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Respiratory Therapy Formulas and Calculations:

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 all the math equations that you could possibly see while in RT School. On top of that, you may see some of these practice questions and answers on your exam(s) in school and even on the TMC Exam once it’s time for you to take your board.

That is why it’s important for you to go ahead and learn these respiratory formulas and calculation now, so when the time comes, you’ll be good to do.

🔒Click Here to get access to our FREE Cheat Sheet with all the best Formulas and Calculations.

QuestionAnswer
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
QuestionAnswer
RawPIP-Plat/Flow (L/sec)
a/A ratioPao2/PAO2
Bicarb correction of base defecitBE-Kg/4
BSA(4xkg)+7/kg+90
CO-Fick methodO2 consumption/CaO2-CvO2
O2 consumption-estimated130xBSA
Dynamic complianceChange in V/change in P
Static compliancechange in volume/PIP-total PEEP
Corrected VtVte-tubing volume
Vd/VtPaCO2-PeCO2/PaCO2
Expired Minute ventilationVtxRR
Alveolar minute ventiation(Vt-Vd)xRR
CaO2(Hbx1.34xSaO2)+(PaO2x.003)
time constantRxC
ventialtor rate needed for a desired PaCO2knownRRxKnownPaCO2/Desired PaCO2
TCTTi+Te
calculate a rate from TCT60sec/TCT in seconds
Calculating TeTCT-Ti
RSBIVt/f
Female IBW105+5(60-HT in inches)
Male IBW106+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 BSA1.7m2
Normal CO4-8L/min
Normal Cdyn30-40mL/cmH2O
Normal Cst40-60mL/cmH2O
Normal Vd/Vt-spontaneous20-40%
Normal Vd/Vt-on mechanical ventilation40-60%
Normal CaO216-20vol%
Normal shunt<10%
Normal RSBI<100
Desired VtActual VtxActual PaCO2/Desired PaCO2
Desired frequencyActual frequencyxActual PaCO2/Desired PaCO2
Normal PaO2/FIO2 ratio500
Normal PvO235-40mmHg
desired FIO2Desired PaO2xKnown FIO2/ KnownPaO2
QuestionAnswer
Cylinder Duration of flowpsig X Tank Factor / flow Full tank is 2200 psi
Dynamic ComplianceExhaled volume/ PIP-PEEP
Static ComplianceExhaled volume/Plateau-PEEP
Alveolar minute ventilation *Shortcutlml per # of body weight (i.e. if someone weighs 110lb, it would be 110ml)
Suction Catheter Size ***ShortcutMultiply the ID size x 2 and use next smallest Fr size cathether
Pulse PressureSystolic – Diastolic
Mean Arterial Pressure (MAP)(2 x Diastolic) + Systolic / 3
Cardiac Output (CO) using stroke volumeHeart rate x stroke volume
Stroke Volume EquationCardiac Output (QT) / Heart Rate
Systemic Vascular Resistance (SVR)(MAP-CVP) / CO
Conversion mm Hg/L/min to Dynes/sec/cmMultiple mm Hg/L/min by 80
Pulmonary Vascular Resistance (PVR)(MPAP- PWP) /CO
VD/VTPaCO2-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 **ShortcutEstimate by subtracting 30 from SaO2
I:E Ratio given both I time and E time are givenDivide both sides by the Inspiratory time.
Inspiratory TimeTotal cycle time = 60/f or RR total cycle time/sum of I:E ratio parts
Ventilator Inspiratory Flow (VCV) ***Short cutadd the I:E parts and then multiply by the minute volume (you can then estimate)
mg/mL10 x % concentration
QuestionAnswers
a/A ratioPaO2/PAO2 norm is 90%, >35%=weaning, reflects efficiency of oxygenation as a percentage, <74%=shunt, V/Q mismatch or diffusion defect
A-aDo2A-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
BPnorm 120/80 (90-140/60-90), <90/60 or >180/110 is inconsistent with weaning
CaO2norm 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
CcO2Content of pulm capillary blood oxygen at 100% FIO2, (Hbx1.34)1 + (PAO2x.003) used in shunt equation
CDdynamic 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).
CIcardiac 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
