Here are all the formulas and calculations you will need to know for Respiratory Therapy school. Use this study guide to practice and get the hang of them. Also, you may see some of these practice questions and answers on your exam(s).

*For a really cool mobile app that will do these calculations for you, check out RespCalc.com.

Respiratory Therapy Formulas and Calculations:

 

Question Answer
What is the formula for minute ventilation? minute volume = respiratory rate x tidal volume
A 36 year old female with a respiratory rate of 12 and tidal volume of 500mL. What would the minute volume be in Liters? 6L/minute
A 78 year old male with a history of COPD has a respiratory rate of 20 and tidal volume of 650mL. What would his minute volume be in Liters? 13L/minute
What is the formula for alveolar ventilation per minute? Alveolar ventilation/min = respiratory rate x alveolar ventilation (tidal volume – dead space volume)
20 year old male weighing 160 pounds with a respiratory rate of 12, tidal volume of 500 mL. What would his alveolar ventilation/minute be? 4080mL/min
A patient with a respiratory rate of 12, tidal volume of 500mL and weight is unknown. What would the alveolar ventilation per minute be? 4200mL/min
The ventilatory parameters for 68kg man with a respiratory rate of 20, tidal volume 480 mL, vital capacity 2.7L. What would his alveolar ventilation/min be in Liters? 6.6L/min
What is a normal carbon dioxide production? 200mL/min
What is the formula for alveolar partial pressure of carbon dioxide (PACO2)? PACO2 = VCO2 X 0.863 divided by VA(alveolar ventilation per minute)
If a patient’s carbon dioxide production is normal and their PACO2 is 80mmHg, what would their alveolar ventilation be? 2.15 L
If a patient’s carbon dioxide production is normal and their PACO2 is 60mmHg, what would their alveolar ventilation be? 2.88 L
What is the formula for dead space/tidal volume ratio? VD/VT = PaCO2 – PeCo2/PaCo2
What is the patient’s VD/VT if their PaCO2 is 40 mm HG with a mixed expired CO2 of 28 mm Hg? 0.3
What is the patient’s VD/VT if their PaCO2 is 58 mm HG with a mixed expired CO2 of 32 mm Hg? 0.45
What is the formula for Boyle’s law? P1V1 = P2V2
What is the formula for Charles law? V1/T1 = V2/T2
What is the formula for Combined Gas law? PV = nRT
An H cylinder is half full (full = 2200) and the patient is receiving oxygen via nasal cannula at 3L/min. How long will the cylinder last in minutes? in hours? 1151 minutes. 19.18 hours
An E cylinder is at 1400psi and the flow rate is 2.5L/minute. How many minutes will the tank last? 156.8 minutes
If a patient has smoked 2 packs of cigarettes daily for the past 35 years, what would their pack year history be? 70 pack year
Question Answer
Raw PIP-Plat/Flow (L/sec)
a/A ratio Pao2/PAO2
Bicarb correction of base defecit BE-Kg/4
BSA (4xkg)+7/kg+90
CO-Fick method O2 consumption/CaO2-CvO2
O2 consumption-estimated 130xBSA
Dynamic compliance Change in V/change in P
Static compliance change in volume/PIP-total PEEP
Corrected Vt Vte-tubing volume
Vd/Vt PaCO2-PeCO2/PaCO2
Expired Minute ventilation VtxRR
Alveolar minute ventiation (Vt-Vd)xRR
CaO2 (Hbx1.34xSaO2)+(PaO2x.003)
time constant RxC
ventialtor rate needed for a desired PaCO2 knownRRxKnownPaCO2/Desired PaCO2
