Danger is in the Air

*Editor's Note: In the May 2009 issue of CRST Europe, we featured articles on the basics of ultrasound and fluidics, focusing on the importance of phaco tip selection and incision size to guarantee good anterior chamber stability. This spurred the addition of Fundamentals, a new column featuring articles about the practical aspects of ophthalmology. The column will alternate between cataract and refractive topics. In this installment, Co-Chief Medical Editor, Khiun F. Tjia, MD, discusses surge, one of the main causes of anterior chamber instability, and the potential for damage caused by air in the aspiration line. Because surge is not directly visible, it is rarely recognized as a probable cause of chamber instability.

At the 2008 Dutch Ophthalmological Society meeting, an interesting case was presented during the complicated cases video symposium. A resident having difficulty managing a hard cataract case stepped aside as the supervising surgeon took over. On occlusion break, half of the iris disappeared into the phaco tip. The audience reacted with a mix of surprise and shock. In the ensuing discussion, not one cataract surgeon (out of approximately 200) could identify what caused the phaco tip to swallow half of the iris.

The cause was surge. Surge is the sudden aspiration of fluid directly after occlusion break, when the compliant tubing springs back into its original shape, from a contracted state (due to vacuum). The temporary exsistence of a high pressure gradient between the aspiration line and the anterior chamber can induce an uncontrollable increase in the amount of liquid aspirated, which escalates the risk of a drop in intraocular pressure and a collapse of the anterior chamber.¹ In the previously mentioned case, when the resident retracted the phaco tip from the eye, a large amount of air was aspirated. The supervising surgeon did not notice air in the tubing and continued the case. The surge of aspirated air forced the iris to vault into the phaco tip.

Every surgeon has probably encountered a similar situation. However, few surgeons, as evidenced by the video symposium, are aware of the dangers of aspirated air. In this article, I suggest how to avoid surge.

OCCLUSION BREAK
When the phaco tip is occluded, the flow of liquid is temporarily interrupted. Once the blockage is relieved, a sudden occlusion break occurs, and the flow resumes. Because surge occurs directly after occlusion break, its volume is dependent on the following factors:

• The volume debit caused by the contraction of the compliant tubing under vacuum. With increasing vacuum, the potential surge flow volume also increases;
• The resistance of the phaco tip to outflow during occlusion break. This is determined by the narrowest part of the phaco tip lumen;
• The resistance of the irrigating sleeve to inflow upon occlusion break; and
• The infusion pressure for inflow, which is determined by bottle height.

When a significant amount of air is inadvertently aspirated, the postocclusion surge response can be dramatically higher because air is more compliant than fluid. The air in the aspiration line will enlarge significantly under vacuum. On occlusion break, the air will return to its original volume, markedly adding to the force of the surge. Air is easily aspirated when the phaco tip is retracted from the eye while in footswitch position 2. This happens often, and many surgeons are unaware of the danger.

AVOIDING SURGE
One can limit postocclusion surge by selecting the appropriate phaco tip and sleeve design in relation to fluidics settings. In general, a beginning phaco surgeon should not use very high vacuum settings.

Manufacturers of phaco machines have tried to improve the fluidics performance of their latest models through hardware solutions (eg, lower-compliance tubing, restricted-lumen tubing, very narrow lumen phaco tips) and software solutions (eg, antisurge algorithm) to decrease postocclusion surge flow. Presently, there is not a phaco machine that can detect and/or completely compensate for aspirated air. In fact, when air is present in the aspiration tubing, manufacturer statements regarding anterior chamber stability do not apply.

CONCLUSION
Air is compliant and can lead to a dramatic postocclusion surge if aspirated. Keep air out of the aspiration line to avoid a surge event. If air is aspirated, be aware and prepared for postocclusion surge until the air has emptied from the aspiration line.

Khiun F. Tjia, MD, is an Anterior Segment Specialist at the Isala Clinics, Zwolle, Netherlands. Dr. Tjia states that he is a consultant to Alcon Laboratories, Inc. He is the Co-Chief Medical Editor of CRST Europe. Dr. Tjia may be reached at e-mail: kftjia@planet.nl.

1. Buratto L, Werner L, Apple D, Zanini M. Phacoemulsification: Principles and Techniques. 2nd Edition. Thorofare, NJ: Slack Incorporated; 2003: 43.