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Vaporizer output calculation
Ventilator disconnect: Detection
Wall O2 failure: Signs
Vaporizer output calculation1,2
Variable bypass vaporizer output depends on fresh gas flow rates, temperature, back pressure and carrier gas.
Output will be less than the dial setting for flow rates less than 250 ml/min because of insufficient turbulence to force vapor molecules out, and at flow rates greater than 15 L/min, due to incomplete mixing to saturate the carrier gas with vapor.
Vapor pressure depends on the identity of the agent, and the temperature. Modern vaporizers automatically correct to maintain output over a range of temperatures.
Intermittent back pressure from positive pressure ventilation can cause the pumping effect, when vapor exits from both the vaporizing chamber outlet and inlet, increasing the delivered concentration.
Carrier gas: N2O less soluble than O2 in vapor - decreased output with N2O
Vaporizer output can be calculated as:
C = (Fv x Pva) / [Fd x (Patm - Pva) + Pv x Patm]
Fv, Gas flow into vaporizer
Fd, Diluent gas flow
C, Vapor output
Pva, Agent vapor pressure
Patm, Atmospheric pressure.
Ventilator disconnect: Detection1,3,4
Disconnection of the patient from the ventilator represents one of the most common forms of preventable anesthetic-related mishaps.
Most common location is at the Y-piece.
Part of ASA Standards For Basic Anesthetic Monitoring 3.2.3:
Wall O2 failure: Signs1,3,5
Usually, the pipeline supply is the primary gas source for the anesthesia machine.
Medical oxygen: stored as cryogenic liquid or compressed gas in central location.
From here, pipes (main lines --> risers --> branches) carry to patient care locations.
Pressure gauges are located downstream of each regulator
Oxygen pipeline pressure should be 50-55 psi.
Mishaps regarding the main supply line from the bulk oxygen reservoir were reported by 16% (5/32) of responding [Ohio] institutions
Loss of pressure is the most frequent problem with pipeline supplies.
Keywords Challenge Question:
The most frequently reported malfunction in medical gas pipeline systems is:
A. Inadequate pressure
B. Cross connection
C. Excessive pressure
D. Alarm dysfunction
E. Contamination of gases
References:
1. Barash PG, Cullen BF, Stoelting RK, et al (eds.). Chapter 26: The Anesthesia Workstation and Delivery Systems Clinical Anesthesia, 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2009.
2. Instruction and Service Manual with Illustrated Parts List, Model 885 Conversion, Ohmeda.
3. Dorsch JA, Dorsch SE. Understanding Anesthesia Equipment, 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2008.
4. ASA Standards For Basic Anesthetic Monitoring, 1986, 2011
5. Stoller JK, Stefanak M, Orens D, et al. The Hospital Oxygen Supply: An O2k Problem. Respiratory Care March 2000 Vol 45 No 3
Keywords Challenge Answer: A
Ventilator disconnect: Detection
Wall O2 failure: Signs
Vaporizer output calculation1,2
Variable bypass vaporizer output depends on fresh gas flow rates, temperature, back pressure and carrier gas.
Output will be less than the dial setting for flow rates less than 250 ml/min because of insufficient turbulence to force vapor molecules out, and at flow rates greater than 15 L/min, due to incomplete mixing to saturate the carrier gas with vapor.
Vapor pressure depends on the identity of the agent, and the temperature. Modern vaporizers automatically correct to maintain output over a range of temperatures.
Intermittent back pressure from positive pressure ventilation can cause the pumping effect, when vapor exits from both the vaporizing chamber outlet and inlet, increasing the delivered concentration.
Carrier gas: N2O less soluble than O2 in vapor - decreased output with N2O
Vaporizer output can be calculated as:
C = (Fv x Pva) / [Fd x (Patm - Pva) + Pv x Patm]
Fv, Gas flow into vaporizer
Fd, Diluent gas flow
C, Vapor output
Pva, Agent vapor pressure
Patm, Atmospheric pressure.
Ventilator disconnect: Detection1,3,4
Disconnection of the patient from the ventilator represents one of the most common forms of preventable anesthetic-related mishaps.