TCT Ti+Te
calculate a rate from TCT 60sec/TCT in seconds
Calculating Te TCT-Ti
RSBI Vt/f
Female IBW 105+5(60-HT in inches)
Male IBW 106+6(60-HT in inches)
Qs/Qt (PAO2-PaO2)x0.003/(CaO2-CvO2)+(PAO2-PaO2)x0.003
PAO2 (Pb-PH2O)xFIO2-PaCO2x1.25
Normal a/A ratio >60%
Average adult BSA 1.7m2
Normal CO 4-8L/min
Normal Cdyn 30-40mL/cmH2O
Normal Cst 40-60mL/cmH2O
Normal Vd/Vt-spontaneous 20-40%
Normal Vd/Vt-on mechanical ventilation 40-60%
Normal CaO2 16-20vol%
Normal shunt <10%
Normal RSBI <100
Desired Vt Actual VtxActual PaCO2/Desired PaCO2
Desired frequency Actual frequencyxActual PaCO2/Desired PaCO2
Normal PaO2/FIO2 ratio 500
Normal PvO2 35-40mmHg
desired FIO2 Desired PaO2xKnown FIO2/ KnownPaO2


Question Answer
Cylinder Duration of flow psig X Tank Factor / flow Full tank is 2200 psi
Dynamic Compliance Exhaled volume/ PIP-PEEP
Static Compliance Exhaled volume/Plateau-PEEP
Alveolar minute ventilation *Shortcut lml per # of body weight (i.e. if someone weighs 110lb, it would be 110ml)
Suction Catheter Size ***Shortcut Multiply the ID size x 2 and use next smallest Fr size cathether
Pulse Pressure Systolic – Diastolic
Mean Arterial Pressure (MAP) (2 x Diastolic) + Systolic / 3
Cardiac Output (CO) using stroke volume Heart rate x stroke volume
Stroke Volume Equation Cardiac Output (QT) / Heart Rate
Systemic Vascular Resistance (SVR) (MAP-CVP) / CO
Conversion mm Hg/L/min to Dynes/sec/cm Multiple mm Hg/L/min by 80
Pulmonary Vascular Resistance (PVR) (MPAP- PWP) /CO
VD/VT PaCO2-PECO2/PaCO2 x 100
P(A-a)02 *Shortcut (FIO2 x 7) – CO2 – PaCO2
A-aDO2 **Shortcut (FIO2 x7- CO2)-PaO2
PAO2 **Shortcut (FIO2 x 7)- PaCO2
PaO2 **Shortcut Estimate by subtracting 30 from SaO2
I:E Ratio given both I time and E time are given Divide both sides by the Inspiratory time.
Inspiratory Time Total cycle time = 60/f or RR total cycle time/sum of I:E ratio parts
Ventilator Inspiratory Flow (VCV) ***Short cut add the I:E parts and then multiply by the minute volume (you can then estimate)
mg/mL 10 x % concentration
Question Answers
a/A ratio PaO2/PAO2 norm is 90%, >35%=weaning, reflects efficiency of oxygenation as a percentage, <74%=shunt, V/Q mismatch or diffusion defect
A-aDo2 A-a gradient, norm 5-10 mmHg on .21, 30-60 on 100%, >350=mech support, <350 weaning. Represents potential to Oxygenate vs. the amount of O2 in the artery. Every 50mmHg is approx. 2 percent shunt above norm of 2-5%
Alveolar Air Equation (pb-47)FIO2-(PaCO2x1.25), norm 80-100mmHg (can reach 675 on 100%), press of O2 in the Alveoli at any given Pb, represents potential for arterial oxygen
BP norm 120/80 (90-140/60-90), <90/60 or >180/110 is inconsistent with weaning
CaO2 norm 16-20 vol% (Hbx1.34)SaO2 + (PaO2x.003) total amount of O2 carried in 100ml of blood, combined content of O2 carried on Hb and dissolved in plasma, (can be reduced by <hb, anemia=”” or=”” <co)<=”” span=””></hb,>
CcO2 Content of pulm capillary blood oxygen at 100% FIO2, (Hbx1.34)1 + (PAO2x.003) used in shunt equation
CD dynamic compliance, aka CLdyn, norm is 0.03-0.04L/cmH2O (30-40mL/cmH20), calc is Vt(L)/(PIP-PEEP) Changes in CD indicate changes in CL or elastance. Up with decreased elasticity (emphysema, down with with stiff (ARDS).
