◆ Volume-controlled modes of mechanical ventilation guarantee flow and tidal volume, while airway pressures are variable.
◆ In volume-controlled modes, the clinician sets the flow pattern, flow rate, trigger sensitivity, tidal volume, respiratory rate, positive end-expiratory pressure (PEEP), and fraction of inspired oxygen (FiO2).
◆ Patient ventilator synchrony can be enhanced by setting appropriate trigger sensitivity and inspiratory flow rate.
◆ I:E ratio is adjusted to improve oxygenation, avoid air trapping and enhance patient comfort.
◆ Low tidal volume ventilation in conjunction with plateau pressure limitation should be employed in patients with acute respiratory distress syndrome (ARDS).
The goal of mechanical ventilation is to achieve adequate gas exchange, while minimizing haemodynamic compromise and ventilator-associated lung injury (VALI). There are a vast and ever increasing number of ventilator modes and settings to choose from. After endotracheal intubation, the first choice facing the clinician is between two basic modes of mechanical ventilation—pressure- and volume-controlled. In pressure control mode, peak inspiratory pressure is selected and tidal volumes are variable. In volume control mode, tidal volume is guaranteed at the expense of variable airway pressures. Volume-controlled ventilation, (also termed volume-cycled or volume-limited) can be delivered via several modes including controlled mechanical ventilation (CMV), assist control (AC) and synchronized intermittent mandatory ventilation (SIMV).
Ventilator settings for volume-controlled ventilation
In controlled mechanical ventilation, the minute ventilation (VE) is determined by the set respiratory rate and tidal volume. This mode is used in patients with no spontaneous effort such as those in comatose states or pharmacologically paralysed. The ventilator does not respond to spontaneous breaths or changes in flow requirements. During AC ventilation, the clinician sets the minimum VE as well, although the patient can increase VE by triggering additional breaths. With each additional breath, the full preset tidal volume is delivered. Therefore, if a patient has a respiratory rate twice that of the preset frequency, the VE will be effectively doubled. In SIMV mode, the mandatory ventilator breaths are synchronized with the patients’ inspiratory effort. Additional inspiratory efforts either receive no additional ventilator assistance or pressure support may augment these breaths. With the exception of mandatory ventilator breaths, the tidal volume delivered is highly variable.
Controlled mechanical ventilation and AC volume-controlled modes control the tidal volume of each breath and provide most of the work of breathing. In patients who are critically ill, where control of ventilation and minimizing respiratory muscle oxygen consumption are paramount, controlled mechanical ventilation and AC may be the preferred modes. SIMV can provide partial or full ventilator support, ostensibly offering improved patient–ventilator synchrony and preservation of respiratory muscle strength . AC and SIMV are the most commonly used modes of volume-controlled ventilation in medical and surgical intensive care units . There are little data available regarding the benefits of one volume-controlled mode over another. In a multicentre observational study of ventilated critically-ill patients, there was no significant difference in clinical outcomes between those receiving AC ventilation versus those receiving SIMV with pressure support .
With volume-controlled modes, the clinician must set the flow pattern, flow rate, trigger sensitivity, tidal volume, respiratory rate, positive end-expiratory pressure (PEEP), and fraction of inspired oxygen (FiO2).
Most mechanical ventilators currently available offer a pressure limited form of mechanical ventilation where the tidal volume is assured. The term utilized for this modality varies according to the designation created by the company. Although comparative studies are lacking the putative advantage of these modalities is to ensure an adequate tidal breath, while utilizing a pressure mode, which may afford greater patient-ventilator synchrony.
Flow pattern and flow rate
While the term volume-controlled ventilation is commonly used, the ventilator actually is controlling the inspiratory flow. Each breath terminates after delivery of the set tidal volume unless a pressure limit is exceeded. Flow can be delivered either in a constant pattern or a decelerating pattern. (Fig. 95.1) A constant flow pattern provides nearly constant gas flow during the inspiratory cycle. It is often called a rectangular or square wave pattern. Tidal volume is delivered to the lungs equally throughout the entire inspiratory time (Ti). The airway pressure increases linearly with time.
The decelerating flow pattern is also termed a descending ramp flow pattern. Inspiratory flow is greatest at the beginning of inspiration and decreases throughout the inspiratory cycle. Most of the tidal volume is delivered early during inspiration and unless the flow rate is increased, the inspiratory time is lengthened with descending ramp flow. Peak inspiratory pressure is higher with constant flow and mean airway pressure is higher with descending ramp flow. An advantage of descending ramp flow is that the longer Ti can enhance alveolar gas distribution, improving oxygenation and ventilation. When switching between constant and descending ramp flow the ventilator must adjust either the peak flow rate or the Ti to maintain the set tidal volume.
In a spontaneously breathing individual, expiration is passive and expiratory time is longer than inspiratory time. Thus, the inspiratory to expiratory (I:E) ratio is less than 1. In patients receiving mechanical ventilation inspiratory time is a function of tidal volume and inspiratory flow. In mechanically-ventilated patients with airways obstruction and air trapping, dynamic hyperinflation can be avoided by shortening the inspiratory time. This may be accomplished on a breath-by-breath basis either by decreasing tidal volume, increasing the flow rate of gas administered, or by decreasing the number of breaths administered per minute.
The I:E ratio during volume-controlled ventilation be deliberately lengthened with important physiological effects. A longer inspiratory time increases mean airway pressure, increases intrathoracic pressure, can decrease cardiac output, particularly if hypovolaemia is present, increases oxygenation in patients with acute respiratory distress syndrome (ARDS), and can lead to air trapping .