Most common location is at the Y-piece.
Part of ASA Standards For Basic Anesthetic Monitoring 3.2.3:
o When ventilation is controlled by a mechanical ventilator, there shall be in continuous use a device that is capable of detecting disconnection of components of the breathing system. The device must give an audible signal when its alarm threshold is exceeded.
Ventilator disconnection may be detected by: o Capnography sudden drop in ETCO2 to near zero may indicate disconnection vs cardiac arrest clinical correlation advised
o Low airway pressure alarm Pneumatic or electronic device, activated when PIP does not exceed pre-set threshold pressure. Therefore, threshold pressure should be set just below PIP auto set feature of our Drager Fabius GS machines. Otherwise, may not detect partial disconnection.
o Low volume alarm infrared, electronic or ultrasonic based, may measure tidal volume (inhaled or exhaled) or minute ventilation.
o Ventilator: ascending bellows will not fill with total disconnection
o Chest auscultation/chest rise
o Visual inspection /Vigilance
o Low airway pressure alarm Pneumatic or electronic device, activated when PIP does not exceed pre-set threshold pressure. Therefore, threshold pressure should be set just below PIP auto set feature of our Drager Fabius GS machines. Otherwise, may not detect partial disconnection.
o Low volume alarm infrared, electronic or ultrasonic based, may measure tidal volume (inhaled or exhaled) or minute ventilation.
o Ventilator: ascending bellows will not fill with total disconnection
o Chest auscultation/chest rise
o Visual inspection /Vigilance
Wall O2 failure: Signs1,3,5
Usually, the pipeline supply is the primary gas source for the anesthesia machine.
Medical oxygen: stored as cryogenic liquid or compressed gas in central location.
From here, pipes (main lines --> risers --> branches) carry to patient care locations.
Pressure gauges are located downstream of each regulator
Oxygen pipeline pressure should be 50-55 psi.
Mishaps regarding the main supply line from the bulk oxygen reservoir were reported by 16% (5/32) of responding [Ohio] institutions
Loss of pressure is the most frequent problem with pipeline supplies.
o Every anesthesia machine has pipeline pressure gauges.
o May result in failure of ventilators which are O2-driven
o Detected by pipeline pressure gauges --> oxygen analyzer --> gas analyzer --> pulse oximetry --> cyanosis
Other problems: o May result in failure of ventilators which are O2-driven
o Detected by pipeline pressure gauges --> oxygen analyzer --> gas analyzer --> pulse oximetry --> cyanosis
o Excess pressure should be prevented by pressure relief valve in main supply line unless that fails, too. Can be detected with anesthesia machine pipeline pressure gauge
o Cross connections detected by oxygen analyzer, gas analyzer, pulse oximetry, cyanosis. If pipeline crossover suspected, MUST turn on backup E-cylinder AND turn off pipeline supply. This is because the pipeline pressure of ~50psi will override the lower cylinder pressure of ~45psi.
o Contamination (bacteria, particulates, water) filters will prevent but limited detection options.
o Cross connections detected by oxygen analyzer, gas analyzer, pulse oximetry, cyanosis. If pipeline crossover suspected, MUST turn on backup E-cylinder AND turn off pipeline supply. This is because the pipeline pressure of ~50psi will override the lower cylinder pressure of ~45psi.
o Contamination (bacteria, particulates, water) filters will prevent but limited detection options.
Keywords Challenge Question:
The most frequently reported malfunction in medical gas pipeline systems is:
A. Inadequate pressure
B. Cross connection
C. Excessive pressure
D. Alarm dysfunction
E. Contamination of gases
References:
1. Barash PG, Cullen BF, Stoelting RK, et al (eds.). Chapter 26: The Anesthesia Workstation and Delivery Systems Clinical Anesthesia, 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2009.
2. Instruction and Service Manual with Illustrated Parts List, Model 885 Conversion, Ohmeda.
3. Dorsch JA, Dorsch SE. Understanding Anesthesia Equipment, 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2008.
4. ASA Standards For Basic Anesthetic Monitoring, 1986, 2011
5. Stoller JK, Stefanak M, Orens D, et al. The Hospital Oxygen Supply: An O2k Problem. Respiratory Care March 2000 Vol 45 No 3
Keywords Challenge Answer: A
Last edited:
Amiodarone: hemodynamic effect
Aortic crossclamp: CV complications
Arterial waveform: Periph vs central
I messed up the crossclamp keyword: instead of CV complications, I did hemodynamic changes. This is still a high yield topic so I left it in.
Amiodarone: hemodynamic effect
Antiarrhythmic, primarily class III = potassium channel blocker
Hemodynamic effects:
Aortic crossclamp: CV complications
Hemodynamic changes depend on level of the cross-clamp, status of the left ventricle, degree of periaortic collateralization, blood volume and distribution, activation of the sympathetic nervous system, and anesthetic agents and techniques.
Higher clamp levels cause more significant hemodynamic changes.
↑ Arterial blood pressure above the clamp*
Due to ↑ afterload
↓ Arterial blood pressure below the clamp*
↑ Segmental wall motion abnormalities
↑ Left ventricular wall tension
↓ Ejection fraction
↓ Cardiac output (inconsistent)
↑ Pulmonary occlusion pressure
↑ Central venous pressure
↑ Coronary blood flow
↓ Renal blood flow
* = most consistent effects
Arterial waveform: Periph vs central
From central aorta to peripheral artery:
Arterial upstroke becomes steeper (reflection from
arteriolar resistance, exaggerated in elderly)
Systolic peak becomes higher
Dicrotic notch appears later
Diastolic wave becomes more prominent
End-diastolic pressure becomes lower
Wider pulse pressure.
Delayed arrival of the peripheral pulse (systolic upstroke = 60 msec later in
radial artery vs aorta)
MAP in the aorta is just slightly greater than MAP in the radial artery (little
resistance to flow in the major conducting arteries.)
Keywords Challenge Question:Which of the following are typically DECREASED with aortic crossclamp?
A. Incidence of regional wall motion abnormalities
B. Central venous pressure
C. Pulmonary artery occlusion pressure (AKA pulmonary capillary wedge pressure)
D. Blood pressure above the clamp
E. Blood pressure below the clamp
References:
1. Miller, Ronald D. Miller's Anesthesia. Philadelphia, PA: Churchill Livingstone/Elsevier, 2010. Print.
2. Barash, Paul G., Bruce F. Cullen, and Robert K. Stoelting. Clinical Anesthesia. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2009. Print.
3. Zipes, Douglas P., Peter Libby, Robert O. Bonow, and Eugene Braunwald. Braunwald's Heart Disease a Textbook of Cardiovascular Medicine. Philadelphia: Elsevier/Saunders, 2005. Print.
Keywords Challenge Question Answer: E
Aortic crossclamp: CV complications
Arterial waveform: Periph vs central
I messed up the crossclamp keyword: instead of CV complications, I did hemodynamic changes. This is still a high yield topic so I left it in.
Amiodarone: hemodynamic effect
Antiarrhythmic, primarily class III = potassium channel blocker
o (Although also class I, II, IV)
Useful for supraventricular and ventricular arrhythmias. Hemodynamic effects:
o More pronounced with IV than PO forms.
o Peripheral and coronary vasodilator
o Antiadrenergic
o Decreases heart rate, systemic vascular resistance, and contractility
o Peripheral and coronary vasodilator
o Antiadrenergic
o Decreases heart rate, systemic vascular resistance, and contractility
Aortic crossclamp: CV complications
Hemodynamic changes depend on level of the cross-clamp, status of the left ventricle, degree of periaortic collateralization, blood volume and distribution, activation of the sympathetic nervous system, and anesthetic agents and techniques.
Higher clamp levels cause more significant hemodynamic changes.
↑ Arterial blood pressure above the clamp*
Due to ↑ afterload
↓ Arterial blood pressure below the clamp*
↑ Segmental wall motion abnormalities
↑ Left ventricular wall tension
↓ Ejection fraction
↓ Cardiac output (inconsistent)
↑ Pulmonary occlusion pressure
↑ Central venous pressure
↑ Coronary blood flow
↓ Renal blood flow
* = most consistent effects
Arterial waveform: Periph vs central
From central aorta to peripheral artery:
Arterial upstroke becomes steeper (reflection from
arteriolar resistance, exaggerated in elderly)
Systolic peak becomes higher
Dicrotic notch appears later
Diastolic wave becomes more prominent
End-diastolic pressure becomes lower
Wider pulse pressure.
Delayed arrival of the peripheral pulse (systolic upstroke = 60 msec later in
radial artery vs aorta)
MAP in the aorta is just slightly greater than MAP in the radial artery (little
resistance to flow in the major conducting arteries.)
Keywords Challenge Question:Which of the following are typically DECREASED with aortic crossclamp?
A. Incidence of regional wall motion abnormalities
B. Central venous pressure
C. Pulmonary artery occlusion pressure (AKA pulmonary capillary wedge pressure)
D. Blood pressure above the clamp
E. Blood pressure below the clamp
References:
1. Miller, Ronald D. Miller's Anesthesia. Philadelphia, PA: Churchill Livingstone/Elsevier, 2010. Print.
2. Barash, Paul G., Bruce F. Cullen, and Robert K. Stoelting. Clinical Anesthesia. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 2009. Print.
3. Zipes, Douglas P., Peter Libby, Robert O. Bonow, and Eugene Braunwald. Braunwald's Heart Disease a Textbook of Cardiovascular Medicine. Philadelphia: Elsevier/Saunders, 2005. Print.
Keywords Challenge Question Answer: E
Last edited:
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Bronchopleural fistula (BPF): Vent mgmt [1,2]
BPF = Communication between bronchial tree and pleural space
Etiology: post-lung resection, chest tube, trauma, malignancy, infection, mech.vent.
Pneumonectomy pts: incidence 211%, mortality 2371%
Diagnosis: usually made clinically after 7d, confirmed by bronchoscopy.
May cause loss of VT or PEEP; transmission of chest tube suction to airways
May have flow through BPF >20L/min!
May need temporizing measures until definitive Tx
Alternative vent modes: HFJV, HFOV, differential lung vent.
Low VT ventilation: Protect effect [3,4,5,6]
Lung protect vent: Pressure goal
Conventional VT: 10-15ml/kg
Ventilator induced lung injury (VILI)
Therefore, generally accepted lung protective strategies include VT 6-8 ml/kg predicted body weight and Pplat < 30 cmH2O.
Keyword Challenge
Which of the following combinations of tidal volume (VT) and plateau pressure (Pplat) are part of a generally accepted lung protective ventilation strategy?
a. VT = 15ml/kg, Pplat < 50 cmH20
b. VT = 6 ml/kg, P¬plat < 50 cmH20
c. VT = 12 ml/kg, P¬plat < 30 cmH20
d. VT = 6 ml/kg, P¬plat < 30 cmH20
e. Whatever the respiratory therapist wants
References:
1. Millers Anesthesia pp. 1866-7
2. Jantz MA, Anthony VB. Pleural Disease in the Intensive Care Unit In: Civetta, Taylor and Kirby's Critical Care, 4th ed. Pp. 2181-2
3. N Engl J Med 1998;338:347-54
4. Halls Principles of Critical Care. Ch. 37
5. Slutsky AS: Mechanical ventilation. American College of Chest Physicians' Consensus Conference. Chest 104:1833, 1993. [PMID: 8252973]
6. The Acute Respiratory Distress Syndrome Network: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med 342:1301, 2000.
Keywords Challenge Answer: D
BPF = Communication between bronchial tree and pleural space
Etiology: post-lung resection, chest tube, trauma, malignancy, infection, mech.vent.
Pneumonectomy pts: incidence 211%, mortality 2371%
Diagnosis: usually made clinically after 7d, confirmed by bronchoscopy.
o Persistent air leak (> 24h), dyspnea, subQ emphysema, tracheal deviation, air-fluid level on CXR, purulent sputum.
