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# Formulas & Norms

### WillWallace Fomulas & Norms

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) |

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 |

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 |

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 |

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. |