SUMMER 2009
Pressure Support Ventilation and the Ophthalmic Surgical Patient
Myra Aultman CRNA
Callahan Eye Foundation Hospital
Birmingham AL
The presence of a “soft eye” is necessary for safe intraocular surgery. Coughing, gagging, hypertension, and movement cause an increase in intraocular pressure, jeopardizing the delicate surgical repair that the surgeon must accomplish in order to offer our patients the best odds for retaining vision. Moreover, patient movement during a microscopic repair can disrupt the surgical field and potentially harm the patient’s opportunity for vision recovery.
The mode of ventilation can play a large role in allowing the patient to avoid the hazards previously described. If MAC is planned, the patient must be maintained at a level so that “startled” movements are avoided. If a general anesthetic is planned, an attempt must be made to reduce the incidence of coughing, gagging, and movement during emergence. Using a laryngeal mask airway (LMA) in place of an endotracheal tube has been one way in which the anesthetist has attempted to reduce airway protective responses. Another method, has
been to incorporate different modes of ventilation, such as synchronized intermittent mandatory ventilation (SIMV), pressure control (PC), pressure support (PS), and most recently, pressure support with assured volume (PSVPro). A third method has been to extubate the patient “deeply”. However, in the current patient population with an ever increasing incidence of obesity and diabetes, aspiration can be a risk with this technique. Moreover, the ophthalmic surgical patient can be a difficult patient to ventilate by mask after a deep extubation due to the presence of an eye shield and the design of the surgical bed.
Clinicians prefer to use an endotracheal tube in an environment of field avoidance. In contrast to a LMA, an endotracheal tube provides a “secure” airway against aspiration. An endotracheal tube also allows the clinician to use muscle relaxants to provide paralysis to prevent patient movement and allow an opportunity to provide positive pressure ventilation (PPV). Paralysis prevents asynchrony during PPV that can occur when the patient is able to breathe spontaneously and placed on a ventilator.
Advocates for the LMA suggest that there is less coughing, gagging, and hypoxia during emergence as compared with an endotracheal tube.1 Moreover, the side effects of muscle relaxants and reversal drugs can be avoided with this device "thereby decreasing the incidence of nausea and facilitating quicker recovery." The asynchrony that has been observed with the interaction of patient and a ventilator can be greatly reduced by using a mode of ventilation such as PSVPro.2 The respiratory depression and increased work of breathing caused by inhalational anesthetics in spontaneously breathing patients can be eliminated by adding PS. 3 Mechanical ventilation via a pre-set alarm setting “back-up mode” can commence should apnea occur during the anesthetic.4
Pressure support ventilation (PSV) is an assist mode that has traditionally been used during the weaning phase of mechanically ventilated patients. Pressure assist with PSV is designed to occur with every spontaneous breath to provide a desired volume, but not to conflict in synchrony with a preset clinician controlled volume or flow as seen in SIMV or manually assisted PPV. The best analogy of this mode is to compare it to a technique used by experienced anesthetists during face mask ventilation (FM). During ventilation via a FM, the anesthetist will partially close the pop-off valve while maintaining a tight seal with the mask. This allows the anesthetist to provide continuous positive airway pressure (CPAP) to the patient. The anesthetist will “assist” the patient when a patient generated breath is sensed (via a “hand on the bag”).This is a comparison to pressure support provided by the ventilator. The difference is that the anesthetist is using the “bag” as the reservoir rather than the machine bellow that is used when a patient is ventilated in the mode PSVPro. PSV is flow cycled and allows the patient to control the duration and depth of inspiration, based on a preset percentage of peak flow that must be achieved by the patient before PS is discontinued in a breath, usually 25%. 5 This ventilation mode essentially allows the ventilator to “push” an inspired gas at the same flow rate that the patient can “pull” the gas.2
Fortunately, the anesthesia team has several methods that can be considered that allow a safe anesthetic acceptable to surgeon, anesthesia clinician, and patient. It is the communication and cooperation between these parties that allows for a good result.
1 Brimacombe JR, Emergence Phase, Ashmore A, Laryngeal Mask Anesthesia, principles and practice, 2nd edition, Philadelphia, Saunders, 2005, 265-280.
2 Amato MB, Barbas CS, Bonassa J, Saldiva PH, Zin WA, deCarvalho CR. Volume assured pressure support ventilation(VAPSV). A new approach for reducing muscle workload during acute respiratory failure. Chest 1992; 102; 1225-1234.
3 Christie JM, Smith RA. Pressure support ventilation decreases inspiratory work of breathing during general anesthesia and spontaneous ventilation. Anesthesia and Analgesia. 1992; 75:167-171.
4 GE Healthcare Aestiva/5 7900 Operation Manual.
5 Pouseman RM, Parmley CL. Endotracheal tube and respiratory care. Benumof’s Airway Management, 2nd edition, Philadelphia, Mosby Elsevier, 2007, 1057-1078.
