What is a defibrillator

2.0: What is a defibrillator.

The goal here is to try to understand what the machine is trying to do. Let’s say your patient pops into a nasty rhythm – not handling it very well, not making much of a blood pressure; and you want to deliver electricity – what do you need?

2.1: The monitor.

you need to be able to see what’s going on. This is of course one of the 

reasons why our patients are monitored at the bedside: so you can see 

what rhythm your patient is in. Defibrillators are built to travel– so they 

have a built in monitor screen.

2.3: The capacitor.

  

Generally you need a battery to run any transportable medical device.

Due to rechargeable batteries these devices are so heavy. The battery

stores electricity,when machine is plugged in to AC. The capacitor fills

up with electricity when you push the button that selects charge .

2.4: The paddles and the pads:

Paddles

Self Adhesive Pads

Internal Paddles

paddles are considered “old-tech” – nowadays the thing to do is to slap on 

sticky defibrillation pads that hook up to the machine – the same ones as 

external pacing pads – then stand back, charge and discharge the machine 

from a few feet away. The shock can be delivered to the heart by means of 

electrode placed on chest of the patient(External defibrillation) or the electrode

may be held directly against the heart when the chest is open

(internal defibrillation). Higher voltage are required for external defibrillation 

than for internal defibrillation.

2.5: Theory of operation.

Schematic diagram of a defibrillator

Above schematic show basic circuit diagram of DC Defibrillator. A Variable auto transformer forms the primary of a high voltage transformer. The output voltage transformer is rectified by a diode rectifier and is connected to vacuum type high voltage change over switch. In position 1, the switch is connected to one end of an oil filled micro farad capacitor. In this position, the capacitor charge to a voltage set by the positioning of the auto transformer. When the shock is delivered to the patient, a foot switch or a push button mounted on the handle of the electrode is operated. The high voltage switch change over to position 2 and the capacitor is discharged across the heart through the electrode. The inductor in the circuit slow down the discharge from capacitor by induced counter voltage. This give the output pulse a physiologically favorable shape. The dis advantage of using inductor is that any practical inductor will have its own resistance and dissipates part of the energy during the discharge process. The shape of waveform that appears across electrodes will depend upon the value of the capacitor and inductor used in the circuit. The discharge resistance which the patient represent for defibrillating pulse may be regarded as purely ohmic resistance of 50-100 Ω approximately for typical electrode size of 80 cm2. The typical discharge pulse of defibrillator is shown in fig.3 b. Using this design, external defibrillation uses: –50 to 100 Joules of energy when electrodes are applied directly to the heart –Up to 400 Joules when applied externally ,Capacitors used range from 10 to 50F .Voltage using these capacitors and max energy (400J) ranges from 1 to 3 kV . Energy loss result in the delivery of less than theoretical energy to the heart

Defibrillator: Rectangular-Wave

 •Capacitor is discharged through the subject by turning on a series silicon- controlled rectifier.

 •When sufficient energy has been delivered to the subject, a shunt silicon- controlled rectifier short-circuits the capacitor and terminates the pulse, eliminating a long discharge tail of the waveform

•Output control can be obtained by varying:

          –Voltage on the capacitor

          –Duration of discharge

•Advantages of this design:

          –Requires less peak current

          –Requires no inductor

          –Makes it possible to use physically smaller electrolytic capacitors

          –Does not require relays ·

Monophasic pulse width is typically programmable from 3.0 to 12.0 msec

Biphasic positive pulse width is typically programmable from 3.0 to 10.0 msec, while the negative pulse is from 1.0 to 10.0 msec ·

Studies suggest that biphasic pulses yield increased defibrillation efficacy with respect to Monophasic pulses.

Reference : http://coep.vlab.co.in

Ventricular Fibrillation  

De-fibrillation - Part1

1.0  What is fibrillation?

Fibrillation is an arrhythmia that affects either the atria as a pair, or the ventricles as a pair, producing “a-fib”, or “v-fib”, respectively. Most cardiac rhythms are organized – they’re regular in some way, producing some sort of regular (as opposed to disorganized), rhythmic motion of the chambers, hopefully producing a blood pressure. In fibrillation, the cardiac tissue of the chambers involved wiggles about like (classic phrase) “a bag of worms”. Does a chamber wiggling like a bag of worms pump any blood, produce a cardiac output, eject any fraction of its contents? No, it does not!

