Flashcards in Phacodynamics Deck (49):
Approximately how much does IOP increase with the pedal in position 1 for each 15cm (6 inch) rise in bottle height?
If the aspiration flow rate is increased while the bottle height is held steady, what will happen to IOP?
IOP will drop
What happens to IOP when the aspiration tip is occluded?
the IOP will rise to the hydrostatic level established by the bottle height
What is the relationship between rotational speed of a flow pump head and the rise time?
What potential issue can result with highly compliant aspiration tubing?
What are the two major classifications of pumps used in phaco?
Flow-dependent (e.g., peristaltic pump) vs. Vacuum-dependent (e.g., Venturi)
How is vacuum defined in the context of phaco?
a pressure that is less than atmospheric pressure (i.e., negative pressure if atmospheric pressure is used as the origin)
What are the sources of vacuum (i.e. pressure less than atmospheric pressure) in a flow-pump system?
1) The small aspiration port
2) Occlusion of the aspiration port
3) The aspiration line tubing
These resistances become significant as aspiration flow rate rises.
Does a flow pump provide direct or indirect control over vacuum generated?
Can a flow pump actually reduce flow rate below the baseline expected due to bottle height?
Yes, flow pumps can act as "flow regulators"
With a flow pump, how does IOP change as aspiration port occlusion level increases (and therefore aspiration flow rate decreases)?
How is vacuum (i.e., negative pressure) buildup beyond a given preset avoided with a flow pump?
Through a venting mechanism or reduction in flow pump rotational speed
What is rise time?
The time required to achieve a given level of vacuum (i.e., negative pressure
How is compliance defined for an aspiration line?
change in volume in response to a change in pressure
How does high compliance of aspiration line tubing affect vacuum rise time?
High compliance of AL tubing increases potential rise time, as the pump must work to overcome the compliance (i.e., deform the tubing) prior to building vacuum against an occluding fragment at the aspiration port
How can patient positioning inadvertently alter the effects of your machine settings?
Height of the patient's eye relative to the bottle affects the pressure generated with a given bottle height setting.
For a machine with linear control of vacuum, what is the maximum potential vacuum that can be generated when the foot pedal is pressed 1/2 way through position 2?
Half of the maximum preset vacuum
Is there open communication between the bottle, AC, and aspiration line in position 1?
Yes. However, remember that there is resistance to flow at the (small) aspiration port.
Is total tip occlusion necessary to build up holding pressure to the maximum preset vacuum with both flow and vacuum pumps?
How is irrigation stopped in position 0?
A pinch valve on the machine occludes the irrigation line. This separates the AC from the full pressure of the bottle height. The location (height) of the pinch valve relative to the patients AC determines the IOP.
What are common examples of vacuum pumps?
1) venturi pump
2) diaphragm pump
3) rotary vane pump
How do vacuum pump designs differ from flow pumps with regard to drainage receptacle?
Aspirated fluid drains into a rigid cassette (the pressure inside which can be controlled) with a vacuum pump.
With a flow pump, the fluid drains into a flexible drainage pouch.
What are sources of resistance in aspiration?
1) length of tubing
2) internal diameter of tubing
3) diameter of aspiration port
Does increased resistance to aspiration (e.g., by adding an I/A handpiece) affect vacuum pump systems or flow pump systems to a greater degree?
Vacuum pump systems
How does bottle height affect aspiration flow rate differently in vacuum pump and flow pump systems?
In flow pump systems, the flow pump acts as a flow regulator, thus leading to a constant flow rate.
In vacuum systems, there is no flow regulation, and the increased pressure differential associated with a rise in bottle height will lead to a greater flow rate.
Is the vacuum (i.e., pressure difference) generated at the aspiration inlet by a vacuum pump equal to that at the cassette?
No, it is less
In which pedal positions can surge occur?
position 2 or position 3
What is surge?
Rush of intraocular contents to the aspiration port after the breaking of a tip occlusion. The rush of contents is due to the high pressure differential (vacuum) that is generated during the occlusion.
If a flow pump is used, can surge be avoided?
Yes, by using a low flow rate, the pump can act as a flow regulator. The disadvantage of this approach is the slow rise time that results.
How can surge be defined in terms of IOP?
Surge can be defined as the integral of actual IOP minus set IOP with respect to time.
What is the range of frequencies of ultrasound power used in phaco?
25 kHz to 60kHz.
With axial oscillation of the phaco needle what is the usual maximum stroke length?
What produces the oscillation in a phaco handpiece?
Current applied to a pizoelectric crystal.
What are three potential mechanisms of phacoemulsificiation?
1) Acoustic breakdown
2) Microcavitation bubbles
3) Mechanical breakdown
What is followability?
The combination of attracting fragments to the tip along with feeding them into the tip against the repulsive action of ultrasound using the combined fluidic parameters of vacuum and flow.
Is the holding ability of a beveled tip greater or less than an 0 degree tip?
Why switch from a phaco handpiece to an IA handpiece after completing nucleus removal?
The standard IA tip's smaller aspiration port size means that it is more easily occluded by the remaining thin cortex than would be the phaco tip.
What is responsible for the buzzing sound noted when gauging ultrasound power?
Subharmonic tones of the handpiece and needle.
What is autoregulation?
A feature of vacuum pump systems wherein flow increases during partial occlusion automatically due to a set level of commanded vacuum.
What is a typical low flow range that would be considered necessary for ultrasound tip cooling?
12-20cc per minute.
What are 3 variables that determine the phaco power needed for a given manuever?
1) Density of the nucleus where the tip is engaged
2) Proportion of tip that is engaged
3) Linear velocity of tip during the manuever
Conceptually, what characteristic of the nucleus determines the amount of energy needed to emulsify a given nuclear fragment?
At what speed should the phaco tip move in a dense nucleus?
With a dense nucleus, why does increasing power require an increase in flow rate (or vacuum)?
The repulsive effect of the phaco power will need to be offset by greater flow or vacuum.
What is divide and conquer?
Sculpting 2 perpendicular grooves to generate 4 nuclear quadrants.
What is phaco chop?
Method whereby the nucleus is impaled and a chopping instrument is used to produce multiple nuclear fragments. f
What is stop and chop?
Sculpting one central groove and then subdividing the nuclear halves with a chopping instrument.
What is the goal in epinucleus removal?
To free the epinucleus as a single piece from the surrounding cortex and capsule.