CI cardiac index, 2.5-4 L/min, <2.1 inconsistent with weaning
Cs (CVAE) static compliance, norm for vent pt is .035-.055L/cmH20 or 35-55mL/cmH20, calc is Vt/(Pplat-PEEP), represents the combination of lung elasticity and chest wall recoil while on vent. Down with stiff lungs, >35ml/cnH2O weaning
*Ct Tube compliance aka compressible volume, volume lost to pt circuit, approx 3ml/cmH20 x PIP, deduct from VT to find actual delivered tidal volume. VT-(PIP x 3) equals actual VT.
CvO2 (Hb x 1.34)SvO2 + (PvO2 x .003) norm is 15 vol%, represents the value of O2 in blood returning to the right side of the heart after tissues have oxygenated.
CVP central venous pressure, norm 2-6 mmHg, 2-6 weaning
Deadspace ventilation in excess of perfusion (pulm emboli)
DO2 O2 Delivery, (CaO2 x CO) x 10, norm is 1000mL/O2/min
FIO2 for weaning <40-50% weaning
Flow (VT/IT) x 60 or VE x (I+E)
Flow required for a 1 second I-time and a VT of 750cc? .750 equals L/sec x 1 or .750L/sec flow. To convert to L/min .750 x 60 equals 45L/m, this is the vent flow setting to deliver a VT in 1 second I-time.
Hb 12-16 g/100ml of blood, (no anemia or >10g for weaning)
HCO3 22-26 torr
HCT 40-50%
HR norm 60-80, 60-120 weaning
I:E ratios/% 1:2=33%, 1:3=25%, 1:4=20% (add the ratio parts, then figure what percent is the sum of the parts, 1+4 is 5 and 1/5 of 100% is 20%)
IBW F 105+5/lb over 60″
IBW M 106+6/lb over 60″
I-time represents relationship for volume (Liters), flow (L/sec) and time (seconds). V eq F x T or F eq V/T
Low press on vent look for leak, cuff or humidifier will be first place
Magic Box TIIVR+TIVER, FRC=TIIVR=TLC, IRV, IC, VC, RV, + TIVER, FRC=TLC, IRV, VT, ERV, RV, FRC
MAP Mean arterial pressure, norm 90 (80-100), 70-130 weaning
MIP/NIF Max Inspiratory Press, norm -80 – -100, > -20 support indicated, <-20 weaning (remember that negative numbers are larger as they become less, -25 < -20)
PaCO2 35-45 torr, >55 indicates support, <50 weaning
PaCO2 to increase with pt on mech vent <pip, <rr,=””>PEEP</pip,>
PaCO2(d) CO2 desired, CO2 is adjusted by changing Resp rate so (VExCO2)/CO2d=VEd or (RRset x CO2)/CO2d=RRd, always round down and go slightly acidic as tissue will oxygenate better.
