LASER and DIATHERMY Flashcards
what are the main features of a laser?
monochromatic = all light is of same wavelength (same energy and frequency)
coherent = all in same phase (dont cancel one another out)
collimated = all parallel (non divergent)
what does LASER stand for?
light amplification by stimulated emission of radiation
how do LASERs work?
consists of a laser medium exposed to flash light and bound either end by reflective material where one end has a small opening for light to escape for use.
- flash lamp provides energy for the electrons of atoms within the lasing medium to be excited to a higher energy level
- when they drop back down in energy level they release the energy as a photon of energy. (spontaneous emission)
- if a photon of energy hits another electron in excited level it causes 2 photons to be released both with equal wavelength, parallel and in phase with one another (stimulated emisiion)
- this results in amplification of light energy as more and more photons of light are released and go on to further stimulate others
- these photons will be reflected back and forth via the reflective material at either end
- creating intense light which is monochromatic, coherant and collimated from a low energy source
- at one end there is a small window allowing a focused beam of light to escape and be used.
why is light in a LASER of one wavelength?
the wavelength is determined by the laser medium / lasting medium
these are various substances depending on purpose of laser e.g. CO2 lasers produce wavelength of 10.6um
depends on the electrons and atoms within that medium and the difference between low and high energy states that determines the energy within photon they release and hence its wavelength.
what is meant by the term ‘lase’
when more electrons are in a higher energy state than ground state , the medium is said to lase.
what are the differences between continuous and pulsed wave lasers?
continous - the source of light is continous, creates very high energy continous output. can be too damaging in surgery
pulsed = output is split into short pulses of varying duration. more controlled confined source of heat and more useful in surgery.
how is the energy of a wave related to its frequency?
E = frequency x planks constant (h)
do you know any specific types of lasers and the wavelength they produce and their uses?
CO2 gas medium = 10.6um = infrared. superficial cutting and precise good for airway surgery
Nd -YAG (solid) = 1060nm = infrared - used for coagulation and cutting in surgery, endoscopic and airway. good for GI bleeds.
Argon Laser - 500nm - blue/green visible light. used for dermatology
ruby - for tattoe removal.
what does the application of a laser depend on?
its wavelength
depth of penetration
what substances absorb it
most lasers are in infrared spectrum, why is a red light seen?
safety feature added so user knows where it is pointing for guidance
how are lasers classified?
based on their power output.
7 classes in total
class 1 is low power. cannot emit radiation of any known hazard for skin or eye
class 1m - same as above as long as not via a lens
class 2 - lower power - not damage unless maintained stare into beam
class 2m - same as above as long as not via a lens
class 3a
class 3b - need eye protection , low risk to skin
class 4 ->0.5Watts power. hazard to skin and eyes. most surgical lasers. eye protection is essential
tell me some uses of surgical laser?
cutting e.g. in airway surgery
removal of moles
breaking down renal stones - laser lithotripsy
retinal eye laser surgery
what is meant by population inversion?
when more than 50% of the atoms are in their excited state.
what are the safety concerns regarding lasers ?
environment
* fires and explosions- provides energy for ignition
staff
* damage to eyes, burns
* e.g. if they hit optic nerve could cause complete blindness, if they hit retina blind spot
patient
* burns, damage to eyes
* unconscious so cant react to pain
* airway fire risk
what precautions are taken to minimise risks of laser?
Staff and general:
protection - goggles
trained staff - laser safety officer present
signs on doors
non reflective material in the room - black matt finish
fire extinguisher available
patient:
if used in the airway, minimum amount of O2 less than 30%
ET tube cuff filled with saline
eye protection
wet swabs to protect tube.
laser resistant tube.
what are the features of a laser safe ET tube?
stainless steel flexible spiral
double cuff
non combustible
fill cuff with saline
how is an airway fire managed?