Inverse ratio ventilation (IRV) can be used as a strategy to improve oxygenation in ARDS. IRV implies a prolonged inspiratory time with an I:E ratios exceeding 1:1. IRV is commonly employed with pressure-controlled ventilation, but can also be performed in volume-controlled ventilation through the use of a low flow rate or the administration of an inspiratory pause on every breath. While, in general, studies have demonstrated that it improves oxygenation, IRV has not been shown to improve clinical outcomes such as mortality or duration of mechanical ventilation. IRV is uncomfortable and often requires high levels of sedation or pharmacologic paralysis .
Triggering mechanism and sensitivity
In AC mode, the patient is able to trigger the ventilator to provide breaths. These breaths can be pressure- or flow-triggered. Pressure-triggered breaths are delivered when the patient exerts a specified negative-pressure deflection below the level of set PEEP or auto-PEEP, when present. Flow triggering occurs when the machine senses a decrease in flow of a set magnitude during exhalation. Triggering mechanism and sensitivity significantly influence inspiratory work of breathing and patient ventilator synchrony. Ineffective triggering is a form of ventilator asynchrony. As a result, triggering sensitivity should be set at the lowest required value that does not lead to auto-cycling or ‘breath stacking’. Conversely, the sensitivity should not be set at such a value above which the patient has to overcome a significant amount of ventilator circuit or airways resistance to trigger a breath. This results in increased work of breathing and discomfort.
For example, a patient with significant air trapping due to obstructive airways disease may develop a significant amount of auto or intrinsic PEEP. If the trigger sensitivity is set at –2 cmH2O and intrinsic PEEP is 10 cmH2O, the patient will have to exert a negative-pressure deflection of –12 cmH2O to trigger a breath. This can be recognized easily at the bedside as the patient will have obvious attempts to inhale with no ventilator breath delivered. The flow rate also is an important factor in work of breathing. The inspiratory flow rate should be set high enough in order to complete inhalation before the patient begins to exhale. Exhalation against a closed exhalation valve is distressing to a patient and best avoided. In general, the average flow rate should be four times the minute ventilation to avoid discomfort.
Tidal volume should be set with individual pulmonary mechanics and physiology in mind. Present practice suggests tidal volumes in the range of 6–8 mL/kg ideal body weight are appropriate for most clinical circumstances. In order to avoid VALI, ideally plateau pressures should not be in excess of 30 cmH2O, lower if possible, as this is a continuous, not a threshold variable. In individuals with normal lungs, tidal volumes such as 10 mL/kg of ideal body weight are generally well-tolerated without generating high airway or plateau pressures. Nevertheless, clinicians should be cognizant of the fact that tidal volumes in excess of 700 mL have been associated with an increased risk of ARDS in post-operative patients with normal lungs. Persons with history of lung resection and pulmonary fibrosis typically can tolerate tidal volumes of 4–8 mL/kg. In ARDS, volume-controlled ventilation with tidal volumes of 6 mL/kg are recommended based upon the ARDS Network study demonstrating a 22% reduction in mortality in those receiving a low tidal volume strategy . Low tidal volumes of less than 6 mL/kg may result in atelectasis; however, this can be offset by increases in PEEP. In controlled mechanical ventilation or AC modes the delivered tidal volume should be equivalent to exhaled tidal volume. Exceptions are the presence of an endotracheal tube cuff leak and pneumothorax with bronchopleural fistula.
The back-up respiratory rate should be chosen in order to satisfy the desired minute ventilation based upon pH and PCO2 goals. In AC mode, however, if the patient triggers each breath, the set back-up rate may have no effect upon VE. Ideally, the back-up rate is 2–4 breaths below the respiratory rate in patients receiving AC, permitting patients to determine their own minute ventilation and PCO2.
PEEP and FiO2
At initiation of mechanical ventilation, in most cases an FiO2 of 1.0 and PEEP of 5 cmH2O are recommended. These variables can be adjusted based upon patient condition and physiological goals. FiO2 is later decreased in order to maintain an arterial saturation of at least 88% on arterial blood gas or 92% on pulse oximetry, allowing for the imprecision of the latter device.
Volume-controlled ventilation is the most commonly used mode in inpatient settings. The major advantage is the delivery of a constant tidal volume, thus minute ventilation is assured. The peak inspiratory pressure will vary based upon compliance and resistance of the respiratory system. The vast majority of patients undergoing mechanical ventilation can be successfully and safely managed using conventional volume-controlled mechanical ventilation. There are no absolute indications for resorting to pressure-controlled modes, including in the initial management of patients with ARDS, where a low tidal volume strategy via volume-controlled ventilation has a proven mortality benefit. In the limited available studies comparing volume-controlled to pressure-controlled ventilation in persons with hypoxic respiratory failure, there have to date been no differences in clinically important variables, such as mortality.
A major limitation of volume-controlled ventilation is that the flow pattern is fixed and will not change with changing demands of the patient. This increases the risk for patient-ventilator dys-synchrony. Pressure modes typically do allow for varied flow depending upon patient demand and may be preferred for this reason. Additional, in poorly compliant, stiff lungs, airway pressures during volume-controlled ventilation may be excessive leading to concern for ventilator-associated lung injury. In these instances, pressure-controlled ventilation may be advantageous.
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