Are you looking for all the **Respiratory Therapy formulas and calculations** that you need to know as a Respiratory Therapy student? If so, then you’re definitely in the right place! You can use this study guide to practice and get the hang of all the math equations and formulas that you will possibly see throughout the Respiratory Therapy program at your school.

On top of that, the information and practice questions found in this study guide can help you prepare for your exams in school, as well as the TMC Exam once it’s time to start preparing for boards.

Even though we may not like it at times (because I mean, who does like doing math?), we still must know and understand each of these equations and formulas if we want to succeed in Respiratory Therapy school. That is why it’s important for you to go ahead and learn each and every one of these respiratory therapy formulas and calculations now, so when the time comes, you’ll be good to go! Are you ready to get started?

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

## 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 |

## 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: