Arterial / Alveolar ratio ( a/A ratio)
PaO2/PAO2 norm is 90%
>35% = weaning, reflects efficiency of oxygenation as a % <74% = shunt, V/Q mismatch or diffusion defect
Alveolar - arterial oxygen tension difference
( A-aDO2 )
A-a gradient, represents potential to Oxygenate vs. the amount of O2 in the artery. Every 50mmHg is approx. 2 percent shunt above norm of 2-5%
Normal A-a gradient is 5-10 mmHg on 21 % and 30-60 mmHg on 100%.
>350 = mech. vent.support,
<350 weaning.
Alveolar Air Equation (PAO2)
pressure of O2 in the Alveoli at any given Pb, represents potential for arterial oxygen.
PAO2 = (Pb-47)FIO2-(PaCO2 x 1.25),
Normal range is 80-100mmHg (can reach 675 on 100%),
Blood Pressure (BP)
Normal BP 120/80
Normal Systolic BP is 90-140
Normal Diastolic BP is 60-90)
<90/60 hypotension or >180/110 hypertension is inconsistent with weaning.
Oxygen Content of arterial blood ( CaO2)
Total amount of O2 carried in 100 mL of blood, combined content of O2 carried on Hb and dissolved in plasma, (can be reduced by <Hb, anemia or <CO)
Normal CaO2 is 16-20 vol%
CaO2 = (Hbx1.34)SaO2 + (PaO2x.003)
Oxygen Content of pulmonary capillary blood (CcO2)
Content of pulm capillary blood oxygen at 100% FIO2, used in shunt equation.
Normal is 5 vol %
CcO2 = (Hbx1.34)1 + (PAO2x.003)
Dynamic Compliance (CD aka CLdyn)
change in dynamic compliance indicates change in compliance and elastance.
increase in CD with decrease in elasticity ( Ex: Emphysema)
decreased in CD with stiff lungs (Ex: ARDS)
CD = Vt / PIP-PEEP
Normal CD is 0.03 to 0.04 L/cmH2O or 30 to 40 mL/cmH2O
Cardiac Index ( CI )
Normal range 2.5 to 4 L/min.
< 2.1 inconsistent for weaning.
Static Compliance ( CS aka CLstatic )
Represents the combination of lung elasticity and chest wall recoil while on vent.
CS decreases with stiff lungs.
> 35 mL/cmH2O weaning.
Tube Compliance aka Compresible volume, volume lost to pt. circuit. ( Ct )
approximately 3 mL/cmH2O x PIP; then deduct from Vt to find actual delivered Vt.
Ct = Vt - (PIP x 3) " equals actual/ corrected Vt".
Oxygen Content of mixed venous blood ( CvO2 )
it represents the value of O2 in blood returning to the right side of the heart before tissues have oxygenated.
Normal CvO2 is 15 vol %
CvO2 = (Hb x 1.34) + (PvO2 x 0.003)
Central Venous Pressure ( CVP )
Normal range 2 to 6 mm Hg, 2 -6 weaning
Deadspace
Ventilation in excess of perfussion; ventilation without perfussion.
Example: Pulmonary Embolism (PE)
Total Oxygen Delivery ( DO2)
Normal DO2 is 1000 mL/O2/min
DO2 = QT x (CaO2 x 10)
DO2 decreases when Saturation, Hb, cardiac output declines.
DO2 increases when Saturation, Hb, Cardiac output improves or normal ranges.
FIO2 for weaning
< 40 to 50 % of FIO2.
Flow ( V )
It is the speed at which a given vol. is delivered.
- directly proportional to E time.
increase V = decrease I time and increase E time.
- inversely proportional to I time.
decrease V = increase I time and decrease E time.
V = Vt / I-time x 60 sec or V =
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.
Hemoglobin ( Hb )
Normal range 12 to 16 g/100 mL of blood ( no anemia )
> 10 g/100mL of blood weaning.
Bicarbonate ( HCO3 )
Normal range 22 to 26 torr.
Hematocrit ( Hct ) or Packed Cell Volume ( PCV )
It is the volume of RBC in 100mL of blood.
Normal range 40 to 50 %.
Heart Rate or Pulse ( HR )
Normal range 60 to 100.
< 60 bpm = bradycardia
> 100 bpm = tachycardia
60 to 120 safe for weaning.
I : E ratios / %
Add the ratios, then figure what % is the sum of the parts.
Example: 1+4 = 5 and 1/5 of 100% = 20 %
1:2 = 33%
1:3 = 25 %
1:4 = 20 %
Ideal Body Weight ( IBW ) female
- pt. height 105 ( 5' )
- remaining inches x 5
- add the 2 #'s, to convert to Kg divide by 2.2 or shortcut 2
Example: pt female 5'7"
105 + (7 x 5) = 105 + 35
= 140 / 2
= 70 kg
Ideal Body Weight ( IBW ) male
- pt height 106 ( 5' )
- remaining inches x 6
- add the 2 #'s, to convert to Kg divide by 2.2 or shortcut 2
Example: pt male 5'9"
106 + (9 x 6) = 106 + 54
= 160 / 2
= 80 Kg
Inspiratory Time ( I time )
represents relationship for volume (L), flow (L/sec), and time (sec)
Vol = flow x time or Flow = vol / time
Low pressure on vent.
look for leak, cuff or humidifier will be first place.
Lung Volumes and Capacities ( Magic Box )
TLC = IC + FRC
IC = IRV + Vt + ERV
FRC = ERV + RV
VC = IRV + Vt + ERV
Mean Arterial Pressure ( MAP )
Normal MAP is 90, acceptable range 80 to 100.