*CtTube 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.
CVPcentral venous pressure, norm 2-6 mmHg, 2-6 weaning
Deadspaceventilation in excess of perfusion (pulm emboli)
DO2O2 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.
Hb12-16 g/100ml of blood, (no anemia or >10g for weaning)
HCO322-26 torr
HCT40-50%
HRnorm 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 F105+5/lb over 60″
IBW M106+6/lb over 60″
I-timerepresents relationship for volume (Liters), flow (L/sec) and time (seconds). V eq F x T or F eq V/T
Low press on ventlook for leak, cuff or humidifier will be first place
Magic BoxTIIVR+TIVER, FRC=TIIVR=TLC, IRV, IC, VC, RV, + TIVER, FRC=TLC, IRV, VT, ERV, RV, FRC
MAPMean arterial pressure, norm 90 (80-100), 70-130 weaning
MIP/NIFMax Inspiratory Press, norm -80 – -100, > -20 support indicated, <-20 weaning (remember that negative numbers are larger as they become less, -25 < -20)
PaCO235-45 torr, >55 indicates support, <50 weaning
PaCO2 to increase with pt on mech ventPEEP
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,
PaO280-100 torr, >60 mmHg on <60% weaning
PaO2 <80 on FIO2 <60V/Q mismatch
PaO2 <80 on FIO2 >60shunt, refractory hypoxemia or venous admixture
PaO2 desiredPaO2 (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/FIO2PaO2/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
PaO2-to increase while pt on mech vent>PIP, >RR, >PEEP, or >FIO2
PAPpulmonary artery pressure, norm 25/10 (20-35/5-15), >35/15 is inconsistent with weaning, pulm hypertension, left vent fail, fluid overload
PCWPpulmonary artery wedge pressure, norm 5-10 mmHg, >18 is inconsistent with weaning, left vent failure, fluid overload
PEEPpositive end expiratory pressure above baseline of 0, norm 3-5 (5-8/ARDS)
PHNorm 7.35-7.45, <7.20 indicates support, >7.35 weaning
PIPPeak pressure, norm is <50cmH2O, the press required to overcome both compliance and resistance
PvO240 torr
QS/QTPulmonary 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
QTcardiac 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 hypoxemiahypoxemia that does not respond to O2 therapy
RRrespiratory rate, norm 12-20, >35 indicates support, 6-30 weaning
RSBIRapid shallow breathing index, RR/VT, <105 weaning, must be calculated during spont breathing, press support reduces predictive value
RVResidual Volume 1.2 L
SaO297-100%
Shuntperfusion in excess of ventilation
SvO275vol%
TCTime 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.
TCTtotal 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
TLCtotal lung capacity 6L
VAAlveolar ventilation, (VT-VD) x RR, represents sum of breaths taking place in gas exchange, norm 4-5 L/min
VCVital capacity, 65-75 mL/kg, <10mL/kg indicates support, >15 mL/kg for weaning
VDDeadspace volume, VD=(VT-VA), norm is 33% of VE, 1 mL/lb IBW or 2.2mL/kg (approx 150 ml)
VD/VT RatioDeadspace 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
VEMinute ventilation, VE=VT x RRset, flow of expired gas in one minute, norm 5-8L/min, < 10 weaning
VEspontVEtot-(VTset x RRset), norm 5-6 L/min,
VO2O2 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.
VTspontVEtot-(VTset x RRset)/(RRtot-RRset) Measured when machine in SIMV mode, represents what the patient is actually breathing on his/her own.
VTnorm is 5-8 mL/kg (400-600 cc), <5mL/kg indicates support, >5 weaning.
QuestionAnswer
a/A ratio-PaO2 of 75, PAO2 of 24575/245=30.6%
Bicarb correction-Base defecit of -3, 79kg-3x79/4=59.25
BSA-average adult1.7m2
BSA-65kg(4x65)+7/65+90=1.67m2
O2 consumption (VO2) of a 70kg pt(4x70)/70+90=1.75m2130x1.75=227.5
normal CO4-8L/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 600600/25-5=30
NL dynamic compliance30-40mL/cmH2O