PaCO2-to decrease with pt on mech vent >PIP, >RR, <peep< span=””></peep<>
PaO2 80-100 torr, >60 mmHg on <60% weaning
PaO2 <80 on FIO2 <60 V/Q mismatch
PaO2 <80 on FIO2 >60 shunt, refractory hypoxemia or venous admixture
PaO2 desired PaO2 (desired) x FIO2 (current)/PaO2 (current)=FIO2 required to bring PaO2 to desired level. Example of pt on 40% FIO2, PaO2 of 53 and wanting PaO2 of 80, calc is (80x.4)/53 equals .60, so increase FIO2 to 60%
PaO2/FIO2 PaO2/FIO2, norm 350-450mmHg, <300 indicates acute lung injury, <200 indicates ARDS, >200=weaning, measures o2 efficiency
PaO2-to decrease while pt on mech vent <pip, <rr,=”” <peep,=”” or=”” <fio2<=”” span=””></pip,>
PaO2-to increase while pt on mech vent >PIP, >RR, >PEEP, or >FIO2
PAP pulmonary artery pressure, norm 25/10 (20-35/5-15), >35/15 is inconsistent with weaning, pulm hypertension, left vent fail, fluid overload
PCWP pulmonary artery wedge pressure, norm 5-10 mmHg, >18 is inconsistent with weaning, left vent failure, fluid overload
PEEP positive end expiratory pressure above baseline of 0, norm 3-5 (5-8/ARDS)
PH Norm 7.35-7.45, <7.20 indicates support, >7.35 weaning
PIP Peak pressure, norm is <50cmH2O, the press required to overcome both compliance and resistance
PvO2 40 torr
QS/QT Pulmonary Shunt equation (CcO2-CaO2)/(CcO2-CvO2) Norm 2-3%, >20% vent indication, <20% weaning, >30% is life threatening. Measures % of QT not exposed to ventilation, shunts caused by atelectasis, edema, pneumonia, pneumothorax, obstructions
QT cardiac output, norm 5L/min (4-8)
RAW (RIAF) Airway Resistance norm is 5-12cmH2O/L/Sec for intubated pt, (PIP-Pplat)/(flow in min/60 sec). Increase in RAW reflects an issue with airways, bronchospasm, secretions, edema etc.
Refractory hypoxemia hypoxemia that does not respond to O2 therapy
RR respiratory rate, norm 12-20, >35 indicates support, 6-30 weaning
RSBI Rapid shallow breathing index, RR/VT, <105 weaning, must be calculated during spont breathing, press support reduces predictive value
RV Residual Volume 1.2 L
SaO2 97-100%
Shunt perfusion in excess of ventilation
SvO2 75vol%
TC Time constant, (Raw x CS)e, where e represents volume exhaled as a percent, 1 is 63%, 2 is 86%, 3 is 95% and 5 is 100% exhaled. TC <3 leads to air trapping.
TCT total cycle time, 60 sec/RRset, amount of time for a single breath cycle both I and E. If I:E is 1:2 then TCT is 3
TLC total lung capacity 6L
VA Alveolar ventilation, (VT-VD) x RR, represents sum of breaths taking place in gas exchange, norm 4-5 L/min
VC Vital capacity, 65-75 mL/kg, <10mL/kg indicates support, >15 mL/kg for weaning
VD Deadspace volume, VD=(VT-VA), norm is 33% of VE, 1 mL/lb IBW or 2.2mL/kg (approx 150 ml)
VD/VT Ratio Deadspace to VT ratio, norm 24-40%, >60 indicates support, <60% wean, increase (w/no change in VE) indicates decreased blood flow to alveoli, usually caused by emboli, excessive PEEP, or emphysema
VE Minute ventilation, VE=VT x RRset, flow of expired gas in one minute, norm 5-8L/min, < 10 weaning
VEspont VEtot-(VTset x RRset), norm 5-6 L/min,
VO2 O2 consumption, norm is 250mL/O2/L/min, [C(a-v)O2 x QT] x 10, the amount of O2 consumed by the body per liter of blood per minute.
VTspont VEtot-(VTset x RRset)/(RRtot-RRset) Measured when machine in SIMV mode, represents what the patient is actually breathing on his/her own.
VT norm is 5-8 mL/kg (400-600 cc), <5mL/kg indicates support, >5 weaning.