Size of BPF = Difference between inhaled and exhaled tidal volumes
May cause loss of VT or PEEP; transmission of chest tube suction to airways
May have flow through BPF >20L/min!
May need temporizing measures until definitive Tx
o Surgical: myoplasty (pec flap Clagett procedure), pneumonectomy; endobronchial closure, chemical pleurodesis
o Goals: maintain adequate chest tube drainage, full lung expansion, decrease pressure gradient from airway to pleural space
o Minimize flow through BPF, allow to heal
o Decrease airway pressures: ↓ VT, ↓ PEEP, ↓ I:E, ↓ RR
o Reduce chest tube suction (lowest level that expands lung)
Considered an absolute indication for double lumen tube. o Goals: maintain adequate chest tube drainage, full lung expansion, decrease pressure gradient from airway to pleural space
o Minimize flow through BPF, allow to heal
o Decrease airway pressures: ↓ VT, ↓ PEEP, ↓ I:E, ↓ RR
o Reduce chest tube suction (lowest level that expands lung)
Alternative vent modes: HFJV, HFOV, differential lung vent.
Low VT ventilation: Protect effect [3,4,5,6]
Lung protect vent: Pressure goal
Conventional VT: 10-15ml/kg
Ventilator induced lung injury (VILI)
o Macroscopic: barotrauma (extra-alveolar air)
PTX, pneumomediastinum, sub-Q emphysema
o Microscopic: epithelial or endothelial injury, alveolar-capillary barrier damage (2/2 overdistention, volutrauma), surfactant dysfunction (2/2 repetitive opening/closing atelectrauma), bronchiolar injury
o Excess alveolar stretch causes cytokine proliferation
1993 consensus conference: VT 5-7ml/kg, Pplat < 35 cmH2Oo Excess alveolar stretch causes cytokine proliferation
o Based on animal studies
1998 NEJM3: Brazilian RCT, 53 pts w/ early ARDS, VT 6ml/kg or 12ml/kg
o Low VT ↓d mortality (38% vs 71%), also less barotrauma, earlier weaning.
2000 NEJM MC RCT6: o High VT: 12ml/kg & Pplat < 50cmH20 vs low VT: 6ml/kg & Pplat < 30 cmH20
o Stopped after 861 pts enrolled 2/2 ↓ mortality in low VT grp (31.% vs. 40%)
This strategy has also been used in H1N1 pts.o Stopped after 861 pts enrolled 2/2 ↓ mortality in low VT grp (31.% vs. 40%)
Therefore, generally accepted lung protective strategies include VT 6-8 ml/kg predicted body weight and Pplat < 30 cmH2O.
Keyword Challenge
Which of the following combinations of tidal volume (VT) and plateau pressure (Pplat) are part of a generally accepted lung protective ventilation strategy?
a. VT = 15ml/kg, Pplat < 50 cmH20
b. VT = 6 ml/kg, P¬plat < 50 cmH20
c. VT = 12 ml/kg, P¬plat < 30 cmH20
d. VT = 6 ml/kg, P¬plat < 30 cmH20
e. Whatever the respiratory therapist wants
References:
1. Millers Anesthesia pp. 1866-7
2. Jantz MA, Anthony VB. Pleural Disease in the Intensive Care Unit In: Civetta, Taylor and Kirby's Critical Care, 4th ed. Pp. 2181-2
3. N Engl J Med 1998;338:347-54
4. Halls Principles of Critical Care. Ch. 37
5. Slutsky AS: Mechanical ventilation. American College of Chest Physicians' Consensus Conference. Chest 104:1833, 1993. [PMID: 8252973]
6. The Acute Respiratory Distress Syndrome Network: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med 342:1301, 2000.
Keywords Challenge Answer: D
Categorical data: Chi square
Categorical data (also called nominal or qualitative) are used to describe characteristics that have no numerical value, such as gender, eye color, ASA status.
Use chi-square to compare frequencies or proportions in two or more groups, and determine if the observed frequencies differ significantly from the expected values.
Often, data will be presented as 2x2 table.
