What are the differences between pressure controlled and volume-controlled ventilation

Volume-controlled and volume-targeted ventilation

Pressure-controlled ventilation (PCV), also referred to as pressure-limited ventilation (PLV) became the standard approach to ventilation in the early days of neonatal care when attempts at volume-controlled ventilation proved to be ineffective in small preterm infants with the equipment available at the time. Despite strong evidence of the benefits of volume-targeted and volume-controlled ventilation, PCV remains the prevailing mode of ventilation in the NICU in many parts of the world, including the USA [70–72], likely because of its simplicity, tolerance of large ETT leaks and simple inertia. Clinicians continue to cling to the belief that directly controlling PIP is important despite clear evidence that excessive volume, not pressure is the direct cause of lung injury, as outlined earlier in this chapter. The main danger of PCV is the inability to directly control delivered VT, which varies with changes in lung compliance, potentially leading to excessively large VT when a rapid improvement in compliance occurs after intubation with lung volume recruitment and surfactant administration. Clinicians typically do not respond to these changes rapidly enough with manual adjustment of inflation pressure and consequently inadvertent hyperventilation and volutrauma commonly occur, resulting in permanent lung and brain damage. Insufficient VT is also detrimental and may develop when lung compliance or spontaneous respiratory effort decrease. Ventilation with an inadequate VT is inefficient due to increased dead-space:VT ratio and results in tachypnea, increased work of breathing and oxygen consumption, agitation, fatigue, atelectasis/atelectrauma, and eventually hypercapnia, possibly increasing the risk of intraventricular hemorrhage (IVH). Thus relatively tight control of VT delivery during mechanical ventilation is highly desirable.

Ventilatory Modes

There has been controversy both in ICU patients and also in patients undergoing OLV, whether PCV or volume-controlled ventilation (VCV) can be preferred as the mode of ventilation, considering their effects on both oxygenation and lung protection.

In VCV, the VT is preset and constant, delivered with a constant flow with a square waveform. In PCV, the inspiratory pressure is set, and the pattern in flow is decelerating. From a physiologic point, the decelerating flow pattern could allow a more homogeneous gas distribution than a square flow pattern. Initial studies had suggested a possible advantage in terms of gas exchange.133 However, several other studies have not observed the alleged benefits of PCV.134–136 Thus the beneficial effect on gas exchange of PCV compared with VCV remains inconclusive.

The main advantage of PCV versus VCV appears to be lower peak airway pressure that might decrease the risk for barotrauma during mechanical ventilation. However, peak airway pressure does not reflect peak alveolar pressure: peak airway pressure is much greater and depends on ETT resistance, inspiratory flow, and the respiratory mechanics of the lung. The correlation between plateau airway pressure and the incidence of barotrauma is much stronger than the peak airway pressure. A plateau pressure (Pplat) of more than 35 mm Hg has been shown to induce (or be associated with) a ventilation-induced barotrauma.137 There is almost no difference in Pplat between VCV and PCV, and, consequently, in lung protection. On the other hand, another study has shown that the use of PCV improved RV function than the use of VCV during OLV.138 When carbon dioxide is insufflated into the operated hemithorax, the VTs that are actually delivered can vary in proportion to the changes in insufflating pressure.

To date, there is no clear evidence to support the advantage of any one of these ventilation modes over the others. It is rather more important to keep the “driving pressure” (plateau pressure—PEEP) as low as possible.109,139

Mechanical Ventilation Mode

During OLV, VCV guarantees that the desired VT is achieved irrespective of external pressures on the ventilated lung. In turn, PCV guarantees that the driving pressure is constant, and peak pressure is lower than during VCV. When switching from TLV to OLV, the pressure-control modus might be useful to avoid overdistension because of inadvertently high VT. However, if the respiratory system compliance is reduced, hypoventilation can occur. In a relatively small clinical trial, the ventilation mode termed “pressure-control with volume guaranty” (PC-VC) reduced airway pressures in patients undergoing thoracotomy, kept VT approximately constant, and decreased the inflammatory response and lung injury compared with volume-controlled ventilation.102

Evidence from large trials is missing to allow recommendations regarding a particular mechanical ventilation mode for OLV. In our experience, PCV can be particularly useful when leakage in the respiratory system is present, for example, when fiberscopes are used because the inspiratory flow is increased and partially compensates the escape of gas. Also PCV might be valuable when the pressure in the airways cannot exceed a given value. PCV is valuable during the use of endobronchial blockers in which the peak pressure should ideally maintain under 30 mm H2O, or during procedures with carbon dioxide insufflation to avoid excessive peak airway pressure. However, PCV, and even PC-VC, may allow reduction of VT over multiple cycles, which can result in atelectasis of the ventilated lung. VCV is less prone to such fluctuations but can lead to higher peak airway pressures if the compliance of the respiratory system is reduced.

