Gas is driven to and from the lung by a pressure difference between alveolus and airway opening. The majority of adult patients are ventilated, at least initially, with a volume-preset mode (i.e., ACV or intermittent mandatory ventilation [IMV]),1 allowing ready determination of the respiratory system mechanics. When a muscle-relaxed patient is mechanically ventilated at constant inspiratory flow, the inspiratory Pao consists of three components: one to drive gas across the inspiratory resistance, the second to expand the alveoli against the elastic recoil of the lungs and chest wall, and the third equal to the alveolar pressure present before inspiratory flow begins (PEEP or auto-PEEP) (Fig. 32-1).
During constant flow, volume-preset ventilation of a passive patient, Pao is composed of resistive and elastic elements, the latter consisting of the end-expiratory pressure (PEEP or auto-PEEP) and a component proportional to the change in volume and the respiratory system compliance. The second breath includes an inspiratory pause allowing determination of the components of Pao.
The contributions of each of these three components can be found by inserting a 0.3- to 0.5-second end-inspiratory pause,2 briefly stopping flow and allowing the pressure to fall from its peak value (Ppeak) to a plateau pressure (Pplat), in order to quantitate the flow-related pressure, Pres (Pres = Ppeak − Pplat). At any point during passive inspiration, Pao reflects the sum of these three components, as follows:
where Pao is the airway opening pressure, Pres is the resistive pressure component, Pel (Pel = Pplat − Total PEEP) is the elastic pressure, Rrs is inspiratory resistance, ΔV is the increment in lung volume, Ers is elastance of the respiratory system, and total PEEP is applied PEEP or auto-PEEP, whichever is higher. Diagnostic and therapeutic information can be gleaned by distinguishing the individual components of the peak Pao (Ppeak) as follows. First, PEEP is set on the ventilator and this value can be used when auto-PEEP is absent. However, auto-PEEP is present in most ventilated critically ill patients,3 and methods for quantitating it are described below. The Ppeak can be apportioned between its two remaining components, Pres and Pel, by stopping flow (end-inspiratory pause) and allowing the Pres to fall to 0. When flow is 0, Pao drops to a lower Pplat. Then:
The final component (Pel = Pplat − Total PEEP) is proportional to the elastance of the respiratory system and the tidal volume.
At normal inspiratory flow rates in the range of 1 L/s, Pres is typically between 4 and 10 cm H2O. Elevated Pres is found with high inspiratory flow or increased inspiratory resistance. At constant flow, a rise in Pres may indicate, for example, increased bronchospasm or endotracheal tube obstruction. Conversely, falling Pres may correspond to a response to bronchodilators. Because the Pres depends on ventilator flow rate as well as inspiratory resistance, when interpreting its value one must be careful to take the set flow rate into consideration. The most dramatic example of potential error in this regard is when the inspiratory flow is set to a decelerating profile (Fig. 32-2).
This is a passive patient with modest airflow obstruction ventilated with a volume-preset mode and square wave flow (panel A) at 60 liters per minute (lpm) or decelerating flow (panel B) beginning at 60 lpm. A 0.4-second end-inspiratory pause is set in order to allow determination of Pplat. Notice that there is a significant difference between Ppeak and Pplat (40 to 22) during square wave ventilation, but not during decelerating flow (27 to 22) because flow is so low during the later parts of the breath.
Since Pel = Δ V × Ers, elevated Pel indicates excessive tidal volume or increased elastic recoil of the lungs or chest wall, as in pulmonary fibrosis, acute lung injury, or abdominal distention. Respiratory system static compliance (Crs) is the inverse of Ers:
which is normally about 70 mL/cm H2O. When the tidal volume is a typical 500 mL, Pel should be only about 7 cm H2O (500 mL/70 mL per cm H2O). Thus a ventilated healthy patient should have a Ppeak of roughly 17, consisting of Pres (5~cm H2O), Pel (7 cm H2O), and applied PEEP (5 cm H2O). Often the cause of ventilatory failure has not been determined by the time of endotracheal intubation. If the Ppeak is not increased in a passive ventilated patient, the physician should suspect impaired drive, neuromuscular weakness, or a transient, now resolved, problem (e.g., upper airway obstruction bypassed by the endotracheal tube) as the cause for ventilatory failure. When the Ppeak is high, partitioning its components into the resistive pressure (Pres), the elastic pressure (Pel), and PEEP can aid the physician to narrow the differential diagnosis (Fig. 32-3 and Table 32-1).
Table 32–1. Differential Diagnosis of Elevated Peak Airway Pressure ||Download (.pdf)
Table 32–1. Differential Diagnosis of Elevated Peak Airway Pressure
|Increased Pres||Increased Pel||Increased Total PEEP|
|High flow||High tidal volume||High applied Peep|
|Secretions|| Rib deformity||Expiratory limb malfunction|
|Kinked or obstructed tubing|| Pleural disease|
|Airway edema|| Abdominal distention|
|Airway foreign body||Lung|
| Interstitial lung disease|
| Lung resection|
| Pulmonary edema|
Both patients have elevated airway pressures. A brief pause inserted at end-inspiration reveals a striking difference between the two records: the left-hand tracing shows that Pao falls dramatically when flow is stopped, indicating elevated Pres (this patient had status asthmaticus); the right-hand tracing shows that Pao falls quite modestly, since Pel is elevated (this patient had a massively distended abdomen and abdominal compartment syndrome). Note also that expiratory flow differs substantially between the two, with low and prolonged expiratory flow in the left-hand tracing.
In addition, such analysis may allow therapy to be tailored specifically to the cause of ventilatory failure. For example, in a patient with COPD and congestive heart failure who fails extubation following colon resection, bronchodilators will not be helpful if Pres is normal and auto-PEEP is 0. Similarly, if auto-PEEP is greatly elevated, measures to decompress the abdomen are not likely to get the patient off of the ventilator.
The inspiratory pressure waveform during pressure-preset modes, such as PSV and PCV, reflects ventilator settings only and reveals nothing about the respiratory system physiology. These waveforms serve mostly to reveal the current ventilator settings as a snapshot (Fig. 32-4) or to demonstrate the impact of certain complex modes on ventilator actions (Fig. 32-5).
Flow and pressure waveforms during PCV, showing the typical linear fall in flow through the breath. The pressure tracing merely reflects the ventilator settings as pressure cycles between PI (32 cm H2O) and PEEP (14 cm H2O).
These waveforms of flow and pressure demonstrate the effect during pressure-regulated volume control mode of increasing the tidal volume. Over the course of several breaths, pressure gradually rises, driving more flow and increasing the tidal volume, until the new target tidal volume is reached.
During either volume-preset or pressure-preset ventilation, analyzing the expiratory pressure [Paw(ex)] is substantially less useful than the inspiratory pressure, since Paw(ex) is largely determined by characteristics of the mechanical ventilator, not the patient.
where PEEP is the applied PEEP (not auto-PEEP), FlowE is expiratory flow rate, and Rexlimb is the resistance of the expiratory limb of the ventilator. It is important to realize that Paw(ex) does not reflect expiratory alveolar pressure or auto-PEEP, and relates to the patient's respiratory system only indirectly through the expiratory flow. Although some ventilators display inspiratory and expiratory pressure-volume plots, only the inspiratory segment gives useful information about the patient.