How should mechanical ventilation be managed for children undergoing anesthesia? In this month’s issue of Anesthesia & Analgesia, Dr. Jeffrey M. Feldman, Department of Anesthesiology, Children’s Hospital of Philadelphia, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, has reviewed current technology used to ventilate children during anesthesia in the article titled “Optimal Ventilation of the Anesthetized Pediatric Patient.”
There are 3 goals for optimal ventilation of the child: 1) provide optimal arterial oxygen tension (PaO2) at the lowest inspired oxygen concentration (FiO2), 2) provide an acceptable arterial carbon dioxide tension (PaCO2), and 3) provide an optimal tidal volume using the least inspiratory pressure.
Based on work published in the early 1960s, the traditional approach to mechanical ventilation during anesthesia has been to use tidal volumes of 10-12 ml/kg. Due to anesthesia machine design limitations, tidal volumes have been difficult to deliver accurately to children. These limitations include circuit compliance, whereby some of the delivered volume is compressed in the breathing circuit, and the interaction between fresh gas flow and delivered tidal volume. Until recently, small tidal volumes were also difficult to accurately measure. For these reasons, pressure-controlled (PCV) rather than volume-controlled ventilation (VCV) has been the norm for pediatric anesthesiologists. The limitation of PCV is that tidal volume is not constant due to intraoperative changes in lung or abdominal compliance. Newer anesthesia ventilators are designed to compensate for compliance within the breathing system and eliminate the interaction between fresh gas flow and delivered tidal volume, providing accurate volume delivery even at small tidal volume settings typical in pediatrics. These newer anesthesia ventilators also measure exhaled tidal volume more accurately than in the past.
It has been shown that adults with acute respiratory distress syndrome (ARDS) do better when ventilated with lower tidal volumes (6-8 mls/kg) plus PEEP, and there is evidence accumulating that adult surgical patients at risk for post-operative pulmonary complications benefit from a similar lung protective ventilation strategy. We do not have similar data in pediatric surgical patients demonstrating the benefits of lung protective ventilation, but VCV and control of tidal volume has become widely used in the NICU to prevent ventilator induced lung injury. The weight of existing evidence supports a ventilation strategy that includes careful control of tidal volume plus PEEP and fortunately, the latest generation of anesthesia ventilators have the ability to deliver even small tidal volumes accurately when using VCV.
PCV can still be advantageous especially when there are leaks due, for example, to use of an uncuffed endotracheal tube or bronchopleural fistula. PCV may also be preferred in very small infants where the tidal volume target is close to or less than the lower limit of the ventilator, typically 20 milliliters. The square wave pressure waveform associated with PCV helps to favor lung recruitment since maximum inspiratory pressure is maintained throughout the inspiratory cycle. This is not true of traditional VCV where the maximum pressure is achieved at the end of inspiration. Many modern anesthesia machines offer a volume mode that provides constant tidal volume and a square wave pressure waveform offering the best of traditional VCV and PCV.
Control of dead space is important when ventilating children irrespective of whether PCV or VCV are used. Any apparatus added to the anesthesia breathing circuit on the patient side of the y-piece will increase dead space and can lead to hypercarbia or the need for excessive minute ventilation to maintain normocarbia.
This well-written review clearly describes the use of mechanical ventilation for children during surgery. It should be read by all anesthesiologists who care for pediatric patients.