DRE Integra

The DRE Integra SL-MRI anesthesia machine (Avante Health Solutions Company, Louisville, KY) is MR conditional for use with a 3-T scanner and up to 1000 G. DRE Integra (Fig. 20.9) offers six ventilation modes: PSV, PCV, SIMV, Synchronized Mandatory Minute Ventilation (SMMV), spontaneous, and auto-PEEP. It includes an AV-S ventilator, A200SP absorber, Penlon sigma delta vaporizer, and an antihypoxic flowmeter. The AV-S ventilator is a versatile unit designed for adult or pediatric use. The ventilator also provides oxygen monitoring, spirometry, and integration with the absorber. The A200SP absorber has a quick-release canister and has a ventilator mode switch triggered by the bag/ventilator selector switch.

Inspiratory flow pattern

The simplest form of CMV uses a constant inspiratory

Although there are no convincing outcome data differentiating these different modes of CMV, PCV is increasingly used when the peak airway pressure (Ppk) is high with VCV. However, the alveolar distending pressure, which is usually inferred from the plateau pressure (Pplat), is no different provided that Ti and VT are the same.6 During PCV, Pres is dissipated during inspiration so Ppk and Pplat are equal, and during VCV Pres accounts for the difference between Ppk and Pplat (Fig. 31.2). Similarly, different CMV

Pressure-regulated volume control (PRVC) is a form of CMV where the VT is preset, and achieved at a minimum pressure using a decelerating flow pattern.

Mode of ventilation

Non-invasive ventilation should not be routinely used in ARDS (see Ch. 37) and most patients require intubated mechanical ventilation. Following intubation, controlled ventilation allows immediate reduction in the work of breathing, and application of PEEP and a controlled

An advantage of assist-control ventilation (as used in the ARDS Network study) is that spontaneous effort generates a controlled VT. Care should be taken with synchronised intermittent mandatory ventilation (SIMV), particularly if pressure support is added to SIMV, as excessive VT may occur during supported breaths. There is an increasing tendency to use pressure-controlled ventilation (PC) or pressure-regulated volume control (PRVC) as Ppk is lower than volume-controlled (VC) ventilation with a constant inspiratory flow pattern. However, the decelerating flow pattern of PC or PRVC means that most of the resistive pressure (Pres) during inspiration is dissipated by end inspiration, which is in contrast to VC with a constant inspiratory flow pattern where Pres is dissipated at end inspiration (see Fig. 31.2). Consequently, with PC and PRVC Ppk ≈ Pplat which is the same as Pplat during VC.81 Both oxygenation, haemodynamic stability and mean airway pressure are no different between PC and VC, and a moderate-sized randomised study found no difference in outcome.82 However, there may be differences in lung stress due to greater viscoelastic build-up with VC.83

Inverse ratio ventilation, often together with PC, has been used in ARDS. However, when PEEPi and total PEEP are taken into account, apart from a small decrease in

A number of other modes of ventilation (see Ch. 31) including airway pressure release ventilation (APRV) and high-frequency oscillation (HFO) have been proposed for use in ARDS. Randomised clinical trials have not shown improved outcomes with APRV,84 despite potential physiological benefits. The small VT used with HFO are appealing, and some centres use HFO as rescue therapy when conventional ventilatory strategies are failing, while others would consider venovenous extracorporeal membrane oxygenation (ECMO; see Ch. 41). Definitive clinical trials of both trials are underway and awaited.

