Vitreoretinal surgery at high altitudes presents a host of unique challenges. Mountainous terrain is found throughout the United States from the Appalachian Mountains with a peak altitude of nearly 2,130 m (7,000 ft) in the east, to the Rocky Mountains in the west with peaks exceeding 4,270 m (14,000 ft). The Rockies are among the highest mountain ranges in North America and extend through the provinces of British Columbia and Alberta in Canada, as well as Idaho, Montana, Wyoming, Colorado, Utah, and New Mexico, affecting the practice patterns of many vitreoretinal surgeons. Our home city of Albuquerque, New Mexico, sits at an altitude of 1,619 m (5,312 ft) above sea level, with a significant proportion of our patient population residing at even higher altitudes of up to 2,740 m (9,000 ft). The following discussion highlights clinical pearls and factors that affect surgical decision-making that we have encountered practicing in this environment.
The surgical challenge most commonly associated with high-altitude vitreoretinal practice relates to intravitreal gas. Boyle’s law predicts that gas expands with increasing altitude because of decreasing atmospheric pressure. This principle should be considered whenever there is a gas-filled or air-filled vitreous cavity. Air travel is therefore contraindicated in the presence of intravitreal gas or air. However, automobile travel remains a gray area, and recommendations may vary depending on the procedure performed, rate of altitude gain, and absolute change in elevation. Consideration must also be given to both peak and final altitude, particularly when mountain passes are encountered along the travel route. We have found it beneficial to review patients’ return travel routes to ensure they avoid excessive changes in elevation.
Figure 1. Principles of peristaltic pump (A) vs Venturi pump (B). Image courtesy of Oertli Instruments.
The efficiency of vitrectomy surgery may also be adversely affected at high altitudes depending on the gauge of the instrumentation and the amount of vacuum desired by the surgeon. Maximum attainable vacuum levels are limited at high altitudes. To understand the influence of altitude on vacuum, it is important to review the different pump systems. A peristaltic pump system uses rollers to compress the machine’s outflow tubing, thereby creating flow (Figure 1A).1 The flow is controlled by the rotational speed of the pump’s rollers. The surgeon uses a foot pedal to modulate the flow, while vacuum is set to the lowest level required to maintain that flow. Vacuum builds only after occlusion of the instrument tip.2 In a Venturi pump system, the flow of nitrogen at a particular pressure and velocity creates vacuum (Figure 1B). Vacuum is controlled proportionally with the foot pedal. Aspirated fluid is delivered via tubing into a rigid cassette.3 This is the type of pump system used by the Constellation (Alcon) and Stellaris (Bausch + Lomb) vitrectomy machines.
When vacuum-based vitrectomy systems are used at higher altitudes, the lower atmospheric pressure reduces the maximum vacuum the pump can generate. A vacuum pump functions by creating a pressure differential between the ambient pressure and the lowest pressure it can achieve internally. At altitude, because the ambient pressure is already low, there is less pressure for the pump to pull against, resulting in a lower maximum achievable vacuum. This may not significantly affect the efficiency of vitreous removal, because maximum vacuum is not always necessary to achieve maximal flow capability of the vitrector probe.
The flow rate is influenced by other factors, including duty cycle, internal diameter of the probe, cutting speed, and characteristics of the vitreous being removed. According to Poiseuille’s law, if the radius of the vitrector probe decreases while all other parameters remain the same (for example, from 23-gauge to 27-gauge) then the flow rate will also decrease. With larger-diameter instrumentation (eg, 23-gauge), the demand for higher flow rate may be limited by the lower maximum vacuum ceiling that can be reached at high altitude. This would result in less efficient vitreous gel removal and longer vitrectomy times.4 The newest vitreoretinal surgery systems by DORC and Alcon (Eva Nexus and Unity, respectively) provide both vacuum-based and flow-based fluidics modes, which may improve surgical efficiency at high altitude by compensating for limitations in maximum achievable vacuum.5
Another consideration when performing vitreoretinal surgery at altitude relates to the use of gas as a tamponade. A “nonexpansile” gas concentration indicates that the gas will not expand when placed into the eye at a certain altitude if the patient remains at that altitude. However, if the patient ascends in altitude then even a nonexpansile gas concentration will expand. Excessive gas expansion may result in severely elevated intraocular pressure (IOP) and central retinal artery occlusion, which can result in irreversible vision loss. Studies have shown that an increase of 305 m (1,000 feet) in altitude will raise IOP by approximately 10 mmHg (Figure 2).6 Research on vitrectomized, gas-filled rabbit eyes in the Mexico City area revealed an estimated 2.0 mmHg increase in IOP for every 100 m (328 feet) of altitude gain, using a hypobaric chamber to simulate increasing elevation.7
Figure 2. Relationship between change in altitude and rise in intraocular pressure (IOP). There is an approximate increase of 10 mmHg per every 305 m (1,000 ft) gain in altitude.