As I always try to point out, all the waves that you see on ECG strips actually represent some kind of physical motion of one or the other set of cardiac chambers, and the trick is to try to visualize what those chambers are doing in any given rhythm situation. Let’s see if a quick review of some strips helps the visualization process.

Here we are familiar Sinus rhythm on image 01. Organized, rhythmic, producing stable contraction of the chambers – first the atria, then the ventricles. So nice orderly motion, first above, then below.

Image 01

Next slide (Image ) atrial flutter. Still organized: the atria are contracting rapidly, sure, at about 300 bpm, and the ventricles are responding to every third or fourth impulse, slowly enough that the ventricular chambers have time to fill up nicely between beats, fast enough to probably maintain a good blood pressure. So I visualize the atria clipping along, with the ventricles contracting every third or fourth time.

Image 02

This one?  Well – is it organized? Actually it is: see the pattern of doubles? It’s a little easier to figure out by looking at the lower part of the strip – this is a sinus rhythm, and after every sinus beat comes a PAC, followed by a compensatory pause. So yes, still organized. “Regularly irregular”.

Image 03

How about this one? , VT. Ugly, scary, but still organized, regular – the chambers (which ones?) are moving in a steady manner. On your mental screen you should see the ventricular walls contracting very rapidly – do they have time to fill? Should we shock this rhythm? It depends…

Image 04

1-1: What is atrial fibrillation?

How about this one? Not organized? Should we shock this rhythm? A-fib for sure can be a shock able rhythm, but look at the QRS rate – in the 70’s. What would have to be happening to make this a shock able situation? What do you visualize here? Atria: bag of worms. Ventricles – occasional, but normally conducted QRS’s. Are they too slow or too fast to make a blood pressure? How do you tell?

           

                                   

Image 05

           

1-2:  What is ventricular fibrillation?

Here’s an ugly one - you probably recognize this one right off. Doesn’t look organized to me! What rhythm is this? Visualize the ventricles – everybody see the worms? What should we do?

Image 06

                       

2-  What is De-fibrillation?

So: all set on organized, and not organized? The treatment for nasty arrhythmia is often electrical, right? The point is: one type of treatment: cardioversion - is for the organized kind of rhythm, and the other is, De-fibrillation is for disorganized rhythms. 

What you want to do is to send a fixed amount of electrical energy along the normal conduction path of the heart: along the Lead II pathway.

Image 07

Here’s a diagram of the normal lead II:  the positive electrode is down near the apex of the ventricles, the negative one is at the atrial end. Everybody remembers that the normal direction that the cardiac impulse takes is from the SA node at the northwest corner, up near Oregon, down and towards the southeast in Florida, where the positive electrode lives? And that the signal moves along the pathway as the cells depolarize, in sequence, along that pathway?

Pulse Wave Transit Time (PWTT)

ECG and SpO2 readings are taken in to consideration,

PWTT is calculated for each beat from the ECG and peripheral pulse wave. The peripheral pulse wave is measured by an SpO₂ probe on the finger or toe.

PEP and a-PWTT

PWTT includes PEP (Pre-Ejection Period) and a-PWTT (Pulse Wave Transit Time in the Artery).

PEP ----------> Pre-ejection period

a-PWTT: -----> Pulse wave transit time at artery

PWTT:--------> PEP + a-PWTT

a-PWTT is the time it takes the pulse wave to travel from the aorta to a peripheral artery. a-PWTT is directly related to blood pressure. Unfortunately, a-PWTT cannot be measured directly. We can only measure PWTT, which also includes PEP.

PEP is the period just before the blood is pumped into the aorta. In general, PEP change over short periods of time is negligible in most cases so we can assume that PWTT corresponds to a-PWTT and therefore to blood pressure.
However, vasoactive and other drugs can cause significant changes in PEP and affect the correlation between PWTT and blood pressure.
Generally in most cases, we can say that PWTT corresponds to a-PWTT and blood pressure.