Assumptions: Observations are sampled randomly and independently
o Not useful if expected frequencies or sample sizes are small (use Fisher exact test)
Power analysis: study design
Power is the probability of rejecting the null hypothesis when it is false, or accepting the alternative hypothesis when it is true.
Type II error = not rejecting the null hypothesis when its false = false negative
β = probability of type II error
Power = 1 β
α = arbitrarily selected, usually 0.05.
p value = probability that the observed difference is due to chance.
Power increases as sample size n increases. The larger the difference between the treatment and control groups, the smaller sample size will be needed to detect a difference.
Sorry this table didnt paste well:
Null Hypothesis:.....|....True......................... ..................|.....False
_________________|________________________________ _|_________________________________
Accepted...............|....Correct decision, with confidence.....|.....False negative (Type II error)
...........................|....p = 1 α.....................................|.....p = β
_________________|________________________________ _|_________________________________
Rejected................|....False positive (Type I error)............|....Correct decision, with power
...........................|....p = α...........................................|. ...p = 1 β
SE vs SD calculation
Standard error is a measure of how much fluctuation occurs due to sampling.
Standard deviation is the most commonly used measure of the dispersion of data around a mean (requires normal distribution).
Standard error = where σ is the standard deviation and n is sample size.
The larger the sample size, the smaller the standard error will be, which means the sample means will be closer to the population (true) mean.
References:
Beth Dawson, Robert G. Trapp. Basic & Clinical Biostatistics, 4th edition. 2004: McGraw-Hill.
Practice Question
ABA 1996A
35. Which of the following statement about the standard error of the mean (SE) is true?
(A) Sample mean + SE has approximately a 95% chance of containing the population mean
(B) The SE describes the precision of the population mean
(C) The SE describes the range of the sample values
(D) The SE is greater than the standard deviation
(E) The SE is obtained by multiplying the sample standard deviation by the square root of the sample size
Categorical data (also called nominal or qualitative) are used to describe characteristics that have no numerical value, such as gender, eye color, ASA status.
Use chi-square to compare frequencies or proportions in two or more groups, and determine if the observed frequencies differ significantly from the expected values.
Often, data will be presented as 2x2 table.
Assumptions: Observations are sampled randomly and independently
o Not useful if expected frequencies or sample sizes are small (use Fisher exact test)
Power analysis: study design
Power is the probability of rejecting the null hypothesis when it is false, or accepting the alternative hypothesis when it is true.
Type II error = not rejecting the null hypothesis when its false = false negative
β = probability of type II error
Power = 1 β
α = arbitrarily selected, usually 0.05.
p value = probability that the observed difference is due to chance.
Power increases as sample size n increases. The larger the difference between the treatment and control groups, the smaller sample size will be needed to detect a difference.
Sorry this table didnt paste well:
Null Hypothesis:.....|....True......................... ..................|.....False
_________________|________________________________ _|_________________________________
Accepted...............|....Correct decision, with confidence.....|.....False negative (Type II error)
...........................|....p = 1 α.....................................|.....p = β
_________________|________________________________ _|_________________________________
Rejected................|....False positive (Type I error)............|....Correct decision, with power
...........................|....p = α...........................................|. ...p = 1 β
SE vs SD calculation
Standard error is a measure of how much fluctuation occurs due to sampling.
Standard deviation is the most commonly used measure of the dispersion of data around a mean (requires normal distribution).
Standard error = where σ is the standard deviation and n is sample size.
The larger the sample size, the smaller the standard error will be, which means the sample means will be closer to the population (true) mean.
References:
Beth Dawson, Robert G. Trapp. Basic & Clinical Biostatistics, 4th edition. 2004: McGraw-Hill.
Practice Question
ABA 1996A
35. Which of the following statement about the standard error of the mean (SE) is true?
(A) Sample mean + SE has approximately a 95% chance of containing the population mean
(B) The SE describes the precision of the population mean
(C) The SE describes the range of the sample values
(D) The SE is greater than the standard deviation
(E) The SE is obtained by multiplying the sample standard deviation by the square root of the sample size
