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A-aDo2: A-a gradient, norm 5-10 mmHg on .21, 30-60 on 100%,
>350mech 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%
Increased A-a= SHUNT
A-a Gradient also know PF ratio
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/A ratio
Anion Gap= the difference in the measured cations and the measured anions in serum, plasma, or urine.
Used to assess Metabolic Acidosis or alkalosis, normal around 8-16 mEq/L. Use MUDPILES to determine cause of metabolic acidosis (high gap)
= ( [Na+] ) − ( [Cl−]+[HCO3−] ) without
potassium = ( [Na+]+[K+] ) − ( [Cl−]+[HCO3
−] ) with potassium
Anion Gap
CaO2: norm 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)
CaO2
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.
C(a-v)O2 = arterial to mixed venous oxygen content difference
Determines how well the tissues take up O2
CvO2
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.
Oxygen Consumption
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.
Ex: Vt 550 – (25 x3) = 475
Tube compliance
Time Constant: The given % of a passively exhaled breath of air will require a constant amount of time to exhale
Depends on the resistance and compliance of the lung
TC= R x C (in liters)
Time Constant
TC: Time constant, (Raw x CS)e, where e represents volume exhaled as a
percent, 1 is 63%, 2 is 86%, 3 is 95% 5 is 100% exhaled. TC <3 leads to air trapping.
Time Constant
DO2: O2 Delivery, (CaO2 x CO) x 10, norm is 1000mL/O2/min
The ability of oxygen to tissues based on cardiac output and Hb
DO2
I-time = Inspiratory Time, E-time = Expiratory time, TCT= total cycle time (I +E)
I-time when compared to E-time will always be a 1: something ratio.
Respiratory rate = 60 /TCT
EXAMPLE: Calculate I:E ratio, rate and TCT if I-time is 1.2 seconds and E-time is 3 seconds.
TCT = 1.2+ 3 = 4.2 Rate = 60/4.2 =14 I:E = TE/TI = 3/1.2 = 2.5, (I:E is 1:2.5)
I:E Calculations
I-time = Inspiratory Time, E-time = Expiratory time, TCT= total cycle time (I +E) I-time when compared to E-time will always be
a 1: something ratio. Respiratory rate = 60 /TCT
Example: The ventilator is set at 12 breaths per minute with an IE ratio of 1:3. How many seconds for inspiratory time?
Seconds per breath = 60 divided by 12 = 5 seconds TI=5/(1+3) =5/4=1.25 seconds
I:E Calculations
Example: The ventilator is set at 12 breaths per minute with an IE ratio of 1:3. How many seconds for inspiratory time?
Seconds per breath = 60 divided by 12 = 5 seconds TI=5/(1+3) =5/4=1.25 seconds
Since I/E = 1:3, the expiratory time = 1.25 • 3 = 3.75 seconds
Note: 1.25 + 3.75 = 5 seconds (the number of
seconds per breath in this case.) ( a breath equals inspiration + expiration)
I:E Calculations
Flow = VE x (I+E)
Example: Calculate flow given: VT 600 Rate 12 IT 1.5 ET 3
600 x 12 = 7.2 L x (1.5 +3) = 32 L
Flow
Mean Airway pressure (Paw): ½ (PIP-peep) (TI/TCT) + PEEP
Average amount of pressure throughout the TCT
AKA: Pmean, MAP, MAWP, PAW
PaW
VE = actual VE x actual PaCO2 desired PaCO2
New rate = Current rate x actual PaCO2 desired PaCO2
New Vt = Current Vt x actual PaCO2 desired PaCO2
Calculation of VE for desired PaCO2
Example: Calculate the new set rate for a patient,
Current set rate: 12 PaCO2 on ABG 48 Desired PaCO2=40
New Rate = 12 x 48 / 40 = 14.4 (14)
Calculation of VE for desired PaCO2
Example: calculate the new VT for a patient with:
Current VT = 500 PaCO2 on ABG 55 PaCO2 desired 40
New Vt = 500 x 55 = 687.5 (688) 40
Calculation of VE for desired PaCO2
Example: Calculate new VE for a patient with:
Current VE = 12 PaCO2 on ABG 60 Desired PaCO2 40
VE = 12 x 60 = 18 40
Calculation of VE for desired PaCO2
Desired FIO2 = (desired PaO2)(known FIO2) / known PaO2
Ex: Calculate the desired FIO2 for a patient with:
PaO2 on ABG 50 Current FIO2 30% Desired PaO2 60
60 x 0.30/ 50 = 36%
Desired FIO2
Cardiac Output (CO) = amt of blood ejected from the heart per minute
CO= Stroke Volume X heart ratenormal 4-8 L/min
QT = cardiac output
Cardiac Index (CI)= amount of blood pumps per minute based on body weight CI= CO/BSA, normal 2.4-4 L/min
Cardiac Output
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
CcO2: Content of pulm capillary blood oxygen at 100% FIO2, (Hbx1.34)1 + (PAO2x.003) used in shunt equation
Shunt Equation
Total Ve (given from vent) –(VT mech x f mech)/ f spont (total rate – mech rate set)
SPONT VOLUME: 8.1 -5.6/10=250 ml Measured when machine in SIMV mode,
represents what the patient is actually breathing on his/her own.
GOAL is 5-7 ml/kg, adjust PSV to achieve this EX: Pt on SIMV 8, total rate is 12, Set VT
500, Ve 8.0 8 –(4) /4 = 1000 ml
Spontaneous VT
VE total: (VT mech x f Mech) + (VT Spont x f spont)
Calculated when patient is in SIMV mode
Example: VT mech 700 ml, f mech 8, VT spont 250 ml, f
spont 10Total Ve –(VT mech x f mech)/ f spontSPONT VOLUME: 8.1 -5.6/10=250 ml
total Ve
VC: Vital capacity, 65-75 mL/kg, <10mL/kg indicates support, 10-15 mL/kgweaning
Maximum inhalation followed by a maximum exhalation
Measured by a Wright Respirometer
Vital Capacity
RSBI: Rapid shallow breathing index,
RR/VT, <105 weaning
must be calculated during spont breathing, press support reduces predictive value
RSBI
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)
MIP
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
CVP: central venous pressure, norm 2-6 mmHg, 2-6 weaning
Hemodynamics
Plateau pressure: The amount of pressure held in the lung during a brief inspiratory pause. This is used to calculate static compliance. The higher this number the worse the patients compliance as it represents distending pressure. Typically less than PIP, but more than MAP.
Plateau Pressure