Anesthesia Ventilators

Current anesthesia machine ventilators are typically pneumatically and electronically powered, ascending bellows, time-cycled, constant flow, and electronically controlled. The lungs of most children can be ventilated very well with such ventilators, notwithstanding differences in compressible volume of the ventilator bellows and breathing circuit. The availability and skilled application of appropriate clinical and instrument monitoring are crucial. The use of a pediatric bellows (300 mL maximum tidal volume) will lessen the compression volume of the bellows itself (4.5 mL/cm H2O compared with 1 to 2.5 mL/cm H2O).16

Controlled ventilation is now much more precise for small patients. Machines now adjust for fresh gas flow and circuit compliance; recent models also allow sampling of the tidal volume measurement at the airway rather than at the expiratory valve, allowing for a better estimate of true tidal volume. Previously, tidal volume was usually measured at the expiratory valve. Thus the measured volume was not only the exhaled gas but also the gas compressed in the circuit during the previous inspiration.

Pediatric anesthesiologists have traditionally preferred pressure-controlled ventilation. Using a preset pressure and assessments such as ETco2 and chest wall excursion, the anesthesiologist was assured of adequate ventilation with a reduced risk of barotrauma. Stayer et al found that flow generated on inspiration did not reach the set peak pressure when using short inspiratory times in a ventilator without constant pressure or piston-driven bellows.17 In addition, changes in chest wall compliance can greatly influence delivered tidal volume during pressure controlled ventilation, therefore, close attention must be paid to changes in ETco2 and chest wall excursion.

Newer ventilators adjust for fresh gas flow and circuit compliance. In Drager machines the fresh gas is not continuously supplied during the expiration phase and is decoupled from the patient by a valve. Therefore, fresh gas flow does not influence the tidal volume during the inspiratory phase.18 Additionally the Drager Apollo ventilator measures the compliance of the breathing circuit during the initial check and compensates for it so as to deliver accurate tidal volumes. The Aisys by General Electric also measures circuit compliance and allows for compensation during the initial check. Some older ventilator models such as the Datex-Ohmeda Smartvent and the GE Avance do not have circuit compliance compensation but allow for flow sensors at the inspiratory valve, thus offering better volume control and measurement. If the ventilator has a decoupling mechanism and circuit compliance compensation, the anesthesiologist should feel comfortable using volume-controlled ventilation since this newer generation of ventilators is less influenced by fresh gas flow and circuit compliance. For the compliance compensation to be accurate, however, machine checkout must occur with each new circuit placed on the machine. Even with the new advances, examination of chest wall excursion, ETco2 and blood gas analysis remain the gold-standard tools for assessing the adequacy of pulmonary ventilation.

MECHANICAL VENTILATION VERSUS SPONTANEOUS BREATHING

During spontaneous breathing, respiratory muscles establish negative intrathoracic and intrapulmonary pressures and, by downward movement of the diaphragm, a positive intra-abdominal pressure. The resulting intrathoracic pressure–to–ambient pressure gradient allows air to flow into the lungs. The physiological mechanism of spontaneous breathing facilitates venous return, thereby supporting hemodynamics. In contrast with spontaneous breathing, mechanical ventilation uses positive pressure to inflate the lungs.

In most patients with ALI or ARDS, either volume-controlled or pressure-controlled ventilation is used. In the volume control mode, a volume is preset on the ventilator, resulting in a variable airway pressure, whereas in the pressure control mode, the inspiratory pressure is preset, resulting in a certain tidal volume. Thus, the airway pressure results from the applied tidal volume or preset inspiratory pressure and on the preset basic end-expiratory volume and depends on lung compliance, airway resistance, and air flow.

During mechanical ventilation, pressure gradients are altered considerably compared with pressure gradients in spontaneously breathing subjects. Intrathoracic, intrapulmonary, and intra-abdominal pressures increase during inspiration and remain positive during the breathing cycle. Only at the end of expiration do they equalize with ambient pressure, when no positive end-expiratory pressure (PEEP) is applied. PEEP usually is applied to prevent the alveoli from collapsing at end expiration. Consequently, mechanical ventilation exerts systemic hemodynamic effects through a complex interaction among intrathoracic pressure, intravascular volume, and cardiac performance. Mechanical ventilation decreases cardiac output by decreasing preload, affecting both left ventricular geometry and pulmonary vascular volume and resistance, and, in addition, increasing right ventricular afterload. Evidence for these proposed mechanisms has been known for decades, based on studies in animal models and human subjects during spontaneous ventilation or controlled mandatory ventilation in combination with PEEP.4,5

