Anesthesia Machines Quiz MCQ Trivia

Oxygen is stored as a gas under high pressure within the cylinder, so as the gas is released, the pressure within the container decreases proportionally. This is an example of Boyle's Law.



Medical Gas Supply, Ch 1, available at: 

http://elsevierhealth.com/media/us/samplechapters/9780443100871/9780443100871.pdf

An arterial line works by using a fluid column to transfer subtle differences in pressure from the patient's circulation to a transducer. Bubbles--even small bubbles-- can interrupt the transferring of these waves and result in a damping of the tracing. Both systolic and diastolic will be affected.

An arterial line works by transferring the patient's pulse pressure through a fluid column against a transducer, which converts the force it feels into a blood pressure tracing.  If the patient's pulse pressure is removed, there is a short time during which the system will still register pressure.  This concept is called "damping," and the mathematical model for how long the signal lingers is called the "damping coefficient."



Anything that extinguishes the vibration of the pulse pressure within the tubing will increase the damping coefficient.  This includes air bubbles, blood clots, kinks in the line, soft tubing, and an extended line.

The worst thing an anesthesia machine can do is deliver a hypoxic gas mixture to the patient, so there are multiple checks to prevent this from happening--including the rotamers, disconnection alarm, and of course the O2 analyzer. The O2 analyzer is on the inspiratory limb of the breathing circuit immediately before the gas goes to the patient, which makes it the last line of defense against a hypoxic mixture.

Failure to oxygenate patients is a leading cause of morbidity in the OR. All of the answer choices can lead to hypoxic gas mixtures being delivered to the patient, but there are many systems in place such as the Diameter Index Safety System to prevent incorrect attachment of gas lines (choices A and B). The most common cause by far is an accidental disconnection between the anesthetic machine and the patient.

OR fires can only happen when the triad of ignition source, fuel, and oxidizer all come together. In this case, the source is the bovie, the fuel is the patient's skin, and the oxidizer is the O2 she is breathing.  Stopping the fire is as simple as removing one part of the triad, so disconnecting the ET tube here will take away the fire's oxygen supply and it should go out immediately.  The patient should then be extubated and the airway inspected by bronchoscopy.

 

The Mapleson D circuit places the fresh gas flow next to the patient and a pressure valve near the ventilator.  This means that excess pressure generated by the ventilator will be released by the valve and the patient will receive the gas flow.







Kaul TK, Mittal G. Mapleson's breathing systems. Indian J Anesthesia, 2013 [cited 2014 Feb 17];57:507-15. Available from: http://www.ijaweb.org/text.asp?2013/57/5/507/120148

Because nitrous oxide is stored as a liquid, the pressure within the canister is the same until all of the gas is used. Like any liquid, N2O exists in equilibrium between liquid and gas within the container. Whenever any of the gas is released to the patient, the momentary drop in pressure from the container gas on the liquid allows more of the liquid N2O molecules to escape into the gaseous state. This is why the pressure gauge is not the way to check how much gas is left as it should always be 745 PSI. N2O volume remaining is checked by weight-- a full canister weighs 8kg, and an empty one weighs 6kg.

 

A full-size E oxygen cylinder contains 680L O2 at 2200 psi, and as it is stored as a gas, the pressure decreases proportionally as the volume is released. The pressure here is 550, which is 25% of the original, so the volume must also be 25% of the starting value. 680 / 4 = 170 L still in the canister. 170 / 4L per minute = 42.5 minutes.

An arterial line works by transferring pressure from the catheter through a fluid-filled tube and against a transducer that can convert the force into a tracing. When the catheter is elevated as in this case, a fluid column forms that pushes on the transducer and creates a higher value than is actually present in the patient.

Think of soda-lime as a base that combines with CO2 to produce CaCO3 and water with some heat. It is made of: 75% Ca(OH)2, 20% water, 3% NaOH, and 1% KOH. The mechanism of the reaction is:



1) CO2 + H2O → CO2 (aq) (CO2 dissolves in water - slow and rate-determining)

2) CO2 (aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)

3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH



The NaOH and KOH are catalysts as they cancel in the final equation.  Now it makes sense that the soda lime is spent when too much water is in the canister. 



 

As he is in a sitting position and having a craniotomy, this patient is already at high risk for a venous air embolism (VAE), and his drop in HR and ETCO2 is consistent with this diagnosis.  While a large amount of air is required most often (5ml/kg in some studies), there are case reports of VAE with as little as 20 ml air.  The amount of air required is thought to be a function of the procedure, the speed of air injection, and the position of the patient.



Transesophageal echo is most sensitive in detecting VAE followed by pre-cordial doppler.  Both of these methods detect VAE before there are any physiologic changes.  Decreased ET-CO2 and PAP can detect VAE when symptoms are still modest.  Decreased CVP and CO are able to detect larger emboli.  SIgns of catastrophic VAE are EKG changes (either bradyarrhythmia or ventricular fibrillation), cardiovascular collapse, and "Millwheel" murmur.

Of the 15 standard machine checks, the ASA 2008 guidelines advise for all 15 at the start of the day and then only 8 before each case. Here is the list of tests to be completed before each procedure:



1. Verify that patient suction is adequate to clear the airway

2. Verify the availability of required monitors including alarms

3. Verify that vaporizers are adequately filled and if applicable that the filler ports are tightly closed

4. Verify that the CO2 absorbent is not exhausted

5. Perform breathing system pressure/leak testing

6. Verify that gas flows properly through the breathing circuit during both inspiration and expiration

7. Document completion of checkout procedures

8. Confirm ventilator settings and evaluate readiness to deliver anesthesia care

Studies of volatile agents at increased altitudes show that while a higher percentage of gas is delivered, the partial pressure remains roughly the same.

The pressure-relief valve (also known as the pop-off valve or the APL -- Adjustable Pressure Limiting valve) is designed to release gas when the system pressure exceeds a certain threshold. When this valve fails, pressure builds within the system as new gas enters each time the machine gives the patient breath. This new gas will fill the areas of lowest pressure first, so some of the tidal volume is diverted from the patient into the reservoir bag as it requires lower force to expand than the patient’s lungs.

The capnograph portion of the monitor shows that this patient’s CO2 is above 0 mmHg even after expiration. The easiest way this can happen is when the CO2 absorbent is exhausted because exhaled CO2 will be able to return to the inspiratory limb of the breathing circuit. This can be corrected by replacing the soda-lime or increasing the gas flow. CO2 is also elevated when there is an incompetent expiratory valve. The expiratory valve is supposed to prevent expired air from passing back to the patient, so if it is broken, the patient’s own previously exhaled CO2 will cause the reading seen on the monitor here.