Positive Pressure Ventilation - 25 Flashcards
Describe VCV
- Tv pre-selected –> adjusts pressure to deliver volume
2. Rate of lung inflation CONSTANT
Describe PCV
- Inflation pressure pre-selected
- DURATION of inflation is adjusted to deliver desired Tv
- Rate HIGH @ onset –> decelerates
Describe End-Inspiratory Pressure during VCV
Ppeak = P(resistance) + P(elastic from lungs & chest wall)
Formula for P(peak)
Ppeak = P(resistance) + P(elastic from lungs & chest wall)
Formula for P(resistance)
P(res) = R x V
R = resistance to flow in airways V = insp flow rate
Formula for P(elastic)
P(el) = elastance x V
P(plateau) - PEEP = P(elastic)
Describe P(plateau)
Expiratory circuit occluded at vey end of lung inflation (inflation hold) –> pressure dec initially then is constant until pt is allowed to exhale (usually 1s)
P(plateau) = P(peak alveolar)
What does the difference b/w Peak and Plateau pressure represent?
Pressure needed to overcome resistance to airflow at any given insp rate
Describe relationship b/w airway pressure and alveolar pressure during PCV
P(airway end-insp) = P(alveolar peak)
There should be no airflow at end of insp (vs. VCV)
Describe ZEEP
No airflow at end expiration
P(alveoli) = atmospheric (0)
Describe applied PEEP
Valve in exp limb closes –> exhalation ceases at pre-selected PEEP level
Prevents collapse of distal airspaces + opens collapsed alveoli
Describe occult/auto PEEP
Airflow at end expiration but lungs do not empty completely –> hyperinflation (COPD, asthma)
Alveolar pressure + even though proximal airways are 0
Occlude expiratory circuit to equilibrate alveoli and proximal airway pressures
Typical values for mean airway pressure:
- Normal lungs
- Obstruction
- Non-compliant lungs
- 5-10 cmH2O
- 10-20
- 20-30
Describe thoracic compliance during VCV
C = Tv / [Pplateau - PEEP]
Normal = 50-80 ml/cmH2O
Pul edema, ARDS = <25 ml/cmH2O
Describe thoracic compliance during PCV
C = Exhaled Tv / [P(airway end-insp) - PEEP]
Formula for inspiratory resistance
R = (Ppeak - Pplateau) / V (insp flow rate)
Pressure needed to overcome resistive forces of lungs and chest wall
Formula for expiratory resistance
R = [Palveoli - PEEP] / PEFR
Describe Volutrauma
Infiltrative lung diseases - PNA, ARDS
- inflation dist to normal regions of lung
- these high volumes given to lungs with dec functional volumes
Describe lung protective ventilation
Tidal volume goal
- Low volume (6ml/kg) predicted body weight
- VCV w/ initial Tv 8ml/kg
- RR match baseline (30 cmH2O & Tv at 6ml/kg –> dec Tv in 1ml/kg increments until P falls to inc RR until pH>7.30, PaCO2 35
- pH RR to 35 –> inc Tv 1ml/kg until >7.15
- pH >7.45 –> dec RR
Describe Atelectrauma
Small airways collapse at end exp - ESP in pulmonary edema, ARDS –> repetitive opening/closing during PPV
Mitigated with PEEP which keeps the small airways open during expiration
Describe Biotrauma
PPV –> proinflammatory cytokine release –> SIRS (even during low lung volume ventilation)
Describe Barotrauma
PPV –> air leaks from any rupture in the airways –> pneumothorax, pneumomediastinum, subcutaneous emphysema, pneumoperitoneum
How does PPV effect preload?
Reduces Preload
- Dec VR: + intrathoracic pressure dec pressure gradient for venous return (VR) into thorax
- Dec ventricular filling: + pressure outside heart dec transmural pressure during diastole
- Inc pulm vac resistance: dec vent filling
How does PPV effect afterload?
Dec LV afterload
acts like a hand squeezing the LV
How does PPV effect cardiac output?
Balance b/w dec vent preload & dec afterload –> determined by:
- Cardiac function
- Low intravascular volumes
- Intrathoracic pressure
How does cardiac function (normal vs. CHF) effect CO?
- Cardiac function
- Normally dec preload > dec afterload but preload minimally effected d/t inc intrabdominal pressure = dec, same or Inc during PPV
- Dec afterload»_space;> dec preload = INC CO
How does low intravascular volume effect CO?
Preload red effect of PPV predominates in both normal and CHF –> CO DEC
Especially when intrathoracic pressures are high