Anesthesia Machines

The Dräger Fabius MRI (Dräger Medical, Telford, PA) anesthesia workstation was FDA approved in 2008 as MRI conditional for the 1.5-T and 3-T environment up to a field strength of 400 G (Fig. 27-4). The Fabius is equipped with a Teslameter that emits an acoustic signal when the machine reaches the 400 G perimeter. The Fabius is equipped with two vaporizers and uses an electronically controlled ventilator able to deliver multiple modes of ventilation in the MRI suite. These include spontaneous, volume- and pressure-controlled, pressure-support mode, synchronized intermittent mechanical ventilation (SIMV), and SIMV with pressure support. The ventilator may be mounted on either the right or left side to accommodate the logistics of a particular MRI environment. The alarms, large and visible, are equally flexible with respect to ease of use and visibility: they are redundant on both the right and left side. The Fabius is on wheels to facilitate easy mobility in the MRI suite. It has castors around each wheel, however, to reduce unwanted movement. Two vaporizers may be mounted on the Fabius simultaneously. The machine can be equipped with either an interlock or an autoexclusion system to ensure that only one vaporizer is turned on at a time. The Fabius is plugged into the wall and accommodates pin indexed E cylinders and gas from a central source. Even without a power supply, the Fabius can still deliver gas and permit the use of vaporizers. The internal battery can function for a minimum of 45 minutes in the event of a power failure. The ventilator is piston driven, so even in the event of a compressed gas supply failure, the ventilator will still function to ventilate the patient by entraining room air. This is a significant advancement in the safe delivery of anesthesia in the MRI environment. Gas-driven bellows ventilators require a supply of compressed gas to function; in the event of a gas supply failure, they would be unable to function. Manual ventilation using the reservoir bag would also not be possible.

High-frequency oscillatory ventilation

During high-frequency oscillatory ventilation (HFOV), a high mean airway pressure is applied to the lungs and very small tidal volumes, typically 1–3 mL/kg, are delivered by an oscillating diaphragm at rates of 3–15 Hz, or 180–900 breaths per minute.85–88 Theoretically, this should be an ideal approach to minimize VILI in ARDS. The small tidal volumes can prevent volutrauma, the high mean airway pressure can recruit collapsed lung and prevent atelectrauma, and the avoidance of high inspiratory pressure swings can prevent barotrauma.40,89

Two early trials of patients with ARDS with PaO2/FiO2 ratio ≤ mm Hg showed that there was no difference in mortality of ventilator-free days when HFOV was compared with pressure-controlled ventilation, suggesting HFOV was safe to use in ARDS.90,91 However, the control groups in these trials did not receive LTV ventilation.

In 2013, Ferguson et al.92 reported the results of the Oscillation for ARDS Treated Early (OSCILLATE) trial that compared HFOV with lung-protective ventilation with 6 mL/kg PBW tidal volumes, Pplat ≤35 cm H2O, and high PEEP in patients with ARDS with PaO2/FiO2 ratio ≤200 mm Hg. The trial was stopped after 548 of the planned 1200 patients had been randomized due to a signal of harm in the HFOV group. In-hospital mortality was 47% in the HFOV group and 35% in the control group (absolute risk 12%; RR 1.33, 95% CI 1.12–1.79; P = .004).

At the same time, Young et al.93 reported the results of their large multicenter Oscillation in ARDS (OSCAR) trial that compared HFOV and conventional lung-protective ventilation in patients with ARDS with PaO2/FiO2 ratio ≤200 mm Hg. The trial included 789 patients and found no difference in 30-day mortality (41.7 vs. 41.1%; P = .85).

In the OSCILLATE trial, the conventional ventilation strategy was strictly protocolized, whereas patients in the conventional arm of the OSCAR trial were managed according to local practice. As a result, tidal volumes in the control group of the OSCILLATE trial were smaller than those in the OSCAR trial (6.1 ± 1.3 mL/kg PBW vs. 8.3 ± 2.9 mL/kg PBW) and PEEP levels were higher in the OSCILLATE trial (18 ± 3.2 cm H2O vs. 11.4 ± 3.6 cm H2O).94

Two meta-analyses of trials examining HFOV suggested that there is no benefit to using HFOV over conventional lung-protective ventilation and suggested that HFOV may actually cause harm.26,95 A recent individual patient data meta-analysis, however, showed significant heterogeneity of treatment effect with HFOV vs. conventional ventilation, with baseline PaO2/FiO2 ratio being an important effect modifier.96 Patients with severe hypoxemia appeared to benefit from HFOV, while those with moderate ARDS may have been harmed.