Question Answer
a/A ratio-PaO2 of 75, PAO2 of 245 75/245=30.6%
Bicarb correction-Base defecit of -3, 79kg -3x79/4=59.25
BSA-average adult 1.7m2
BSA-65kg (4x65)+7/65+90=1.67m2
O2 consumption (VO2) of a 70kg pt (4x70)/70+90=1.75m2130x1.75=227.5
normal CO 4-8L/min
CO-70kg patient, CaO2 15vol%, CvO2 10vol% 227.5/5%227.5/.05=4550mL (4.5L)
dynamic compliance-PEEP 5, PIP 25, corrected Vt 600 600/25-5=30
NL dynamic compliance 30-40mL/cmH2O
Corrected Vt-Vte .550, Tubing volume 50mL 550-50=500mL
NL Vd/Vd-intubated pt 40-60%
NL Vd/Vt-non intubated pt 20-40%
Vd/Vt- PaCO2 45, PeCO2 35 45-35/45=22%
FIO2 needed for a COPD pt- desired PaO2 60, RA PaO2 35 12%+(60-35)/3=60-35=25/3=8.338.33+21=29.33%
Minimum flow rate needed-minute volume 15, I:E 1:3 15x4=60L.min`
I:E ratio- Ti% 25% 25%/25%:1-25%/25%=1:3
Mean Airway Pressure-RR 25, Ti .5, PIP 35, PEEP 8 25x.5=12.5/60=.20835-8=27x.208=5.62+8=13.6cmH2O
anatomic dead space-estimated of a 65kg pt 65mL
alveolar ventilation-minute volume 8,78kg pt, RR 25 (8/25)=.320L(320-78)=242x25=6050mL
minute volume-RR 20, Vt 500 .500x20=10
NL CaO2 16-20vol%
arterial oxygen content-Hb 10g%, SaO2 95%, PaO2 95 (10x1.34x.95)+(95x.003)12.73+.285=12.015vol%
overall oxygen carying capability of the blood CaO2
Can be found also with an expiratory hold Mean airway pressure
Question Answer
a/A ratio PaO2/PAO2 norm is 90%, >35%=weaning, reflects efficiency of oxygenation as a percentage, <74%=shunt, V/Q mismatch or diffusion defect
A-aDo2 A-a gradient, norm 5-10 mmHg on .21, 30-60 on 100%, >350=mech support, <350 weaning. Represents potential to Oxygenate vs. the amount of O2 in the artery. Every 50mmHg is approx. 2 percent shunt above norm of 2-5%
Alveolar Air Equation (pb-47)FIO2-(PaCO2x1.25), norm 80-100mmHg (can reach 675 on 100%), press of O2 in the Alveoli at any given Pb, represents potential for arterial oxygen
BP norm 120/80 (90-140/60-90), <90/60 or >180/110 is inconsistent with weaning
CaO2 norm 16-20 vol% (Hbx1.34)SaO2 + (PaO2x.003) total amount of O2 carried in 100ml of blood, combined content of O2 carried on Hb and dissolved in plasma, (can be reduced by <hb, anemia=”” or=”” <co)<=”” span=””></hb,>
CcO2 Content of pulm capillary blood oxygen at 100% FIO2, (Hbx1.34)1 + (PAO2x.003) used in shunt equation
CD dynamic compliance, aka CLdyn, norm is 0.03-0.04L/cmH2O (30-40mL/cmH20), calc is Vt(L)/(PIP-PEEP) Changes in CD indicate changes in CL or elastance. Up with decreased elasticity (emphysema, down with with stiff (ARDS).
CI cardiac index, 2.5-4 L/min, <2.1 inconsistent with weaning
Cs (CVAE) static compliance, norm for vent pt is .035-.055L/cmH20 or 35-55mL/cmH20, calc is Vt/(Pplat-PEEP), represents the combination of lung elasticity and chest wall recoil while on vent. Down with stiff lungs, >35ml/cnH2O weaning
*Ct Tube compliance aka compressible volume, volume lost to pt circuit, approx 3ml/cmH20 x PIP, deduct from VT to find actual delivered tidal volume. VT-(PIP x 3) equals actual VT.
CvO2 (Hb x 1.34)SvO2 + (PvO2 x .003) norm is 15 vol%, represents the value of O2 in blood returning to the right side of the heart after tissues have oxygenated.