70 to 130 for weaning.
Maximal Inspiratory Pressure ( MIP / NIF )
Normal range - 80 to -100
> than -20 support indicated
< than -20 weaning recomended
* remember negatives #'s are larger as they become smaller
Example: -25 < -20
PaCO2
Acceptable range 35 to 45 torr
> 55 indicates support.
< 50 indicates weaning.
* To increase PaCO2 with pt. on mech. vent.
- < PIP, < RR, > PEEP.
* to decrease PaCO2 with pt. on mech. vent.
- >PIP, > RR, < PEEP.
Desired CO2 ( PaCO2 (d) )
Adjust by changing RR so
VE x CO2 / CO2 (d) = VE (d)
or
RR x CO2 / CO2 (d) = RR (d)
* always round down and go slightly acidic as tissue will oxygenate better.
PaO2
80 - 100 torr Normal
60 - 79 torr Mild
40 - 59 torr Moderate
< 39 torr Severe
* > 60 mmHg on < 60% FIO2 weaning
* PaO2 < 80 on FIO2 < than 60% ( V/Q mismatch )
* PaO2 < 80 on FIO2 > than or equal to 60% ( Shunt, refractory hypoxemia or venous admixture.
PaO2 desired
FIO2 required to bring PaO2 to desired level.
FIO2 = PaO2 (desired) x FIO2 (current) / PaO2 (current)
Example: of pt on 40% FIO2, PaO2 of 53 and wanting PaO2 of 80.
FIO2 = (80x.4) / 53
= .60 so increase FIO2 to 60%
PaO2 / FIO2
Normal 350 to 450 mmHg
< than 300 indicates Acute Lung Injury.
< than 200 indicates ARDS.
> than 200 = weaning, measures O2 efficiency.
To decrease PaO2 while pt. on mech .vent.
< PIP, < RR, < PEEP, < FIO2
to increase PaO2 while pt. on mech. vent.
> PIP, > RR, > PEEP, > FIO2
Pulmonary Artery Pressure ( PAP )
Normal Mean PAP 25 / 10
Normal PAP Systolic 20 to 35
Normal PAP diastolic 5 to 15
> than 35 / 15 is inconsistent with weaning.
- indicates pulmonary hypertension.
- Left ventricular failure.
- Fluid overload.
Pulmonary Capillary Wedge Pressure ( PCWP )
Normal PCWP 5 - 10 mmHg
> 18 is inconsistent with weaning.
- indicates Left ventricular failure.
- Fluid overload.
Positive End Expiratory Pressure ( PEEP )
above baseline of 0.
Normal 3 - 5 of PEEP.
ARDS 5 - 8 pf PEEP.
pH
Normal range 7.35 to 7.45
< 7.20 indicates support
> 7.35 weaning.
Peak Inspiratory Pressure ( PIP )
Normal PIP is < 50 cm H2O; the pressure required to overcome both CL and Raw.
PvO2
Normal 40 torr.
Pulmonary Shunt Equation ( QS / QT )
Measures % of QT not exposed to ventilation.
Example: shunts caused by
-Atelectasis, Edema, Pneumonia, Pneumothorax,
Obstructions.
QS / QT = CcO2 - CaO2 / Cco2 - CvO2
Normal Pulmonary Shunt 2 - 3 %
> 20% mech. vent. indication.
< 20% weaning.
> 30% is lif
Cardiac Output ( QT )
Normal 5 L/min range 4 to 8 L/min
Airway Resistance ( Raw )
Normal is 5 to 12 cmH2O/L/sec for intubated pts.
Raw = PIP - Pplat / Flow in min/60 sec.
Increase with Raw reflects issue with:
- airways
- bronchospam
- secretion
- edema
Residual Volume ( RV )
Normal RV is 1.2 L
SaO2
97 to 100 %
Shunt
Perfusion in excess of ventilation; perfusion without ventilation.
SvO2
75 vol%
Time Constant ( TC )
TC = (Raw x CS)
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.
Total Cycle Time ( Tct )
Tct = 60 sec / RR
Set amount of time for a single breath cycle both I and E.
Example: If I:E is 1:2 then TCT is 3
Total Lung Capacity ( TLC )
Normal TLC is 6 L
Alveolar Ventilation ( VA )
Represents sum of breaths taking place in gas exchange,
VA = (VT-VD) x RR
Normal 4-5 L/min
Vital Capacity ( VC )
VC 65-75 mL/kg,
<10mL/kg indicates support.
>15 mL/kg for weaning
Deadspace Volume ( VD )
VD = (VT-VA)
Normal is 33% of VE, 1 mL/lb IBW or 2.2mL/kg (approx 150 ml)
Deadspace to Tidal Volume ratio
( VD/VT Ratio )
Normal VD / VT ratio is 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
MInute Ventilation ( VE )
Set flow of expired gas in one minute
VE = VT x RR
Normal VE is 5-8L/min.
< 10 weaning
Spontaneous Minute Volume ( VEspont )
VEspont = VEtot-(VTset x RRset).
Normal 5-6 L/min
Oxygen Consumption ( VO2 )
The amount of O2 consumed by the body per liter of blood per minute.
Normal VO2 is 250mL/O2/L/min.
VO2 = [C(a-v)O2 x QT] x 10.
Spontaneous Tidal Volume ( VTspont )
Measured when machine in SIMV mode, represents what the patient is actually breathing on his/her own.
VTspont = VEtot-(VTset x RRset) / (RRtot-RRset)
Tidal Volume ( Vt )
Normal Vt is 5-8 mL/kg (400-600 cc).
<.5 mL/kg indicates support.
> 5 mL/Kg indicates weaning.