Both the overall gain in altitude as well as the rate of change in altitude are important considerations for patients with gas-filled eyes. A gradual rise in altitude is better tolerated. If a patient’s route home involves sudden large elevation changes, explicit instructions are given to drive slowly in the right lane with the hazard lights activated. Strict instructions are given to monitor symptoms during ascent; if mild pain develops, the patient should stop ascending and wait 30 minutes for the eye to acclimatize. Once symptoms resolve, it is safe to continue. However, if pain becomes unbearable or vision blacks out, then the patient should descend immediately until symptoms improve. Pretreatment with oral acetazolamide and topical aqueous suppressant drops are also recommended.
It is critical to educate patients on the signs and symptoms of increased IOP, such as decreased vision, eye pressure and/or pain, brow ache, headache, nausea, and vomiting. It can also be useful for patients to track changes in altitude during their car travel. The Compass application on the iPhone and Google Maps biking routes can show live changes in altitude and are free and easy to use.
Air travel is contraindicated in the presence of intravitreal gas and air. Federal Aviation Regulation Part 25 requires aircraft cabin pressure altitude to remain below 2,440 m (8,000 ft); therefore, the pressure experienced by the eye is not equivalent to the aircraft’s cruising altitude. However, cabin pressure changes rapidly during ascent, and an intraocular gas bubble can expand by 30% to 40% within minutes.8 Cases of severe pain, loss of light perception, and glaucomatous optic neuropathy have been reported in patients who flew with intraocular gas or air tamponade.9,10
There are scenarios where limited air travel may be possible. In Hawaii, short-duration, low-altitude flights between islands may be safe once the gas fill reaches 50% or less. These flights typically reach a maximum altitude of approximately 900 to 1,200 m (3,000 to 4,000 feet).11 Careful consideration of preoperative IOP and patient history, including anatomically narrow angles, glaucoma history, or a “disc-at-risk” optic nerve, should be undertaken before permitting air travel.12 Short helicopter flights at low altitudes may also be tolerated once the gas bubble is at 50% or less. One interesting case study assessed the IOP of a patient with a 50% C3F8 fill during a helicopter ride from sea level to a maximum altitude of 800 m (2,600 ft) over 15 minutes. The IOP did rise by 10 mmHg per 305 m (1,000 ft) of ascent, although the patient did not experience pain despite a peak IOP of 49 mmHg. He did notice the meniscus of the gas bubble flatten as the maximum altitude was approached.6
It is vital to familiarize yourself with the local geography to assist patients with planning their route home from surgery. Elevation maps are readily available online at no cost. It may be advisable to leave the IOP slightly lower at conclusion of surgery if the patient will be returning home to a higher altitude right after the surgery. Sometimes the choice of tamponade needs to be modified, and silicone oil may be preferable to gas. As a rule of thumb, we use silicone oil for patients who will immediately be returning to an altitude greater than approximately 2,285 m (7,500 ft), where the change in altitude relative to Albuquerque is over 600 m (2,000 ft). However, if patients living at these higher altitudes could stay at an intermediate altitude zone overnight, then gas could theoretically be used because they would have time to acclimatize.
An additional consideration is the location of post-operative visits. If patients undergo surgery at a lower altitude and then return home to higher elevation, it is preferable that they avoid significant descents for their postoperative visits. A descent will decrease the total gas bubble size, resulting in reduced tamponade, which may be suboptimal in cases with inferior pathology. Positioning during travel can also be more challenging, reducing effective bubble apposition to the region of pathology. A shrinking bubble may also result in hypotony, increasing risk of dangerous sequelae such as suprachoroidal hemorrhage.
Although this discussion has focused primarily on vitrectomy, gas is also used in smaller volumes during pneumatic retinopexy. In this setting, the gas expansion with increasing altitude can be advantageous. During pneumatic retinopexy, pure undiluted expansile concentrations of SF6 or C3F8 are used with the expectation of gas expansion over the coming days. SF6 gas expands to twice its size within about 36 hours, and C3F8 quadruples in size in 3 to 4 days.13 A bubble that expands further with an increase in altitude would afford a greater degree of imprecision with positioning. Retinal breaks separated by more than 1 clock hour that otherwise would have required alternating positioning may be covered simultaneously by the expanded bubble. Also, if it is known that the patient will be traveling to a higher altitude following the procedure, then a smaller gas volume can be injected to avoid multiple paracenteses to normalize the IOP.
Gas also takes longer to dissolve at higher altitude because of Boyle’s law.6 The reduced atmospheric pressure diminishes the diffusion gradient driving gas from the vitreous cavity into the bloodstream, resulting in slower diffusion and a longer-lasting gas bubble. At sea level, a full fill of SF6 gas lasts about 2 weeks in the eye, whereas C3F8 lasts about 6 to 8 weeks.14 Even longer durations may occur at higher altitudes. One strategy to mitigate this effect is to use a lower gas concentration during vitrectomy.
Conclusion
Vitreoretinal surgeons should understand how altitude affects surgical planning and postoperative management. Even when the procedure is not performed at high altitude, patient travel can meaningfully influence outcomes due to significant elevation changes. Many patients must travel to receive retina care or may wish to fly shortly after surgery. Surgeons should be familiar with the local geography and travel routes so that appropriate guidance can be provided. By anticipating altitude-related risks and counseling patients accordingly, surgeons can help ensure safe postoperative recovery and successful outcomes while also reducing avoidable complications. RP
References
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