Corrected Vt-Vte .550, Tubing volume 50mL550-50=500mL
NL Vd/Vd-intubated pt40-60%
NL Vd/Vt-non intubated pt20-40%
Vd/Vt- PaCO2 45, PeCO2 3545-35/45=22%
FIO2 needed for a COPD pt- desired PaO2 60, RA PaO2 3512%+(60-35)/3=60-35=25/3=8.338.33+21=29.33%
Minimum flow rate needed-minute volume 15, I:E 1:315x4=60L.min`
I:E ratio- Ti% 25%25%/25%:1-25%/25%=1:3
Mean Airway Pressure-RR 25, Ti .5, PIP 35, PEEP 825x.5=12.5/60=.20835-8=27x.208=5.62+8=13.6cmH2O
anatomic dead space-estimated of a 65kg pt65mL
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 CaO216-20vol%
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 bloodCaO2
Can be found also with an expiratory holdMean airway pressure
QuestionAnswer
a/A ratioPaO2/PAO2 norm is 90%, >35%=weaning, reflects efficiency of oxygenation as a percentage, <74%=shunt, V/Q mismatch or diffusion defect
A-aDo2A-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
BPnorm 120/80 (90-140/60-90), <90/60 or >180/110 is inconsistent with weaning
CaO2norm 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
CcO2Content of pulm capillary blood oxygen at 100% FIO2, (Hbx1.34)1 + (PAO2x.003) used in shunt equation
CDdynamic 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).
CIcardiac 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
*CtTube 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.
CVPcentral venous pressure, norm 2-6 mmHg, 2-6 weaning
Deadspaceventilation in excess of perfusion (pulm emboli)
DO2O2 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.
Hb12-16 g/100ml of blood, (no anemia or >10g for weaning)
HCO322-26 torr
HCT40-50%
HRnorm 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 F105+5/lb over 60″
IBW M106+6/lb over 60″
I-timerepresents relationship for volume (Liters), flow (L/sec) and time (seconds). V eq F x T or F eq V/T
Low press on ventlook for leak, cuff or humidifier will be first place
Magic BoxTIIVR+TIVER, FRC=TIIVR=TLC, IRV, IC, VC, RV, + TIVER, FRC=TLC, IRV, VT, ERV, RV, FRC
MAPMean arterial pressure, norm 90 (80-100), 70-130 weaning
MIP/NIFMax Inspiratory Press, norm -80 – -100, > -20 support indicated, <-20 weaning (remember that negative numbers are larger as they become less, -25 < -20)
PaCO235-45 torr, >55 indicates support, <50 weaning
PaCO2 to increase with pt on mech ventPEEP
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,
PaO280-100 torr, >60 mmHg on <60% weaning
PaO2 <80 on FIO2 <60V/Q mismatch
PaO2 <80 on FIO2 >60shunt, refractory hypoxemia or venous admixture
PaO2 desiredPaO2 (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/FIO2PaO2/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
PaO2-to increase while pt on mech vent>PIP, >RR, >PEEP, or >FIO2
PAPpulmonary artery pressure, norm 25/10 (20-35/5-15), >35/15 is inconsistent with weaning, pulm hypertension, left vent fail, fluid overload
PCWPpulmonary artery wedge pressure, norm 5-10 mmHg, >18 is inconsistent with weaning, left vent failure, fluid overload
PEEPpositive end expiratory pressure above baseline of 0, norm 3-5 (5-8/ARDS)
PHNorm 7.35-7.45, <7.20 indicates support, >7.35 weaning
PIPPeak pressure, norm is <50cmH2O, the press required to overcome both compliance and resistance
PvO240 torr
QS/QTPulmonary 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
QTcardiac 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 hypoxemiahypoxemia that does not respond to O2 therapy
RRrespiratory rate, norm 12-20, >35 indicates support, 6-30 weaning
RSBIRapid shallow breathing index, RR/VT, <105 weaning, must be calculated during spont breathing, press support reduces predictive value
RVResidual Volume 1.2 L
SaO297-100%
Shuntperfusion in excess of ventilation