CVP central venous pressure, norm 2-6 mmHg, 2-6 weaning
Deadspace ventilation in excess of perfusion (pulm emboli)
DO2 O2 Delivery, (CaO2 x CO) x 10, norm is 1000mL/O2/min
FIO2 for weaning <40-50% weaning
Flow (VT/IT) x 60 or VE x (I+E)
Flow required for a 1 second I-time and a VT of 750cc? .750 equals L/sec x 1 or .750L/sec flow. To convert to L/min .750 x 60 equals 45L/m, this is the vent flow setting to deliver a VT in 1 second I-time.
Hb 12-16 g/100ml of blood, (no anemia or >10g for weaning)
HCO3 22-26 torr
HCT 40-50%
HR norm 60-80, 60-120 weaning
I:E ratios/% 1:2=33%, 1:3=25%, 1:4=20% (add the ratio parts, then figure what percent is the sum of the parts, 1+4 is 5 and 1/5 of 100% is 20%)
IBW F 105+5/lb over 60″
IBW M 106+6/lb over 60″
I-time represents relationship for volume (Liters), flow (L/sec) and time (seconds). V eq F x T or F eq V/T
Low press on vent look for leak, cuff or humidifier will be first place
Magic Box TIIVR+TIVER, FRC=TIIVR=TLC, IRV, IC, VC, RV, + TIVER, FRC=TLC, IRV, VT, ERV, RV, FRC
MAP Mean arterial pressure, norm 90 (80-100), 70-130 weaning
MIP/NIF Max Inspiratory Press, norm -80 – -100, > -20 support indicated, <-20 weaning (remember that negative numbers are larger as they become less, -25 < -20)
PaCO2 35-45 torr, >55 indicates support, <50 weaning
PaCO2 to increase with pt on mech vent <pip, <rr,=””>PEEP</pip,>
PaCO2(d) CO2 desired, CO2 is adjusted by changing Resp rate so (VExCO2)/CO2d=VEd or (RRset x CO2)/CO2d=RRd, always round down and go slightly acidic as tissue will oxygenate better.
PaCO2-to decrease with pt on mech vent >PIP, >RR, <peep< span=””></peep<>
PaO2 80-100 torr, >60 mmHg on <60% weaning
PaO2 <80 on FIO2 <60 V/Q mismatch
PaO2 <80 on FIO2 >60 shunt, refractory hypoxemia or venous admixture
PaO2 desired PaO2 (desired) x FIO2 (current)/PaO2 (current)=FIO2 required to bring PaO2 to desired level. Example of pt on 40% FIO2, PaO2 of 53 and wanting PaO2 of 80, calc is (80x.4)/53 equals .60, so increase FIO2 to 60%
PaO2/FIO2 PaO2/FIO2, norm 350-450mmHg, <300 indicates acute lung injury, <200 indicates ARDS, >200=weaning, measures o2 efficiency
PaO2-to decrease while pt on mech vent <pip, <rr,=”” <peep,=”” or=”” <fio2<=”” span=””></pip,>
PaO2-to increase while pt on mech vent >PIP, >RR, >PEEP, or >FIO2
PAP pulmonary artery pressure, norm 25/10 (20-35/5-15), >35/15 is inconsistent with weaning, pulm hypertension, left vent fail, fluid overload
PCWP pulmonary artery wedge pressure, norm 5-10 mmHg, >18 is inconsistent with weaning, left vent failure, fluid overload
PEEP positive end expiratory pressure above baseline of 0, norm 3-5 (5-8/ARDS)
PH Norm 7.35-7.45, <7.20 indicates support, >7.35 weaning
PIP Peak pressure, norm is <50cmH2O, the press required to overcome both compliance and resistance
PvO2 40 torr
QS/QT Pulmonary Shunt equation (CcO2-CaO2)/(CcO2-CvO2) Norm 2-3%, >20% vent indication, <20% weaning, >30% is life threatening. Measures % of QT not exposed to ventilation, shunts caused by atelectasis, edema, pneumonia, pneumothorax, obstructions
QT cardiac output, norm 5L/min (4-8)
RAW (RIAF) Airway Resistance norm is 5-12cmH2O/L/Sec for intubated pt, (PIP-Pplat)/(flow in min/60 sec). Increase in RAW reflects an issue with airways, bronchospasm, secretions, edema etc.
