Lung Mechanics & Mechanical Ventilation

By Donald R. Elton, MD, FCCP
Lexington Pulmonary and Critical Care

Who Cares?
Why do we care about lung mechanics during mechanical ventilation? We all should if we want to provide adequate ventilatory support with a minimum of 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. This paper will explore the theory and application of a few simple bedside maneuvers that anyone can perform either with or without patient cooperation to assess lung mechanics.

Compliance is a measurement of the distensibility of the lung. It is expressed as a change in volume divided by a change in pressure. The standard units of 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 need to deliver a given volume of gas to a patient. Disease states resulting in low compliance include the 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.

Airway Resistance
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 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 use of artificial airways, asthma, emphysema with airway collapse, mucus plugging, vocal cord paralysis, and endobronchial obstruction either from tumors or foreign bodies.

Time Constant
The Time Constant of the lung (TC) is a concept borrowed from electrical engineering which describes the phenomenon whereby a given percentage of a passively exhaled breath of air will require a constant amount of time to be exhaled regardless of the starting volume given constant lung mechanics. That's quite a mouth-full of a definition but consider what determines how long it takes to exhale a tidal breath passively. At the start of exhalation, the initial flow of gas out of the lung depends upon the driving pressure (i.e. alveolar pressure - mouth pressure) and it depends on the airway resistance. For any given volume of gas, the alveolar pressure at the start of exhalation is only dependent upon the lung compliance. Mathematically, the time constant is defined as compliance multiplied by the airway resistance and the resulting value has units of seconds of time..

Airway Pressure & Alveolar Pressure
Airway pressure is the pressure measured at the patient's airway during mechanical ventilation. Airway pressure is determined by the sum of the alveolar pressure and the pressure required to deliver flow across the airways which is determined by the airway resistance. 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 equal to alveolar pressure when there is no flow occurring. At the end of a mechanical inspiration, flow to the distal parts of the lung 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. Measuring Compliance To measure 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. We normally assume that alveolar and airway pressure start 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. If precise measurement is necessary then the pressure should be the plateau pressure minus any end expiratory pressure (or the pleural pressure or Auto-PEEP if it is available) and the volume should be either measured at the airway itself or should be corrected for compressible volume loss. In most cases, approximate values are adequate for clinical use so the plateau pressure minus the end expiratory pressure is divided into the exhaled tidal volume as measured by the ventilator. This compliance measurement is sometimes called the static compliance since it is measured after an inspiratory hold such that there is no gas flow during its measurement.

Measuring Resistance
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 such that you can read the control 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 following 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 (or useless) as the dynamic compliance calculation. A third way to estimate airway resistance can be used if the patient is exhaling passively. This method works based on the time constant. The practitioner times how long it takes for the patient to exhale completely and then divides this result by 3 to estimate the time constant. The lung compliance is then measured and divided into the time constant to result in the airway resistance thus: Raw = Time Constant / Clung

Auto PEEP is the popular name used to describe increased alveolar pressure caused by gas trapping during mechanical ventilation. Gas trapping occurs when there is inadequate time to exhale the mechanical tidal volume. Recall that the time constant determines the length of time needed for a passive exhalation and that the time constant is the product of airway resistance and lung compliance. The lower the compliance, the higher the driving pressure pushing gas out of the lungs during exhalation; the lower the resistance, the higher the expiratory flow rate can be when driven by the alveolar pressure. If the time constant is known (or can be estimated) then the maximum mechanical respiratory rate that can be used before Auto PEEP results can be estimated. Consider that at least 3 time constants are required to exhale passively any volume of gas. The combination of inspiratory and expiratory time leads to a give respiratory rate such that:

Total Breath Time = Insp time + Exp time

Respiratory Rate = 60 / Total Breath Time

Maximum Rate = 60 / (Insp time + 3 x TC)

A patient with a compliance of 0.05 L/cm H20 and an airway resistance of 30 cm H20/L/sec. This would give a time constant of 1.5 seconds. A complete exhalation would take around 4.5 seconds. If inspiratory time is 1 second then total breath time is 5.5 seconds and the maximum respiratory rate without gas trapping would be 11 breaths per minute. When gas trapping occurs, the functional residual capacity (FRC) is increased. As the FRC increases, the alveolar pressure increases by an amount of pressure determined by the patient's lung compliance. As the FRC rises in relation to the total lung capacity (TLC), the lung compliance will decrease. This decrease in lung compliance shortens the time constant for the next breath and thus shortens the time required to exhale the next breath and lessens the amount of trapping that will occur with each subsequent breath until the time constant shortens enough that gas trapping no longer occurs. When this steady state is reached, the FRC is at its maximum and the auto-PEEP is also at its maximum. This fact gives us a way to measure how much auto-PEEP exists since we can serially measure exhaled tidal volumes and then interrupt ventilation (by turning the respiratory rate to zero for several seconds) and measuring how much gas the patient exhales as the patient exhales back to the FRC level that existed prior to ventilation. If we take the difference between the exhaled volume during ventilation and the exhaled volume after interrupting ventilation then we have the amount of gas that was trapped. If we divide this volume by the lung compliance we will have calculated the amount of auto-PEEP applied to the alveoli during ventilation. Normally we are more interested in avoiding auto-PEEP  than in measuring it though there are many patients in whom it cannot be avoided so it is useful to be able to quantitate it. Another way to detect auto-PEEP is to watch a patient's chest movement and/or breath sounds during exhalation to see if exhalation stops prior to initiation of inspiration by the ventilator. If exhalation doesn't finish then auto-PEEP is occurring. When exhaled tidal volumes cannot be measured (which is seldom with modern ventilators) the level of auto-PEEP can be very roughly estimated by interrupting exhalation just prior to initiation of inspiration and watching to see if there is a pressure increase at the airway as exhalation continues into the circuit between the patient and the point of your interruption. This is not an accurate measurement since the interruption necessarily cuts exhalation shorter than it would normally be and because the circuit volume dampens the pressure measurement but this technique can be useful if you are unable to use the more reliable methods outlined above.

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