SvO275vol%
TCTime 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.
TCTtotal 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
TLCtotal lung capacity 6L
VAAlveolar ventilation, (VT-VD) x RR, represents sum of breaths taking place in gas exchange, norm 4-5 L/min
VCVital capacity, 65-75 mL/kg, <10mL/kg indicates support, >15 mL/kg for weaning
VDDeadspace volume, VD=(VT-VA), norm is 33% of VE, 1 mL/lb IBW or 2.2mL/kg (approx 150 ml)
VD/VT RatioDeadspace 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
VEMinute ventilation, VE=VT x RRset, flow of expired gas in one minute, norm 5-8L/min, < 10 weaning
VEspontVEtot-(VTset x RRset), norm 5-6 L/min,
VO2O2 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.
VTspontVEtot-(VTset x RRset)/(RRtot-RRset) Measured when machine in SIMV mode, represents what the patient is actually breathing on his/her own.
VTnorm is 5-8 mL/kg (400-600 cc), <5mL/kg indicates support, >5 weaning.
QuestionAnswer
Equation O2 Cylinder DurationE Cyl=0.28× psig/liter flow H Cyl=3.14× psig/liter flow **Full cyl= 2200psig (1800-2400)
Equation FiO2FiO2=(O2 liter flow× 4)+20
Equation Aveolar AirPAO2= [(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 PaO2PaO2= 105-(Age/2) or 100-(Age/3) *PaCO2 must be normal
Equation PaO2 to FiO2 RatioPaO2/FiO2 <200 is a critical value
Equation Minute VentilationVe= Vt× f Vt=Tidal Volume f=Frequency or rate *Can be manipulated to determine other values
Equation Frequency needed for desired PaCO2f= 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 RatioVd/Vt= (PaCO2-PetCO)/PaCO2 *Pet= Exhaled CO *Deadspace= Ventilation w/o Perfusion
Equation Static Lung ComplianceCst= Delivered Vt/(Plateau-PEEP) *Normal 100ml/cmH2O
Equation Dynamic Lung ComplianceCdyn= Delivered Vt/(PIP-PEEP)
Equation Airway ResistanceRaw= (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 WeightMen: 106+6(H”-60)=IBW# Women: 105+5(H”-60)=IBW# *Convert to metric #/2.2=Kg
Equation Tidal VolumeVt (min)= IBW(Kg)× 10 Vt (max)= IBW(Kg)× 12
Equation Total Cycle TimeTCT= 60sec/f I(time)= TCT/I+E E(time)= TCT-I
Equation Temperature ConversionTemp F°=(C°× 1.8)+32 Temp C°=(F°-32)÷ 1.8
Suction Catheter Sizing(OETT Size/2)x 3= Cath Sz (French)
TermDefinition
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
QuestionAnswer
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= QTHR 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/QTcco2-Cao2 _____________________ cco2-cvo2
Desired Pao2Current Fio2 x Desired Pao2 _______________________________ Current pao2
P/F ratioPao2/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 tankgauge pressure (psi) x tank factor __________________________________ liter flow
Tank factor for E tank.28/ L psi (0.3)
Tank factor for H tank3.14 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 tubeVt-Vte= Lost Vt

Final Thoughts

So there you have it! That wraps up our massive list of most of the formulas and calculations that you will see in Respiratory Therapy School. You can go through this list to practice learning all those pesky math equations that you must know in order to succeed in RT School. Also, feel free to bookmark this page so that you can use it as a reference for every time you get stuck on one to the formulas or calculations, you can use this guide to help get back on track.

And be sure to download a copy of our FREE cheat sheet that breaks down the formulas and calculations even further. Particularly, the ones you need to know for the TMC Exam.

You can get access to the cheat sheet by using the link below:

Formulas and Calculations

All the formulas and calculations that you need to know for the TMC Exam.