Dr. Joseph Beckham and Dr. Ana Crawford (not pictured) are residents in Anesthesiology and Pain Management, University of Texas Southwestern Medical Center.
Many advances have occurred in mechanical ventilators in recent years and these newer technologies are beginning to be incorporated into anesthesia ventilators. This has reduced the differences between intensive care ventilators and anesthesia ventilators. Among the advances are advanced ventilator modes (e.g., pressure-controlled ventilation [PCV], pressure-support ventilation [PSV], synchronized intermittent mandatory ventilation [SIMV]) and improved monitoring of respiratory mechanics (i.e., pressure-volume loops).
These newer ventilator features should improve intraoperative management of challenging patients including the obese and those with pulmonary disease (such as asthma and chronic obstructive pulmonary disease) as well as management of challenging situations (such as severe hypoxemia and hypercarbia). However, the acceptance of these newer modalities is limited, probably due to the lack of understanding of their potential benefits. To that end, we provide a brief overview of several newer ventilation modes and the advances in the real-time monitoring of pulmonary mechanics to help guide perioperative ventilation strategies.
Pressure-Controlled Ventilation (PCV)
In PCV, the airway pressure is fixed and the tidal volume changes with resistance and compliance as well as duration of inspiration. During PCV the peak pressure is achieved rapidly and maintained for the duration of inspiration. This allows delivery of tidal volumes that are similar to volume control ventilation, but at lower peak inspiratory pressure (PIP), assuming similar compliance. This benefit is largely determined by how rapidly the ventilator is able to reach the predetermined inspiratory pressure, which is variable between ventilator types.
In addition, the decelerating gas flow during PCV improves distribution of gas flow and a homogenous tidal volume, which increases lung compliance and improves oxygenation. Overall, PCV may be used as a part of lung protective strategy, albeit clinical studies are lacking.
In contrast to traditional volume control mode, where changes in resistance and compliance are reflected by changes in PIP, PCV reflects resistance and compliance changes with changes in tidal volumes. This requires increased vigilance because the intraoperative lung compliance is highly variable (e.g., from changes in degree of neuromuscular blockade, abdominal packing, surgeon’s hand on the chest) and increased resistance can decrease tidal volumes and contribute to the development of hypercarbia and atelectasis.
Further, unlike volume control — in which the tidal volume is predetermined — inspiratory pressures must be individualized for each patient to ensure adequate ventilation while using PCV. Volume-guaranteed PCV is a new ventilatory mode that may prove useful in addressing one of the major limitations to the routine use of PCV.
Pressure Support Ventilation (PSV)
Though well utilized in the intensive care setting for quite some time, PSV is a relatively new intraoperative mode of mechanical ventilation. PSV augments the patient's spontaneous breaths (increases tidal volumes), improves gas distribution and reduces work of breathing.
PSV improves gas exchange and prevents perioperative atelectasis in patients breathing spontaneously through supra- laryngeal devices (such as a laryngeal mask airway) or tracheal tube. In addition, PSV with or without SIMV, may reduce the need for neuromuscular blockers and “deep” anesthesia to prevent coughing and “bucking.” Furthermore, PSV may be used at the end of surgery while the patient is recovering from
residual anesthesia and muscle relaxants to maintain oxygenation and prevent hypercarbia as well as washout inhaled anesthetics.
Just as with PCV, the efficacy of PSV to deliver an adequate tidal volume is dependent on patients' respiratory mechanics and therefore vigilance is paramount. Volume-guaranteed PSV is a new ventilatory mode designed to address this issue and adjusts the pressure support to deliver a preset tidal volume. Although promising, the literature is mixed with regards to clinical validation of volume-guaranteed PSV.
Positive End-Expiratory Pressure (PEEP)
PEEP applied in amounts greater than the closing pressure can improve compliance, increase functional residual capacity, decrease alveolar collapse (pulmonary atelectasis), reduce reopening injury and avoid increases in pulmonary vascular resistance. It is suggested that PEEP may be beneficial in the obese patient and in those with acute lung injury; however, the putative benefit in healthy patients is less obvious. Of note, excess PEEP can be deleterious in patients with reduced compliance and in patients dependent on preload for effective cardiac output. Therefore, although PEEP clearly benefits some populations, it must be selected on an individual basis.
Pressure-Volume (PV) and Flow-Volume (FV) Loops
The utilization of pressure-volume and flow-volume loops to characterize pulmonary mechanics and to understand the changes induced by various pulmonary pathological entities has been well documented. Real-time monitoring of pulmonary mechanics can allow for the individualization of ventilator settings including determination of “best” PEEP that would increase alveolar recruitment and improve oxygenation.
Another aspect of pulmonary mechanics, resistance can be assessed through the use of FV loops. FV loops can help identify cases of pulmonary obstruction, monitor the efficacy of various therapeutic interventions and can also be used to detect air leaks. Although monitoring of pulmonary mechanics offers the potential to tailor ventilatory strategies based on individual patient needs, there is no absolute consensus on how such information should be interpreted or applied to clinical decision-making.
Summary
The recent advances in intraoperative ventilation modalities and the ability to better monitor respiratory mechanics offer the perioperative anesthesiologist many opportunities to customize the ventilation strategy to the needs of individual patients. With this advanced technology, however, comes the need for better understanding of pulmonary physiology and mechanics to assess which ventilation strategy would improve patient outcomes.