Refractory hypoxemia hypoxemia that does not respond to O2 therapy
RR respiratory rate, norm 12-20, >35 indicates support, 6-30 weaning
RSBI Rapid shallow breathing index, RR/VT, <105 weaning, must be calculated during spont breathing, press support reduces predictive value
RV Residual Volume 1.2 L
SaO2 97-100%
Shunt perfusion in excess of ventilation
SvO2 75vol%
TC Time constant, (Raw x CS)e, where e represents volume exhaled as a percent, 1 is 63%, 2 is 86%, 3 is 95% and 5 is 100% exhaled. TC <3 leads to air trapping.
TCT total cycle time, 60 sec/RRset, amount of time for a single breath cycle both I and E. If I:E is 1:2 then TCT is 3
TLC total lung capacity 6L
VA Alveolar ventilation, (VT-VD) x RR, represents sum of breaths taking place in gas exchange, norm 4-5 L/min
VC Vital capacity, 65-75 mL/kg, <10mL/kg indicates support, >15 mL/kg for weaning
VD Deadspace volume, VD=(VT-VA), norm is 33% of VE, 1 mL/lb IBW or 2.2mL/kg (approx 150 ml)
VD/VT Ratio Deadspace to VT ratio, norm 24-40%, >60 indicates support, <60% wean, increase (w/no change in VE) indicates decreased blood flow to alveoli, usually caused by emboli, excessive PEEP, or emphysema
VE Minute ventilation, VE=VT x RRset, flow of expired gas in one minute, norm 5-8L/min, < 10 weaning
VEspont VEtot-(VTset x RRset), norm 5-6 L/min,
VO2 O2 consumption, norm is 250mL/O2/L/min, [C(a-v)O2 x QT] x 10, the amount of O2 consumed by the body per liter of blood per minute.
VTspont VEtot-(VTset x RRset)/(RRtot-RRset) Measured when machine in SIMV mode, represents what the patient is actually breathing on his/her own.
VT norm is 5-8 mL/kg (400-600 cc), <5mL/kg indicates support, >5 weaning.
Question Answer
Equation O2 Cylinder Duration E Cyl=0.28× psig/liter flow H Cyl=3.14× psig/liter flow **Full cyl= 2200psig (1800-2400)
Equation FiO2 FiO2=(O2 liter flow× 4)+20
Equation Aveolar Air PAO2= [(Pb-P H2O)×FiO2]-(PaCO2/0.80) or PAO2= [(Pb-p H2O)×FiO2]-(PaCO2×1.25) *Pb= Barometric Press (760 when not spec) *P H2O= 47 (mmHg)
Equation Predicted PaO2 PaO2= 105-(Age/2) or 100-(Age/3) *PaCO2 must be normal
Equation PaO2 to FiO2 Ratio PaO2/FiO2 <200 is a critical value
Equation Minute Ventilation Ve= Vt× f Vt=Tidal Volume f=Frequency or rate *Can be manipulated to determine other values
Equation Frequency needed for desired PaCO2 f= current rate× PaCO2/Desired PaCO
Equation RSBI (Rapid Shallow Breathing Index) RSBI= f/Vt *Must be less than 100 for successful weaning
Equation Deadspace to Tidal Volume Ratio Vd/Vt= (PaCO2-PetCO)/PaCO2 *Pet= Exhaled CO *Deadspace= Ventilation w/o Perfusion
Equation Static Lung Compliance Cst= Delivered Vt/(Plateau-PEEP) *Normal 100ml/cmH2O
Equation Dynamic Lung Compliance Cdyn= Delivered Vt/(PIP-PEEP)
Equation Airway Resistance Raw= (PIP-Plateau)/(Flow÷60) *Must be whole number *Answer given in Liters per second *Normal 0.6-2.4cmH2O/L/sec *Normal Vented 5-10cmH20/L/sec
