathing for patients with high airway resistance by creating a back-pressure within the airways causing them to split open during exhalation. When the airways remain splinted open during exhalation more air is allowed to be exhaled. Pursed-lip breathing is a natural way of creating PEEP during exhalation, which makes work of breathing easier by allowing trapped air to escape.

Prompt

You have just admitted a 70-kg (154-lb) 69-year-old male with COPD. He was intubated with an 8.5mm ETT in the ED for acute on chronic respiratory failure and is now apneic. His initial vitals after intubation were as follows:

Pre Intubation

Post Intubation

pH

7.27

7.34

PaCO2

58 torr

47 torr

PaO2

53 torr

68 torr

HCO3

33 mEq/L

33 mEq/L

Vent settings include SIMV, Vt 300 ml, RR 30, FiO2 40%, PEEP 10

The patient’s respiratory mechanics are as follows:

10:00 AM

12 Noon

Peak airway pressure

45 cmH2O

65 cmH2O

Static pressure

24cmH2O

44 cmH2O

Before answering the discussion question below review the data concerning the patient scenario above. Be certain that you have a clear understanding of the patient’s data.
•Interpret the ABG’s.
•What does the information above indicate?
•What are some possible causes for this patient’s measurements?
•What are some possible solutions to improve this patient’s condition?

Modern ventilators compared to earlier models are capable of providing clinicians with valuable information about the patient’s condition while being ventilated. Do you feel that modern technology has improved patients survival rate? How?