Relationship between blood pressure and pulse wave speed

When the heart pumps blood into the aorta, it also generates a pressure wave that travels along the arteries ahead of the pumped blood. This is the pulse wave.The speed of the pulse wave depends on the tension of the arterial walls. When the blood pressure is high, the arterial walls are tense and hard and the pulse wave travels faster. When the blood pressure is low, the arterial walls have less tension and the pulse wave travels slower.
This can also be understood by the following example. When a ping-pong ball is thrown against a hard table, the rebound is strong and fast. If the ping-pong ball is thrown against a soft blanket, the blanket absorbs the force and the rebound is weak and slow.

How PWTT detects change in blood pressure

 

Although the actual blood pressure itself cannot be determined from the speed of the pulse wave, a change in blood pressure is indicated by change in the speed of the pulse wave. Therefore, PWTT is used to detect change in pressure.
Change in PWTT indicates potential change in blood pressure. PWTT for each beat is compared to the PWTT of the last NIBP measurement. When PWTT change exceeds a threshold, it triggers NIBP measurement to measure the actual blood pressure.

You can increase or decrease the PWTT threshold to respond to larger or smaller blood pressure changes and trigger less frequent or more frequent NIBP measurements.



Source: http://www.nihonkohden.com

PHOTO-THERAPY

PHOTO-THERAPY

Phototherapy lights emit light in the blue-green spectrum (wavelengths 430-490nm).  It is NOT ultraviolet light.

 "CONVENTIONAL" AND "INTENSIVE" Phototherapy?

"Intensive phototherapy" means the irradiance of the light is at least 30µW/cm2 per nm as measured at the baby's skin below the center of the phototherapy lamp.  A hand-held

Radiometer

radiometer can be used to measure the spectral irradiance emitted by the light.  Because measurements taken directly under the lights will be higher, measurements should ideally be made at several locations and averaged.  The appropriate radiometer will vary based on the phototherapy system used, so manufacturer recommendations should be followed.

With "Conventional phototherapy" the irradiance of the light is less, but actual numbers vary significantly between different manufacturers.  In general, it is not necessary to rountinely measure irradiance when administering phototherapy, but units should be checked periodically to ensure that the lamps are providing adequate irradiance, according to the manufacturer's guidelines.

In adults, prolonged exposure to blue light can cause retinal damage.  Although retinal damage from phototherapy has not been reported, eye covers for newborns are standard prophylaxis.

some people who are around blue lights for prolonged periods will feel nauseated.  Yellow plastic placed on the outside of the isolate may mitigate this effect.  

 There are no specific guidelines for when to discontinue phototherapy.  Evidence of hemolysis and age of the infant will impact the duration.  In some cases, phototherapy will only be needed for 24 hours or less, in some cases, it may be required for 5 - 7 days.  The AAP Guidelines suggest that an infant readmitted for hyperbilirubinemia, with a level of 18 mg/dL or more, should have a level of 13 - 14 mg/dL in order to discontinue phototherapy.  In general, serum bilirubin levels should show a significant decrease before the lights are turned off. 

HOW CAN PHOTOTHERAPY BE MAXIMIZED?

 

The effectiveness of phototherapy is determined largely by the distance between the lamps and the infant, so phototherapy can easily be intensified by bringing the lamps closer to the infant.  Because a closed isolette does not allow the lamps to be moved in close, if there is a concern about the effectiveness of phototherapy, an isolette should not be used.  With the infant in an open bassinet, it is possible to bring the lamps to within 10 cm of the infant.  An undressed term infant with not be overheated with this arrangement, however is is important that halogen spotlights NOT be used.  Halogen lights can get hot, and burns may result if used this way.  Special blue, regular blue, and cool white lights are all acceptable alternatives.

Increasing the skin surface area exposed to phototherapy will also maximize treatment.  Commonly, an overhead phototherapy unit is combined with a bili blanket that can be place under the infant.  Some of these blankets or pads are rather small, so 2 or 3 of these units may be needed to supply more complete coverage from below.  Lining the sides of the bassinet with white blankets or aluminum foil can also increase the effectiveness of phototherapy.

Sourse Links:

http://www.d-rev.org/about.html