Equation Ideal Body Weight Men: 106+6(H”-60)=IBW# Women: 105+5(H”-60)=IBW# *Convert to metric #/2.2=Kg
Equation Tidal Volume Vt (min)= IBW(Kg)× 10 Vt (max)= IBW(Kg)× 12
Equation Total Cycle Time TCT= 60sec/f I(time)= TCT/I+E E(time)= TCT-I
Equation Temperature Conversion Temp F°=(C°× 1.8)+32 Temp C°=(F°-32)÷ 1.8
Suction Catheter Sizing (OETT Size/2)x 3= Cath Sz (French)
Term Definition
Pack Years = # Packs/Day x # of years smoked
Cerebral Perfusion Pressure (CPP) = CPP = MAP – ICP
Mean Arterial Pressure(MAP) = (2 x Diastolic) + Systolic —————————— 3
Fick Equation(cardiac output)(Qt) = VO2/ C(a-v)O2 x (10)
Cardiac Output (Qt) = Heart rate x Stoke volume
Cardiac Index (CI) = Qt/BSA or Cardiac Output/ Body Surface Area
Pulse Pressure = Systolic – Diastolic Pressure
Stroke Volume = Qt/Heart Rate
Systemic Vascular Resistance (SVR) = (MAP-CVP)/Cardiac Output (CO)
Pulmonary Vascular Resistance (PVR) = (MPAP – PWP)/ Cardiac Output (CO)
Converting to Dynes for PVR/SVR = Multiply mmHg/L/Min by 80


Question Answer
Pack years= # of packs/day x # of years smoked.
MAP= (2x diastolic pressure) + Systolic ________________________________ 3
Fic equation: Cardiac output/QT= V02 ___________________________________ C(a-v)o2 (10)
Cardiac output= QT HR x SV
Stroke volume = QT __________________________________ HR
SVR= (MAP-CVP) _____________________ x 80 QT
PVR= MPAP-PWP _____________________ x 80 QT
Ca02 (Hbg x 1.34 x Sa02) + (Pa02 x 0.003) Oxygen in RBC 02 in plasma
Cvo2= (Hbg x 1.34 x Svo2) + (Pvo2 x 0.003)
Vo2= QT x C(a-v)02 x 10
QS/QT cco2-Cao2 _____________________ cco2-cvo2
Desired Pao2 Current Fio2 x Desired Pao2 _______________________________ Current pao2
P/F ratio Pao2/fio2
Pediatric drug dosage ESD= wt (lbs) x normal adult dose _____________________________ 150
Amount of drug in mg. Drug% x 10 x mL= number of mg in 1mL of the drug.
Minute ventilation= Vt x F
Dynamic Compliance= Exhaled volume __________________________ peak insp pressure – peep
Static Compliance= Exhaled volume __________________________ Plateau pressure – peep
Raw= Peak pressure-Plateau pressure
IDBW for Males= 106lbs + 6lbs/inch over 5ft
IDBW for Females= 105 lbs + 5lbs/inch over 5ft
Calculating minimal flow rate= (Vt x rate) x ( I+E)
RSBI= RR/VT=
Duration of flow in tank gauge pressure (psi) x tank factor __________________________________ liter flow
Tank factor for E tank .28/ L psi (0.3)
Tank factor for H tank 3.14 L psi (3.0)
Total flow= factor of tank used x liter flow
Calculating patients max heart rate. 220- HR
calculating volume lost through chest tube Vt-Vte= Lost Vt

Check out this book for detailed lessons on all of the formulas and calculations in Respiratory Therapy.

(The Kindle Edition on Amazon.com is offered at a pretty good price.)

Respiratory Care Calculations

Third Edition