- declare incident
- stop laser
- call for help
- flood site with saline
- switch off oxygen source
- remove tube
- ventilate with bag/mask
- maintain anaesthesia with TIVA
- once fire is out, examine airway with fibreoptic, likely to need intubating again and follow up for swelling / injury
- ITU admission - associated with ARDS, ABG CXR. give dexamethasone
what can an airway fire result in?
swelling of the airway - hence need intubating to protect airway from obstruction
ARDS like picture - gradually worsening hypoxaemia over next 48hrs
how does diathermy work?
use of high freq AC current applied over small area fo cut and coagulate tissues during surgery
when high frequency AC current is passed over a small area a high current density is produced for precise cutting
can be classed into monopolar and bipolar
what are the different cutting patterns seen in diathermy?
the heating effect depends on current density and duration e.g.
continuous - cut mode 0.5MHz
* high current continuous sine wave
* low voltage
* overall higher power - 100-400 watts because continous
pulsed wave - coagulation mode 1-1.5MHz
* high voltage but pulses
* overall less power - 40watts
* less heating than cutting therefore coagulation.
blended mode
- may be 50:50 ratio
- allows some haemostasis as cutting occurs.
what frequency of AC current does diathermy work at?
0.5-1.5 MHz
describe the basis of monopolar surgical diathermy..
monopolar diathermy circuit consists of an electrode, diathermy machine, neural pad electrode that is much larger.
for current to flow at the tip of diathermy instrument a circuit for current flow is completed with a larger diathermy plate usually placed on back or thigh.
the current comes out at a small point on the instument and causes local tissue heating cutting/coagulation, the current then dissipitates through body and is returned via the larger plate (10cmx 10cm) .
it is only at the tip of the diathermy electrode that the current density is high enough to cause damage, after this it soon disperses and is too low to cause damage.
overall power = 100 to 400 wats
patient is earthed and diathermy ciruit is isolated from mains by a transformer.
why is high frequency current used in diathermy?
biological tissue is most affected by frequency around 50 Hz - i.e. most likely to cause VF
therefore by using frequencies well above this, less likely to cause interference.
high frequency current also creates more energy for cutting/ coagulation
Describe the principle behind bipolar diathermy?
2 diathermy electrodes making up a forecep connected to a diathermy machine
the current travels through one electrode and back up the other - high current density at the point the electrodes are used.
current doesnt travel through the body.
good for digits and small areas and for those with pacemakers.
overall power 40watts
good for coagulation but not so much for cutting.
why is the diathermy pad checked at the end of surgery?
ensure contact was good and no burns of skin - poor contact, increases current density, more likely to burn.
what does diathermy do on a cellular level?
microscopic effects - coagulation
2 macroscopic - widespread tissue destruction and cutting.
pros and cons of diathermy?
pros - allows precise cutting plus coagulation so better for blood loss in theatre
cons- risk of burns, risk of electrucution, interference with monitoring. risk with pacemakers
what are the hazards associated with diathermy and how are these minimised?
burns:
* incorrect attachment of diathermy pad. many modern machines monitor impedence and stop the current flowing if impedence is too high
* if no diathermy pad, current could flow through ecg electrodes
* ignition of skin preps - ensure good drying time
risk of electric shock
* uses currents so risk of microshocks - however frequencies are much hgiher than those likely to cause interference to biological potentials. also floating circuit to prevent mains stray currents.
interference
* ecg interference - filters can help with this
* pacemaker interference - avoid monopolar and disable ICD
* oesophageal doppler - the current can vibrate crystals - piezoelectric effect
fire
* source of ignition for fires
* smoke can be toxic (chemicals, HPV, carcinogens) - suctioned away , surgeons wear mask to protect
what are the risks of pacemakers when using diathermy? how are these minimised?
avoid diathermy in those with pacemakers / ICD - this is because they may stop pacemakers from working, activate ICD unneccessarily delivering shock, risk of burns around the device
minimising risks..
* bipolar is better
* turn off ICD e.g. magnets- attach defib pads instead
* have external pacing available
* if monopolar has to be used, place pad as far from pacemaker as possible and use diathermy in short bursts.
* inform rest of the team of risks
what design features are incoorporated into diathermy to minimise risk of electric shock?
isolated patient circuit using isolating transformer
isolating capacitor - capacitor prefer high freq AC and filter out lower frequency (e.g. mains 50Hz )current . hence protect from more damaging frequencies.
previously diathermy would be earthed and other faulty currents would travel through patient circuit. now isolating transformer and capacitor separates patient.
what is the heating effect in diathermy proportional too?
H ∝ I^2 / A
current squared
divided by area
why should a patient not touch the bedframe when diathermy is used?
allows another path for current to flow, higher current density through arm and hand can cause burns.
