Positive Pressure Ventilation - 25

Question 1

Describe VCV

Answer 1

1. Tv pre-selected –> adjusts pressure to deliver volume

2. Rate of lung inflation CONSTANT

Question 2

Describe PCV

Answer 2

1. Inflation pressure pre-selected
2. DURATION of inflation is adjusted to deliver desired Tv
3. Rate HIGH @ onset –> decelerates

Question 3

Describe End-Inspiratory Pressure during VCV

Answer 3

Ppeak = P(resistance) + P(elastic from lungs & chest wall)

Question 4

Formula for P(peak)

Answer 4

Ppeak = P(resistance) + P(elastic from lungs & chest wall)

Question 5

Formula for P(resistance)

Answer 5

P(res) = R x V

R = resistance to flow in airways
V = insp flow rate

Question 6

Formula for P(elastic)

Answer 6

P(el) = elastance x V

P(plateau) - PEEP = P(elastic)

Question 7

Describe P(plateau)

Answer 7

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)

Question 8

What does the difference b/w Peak and Plateau pressure represent?

Answer 8

Pressure needed to overcome resistance to airflow at any given insp rate

Question 9

Describe relationship b/w airway pressure and alveolar pressure during PCV

Answer 9

P(airway end-insp) = P(alveolar peak)

There should be no airflow at end of insp (vs. VCV)

Question 10

Describe ZEEP

Answer 10

No airflow at end expiration

P(alveoli) = atmospheric (0)

Question 11

Describe applied PEEP

Answer 11

Valve in exp limb closes –> exhalation ceases at pre-selected PEEP level

Prevents collapse of distal airspaces + opens collapsed alveoli

Question 12

Describe occult/auto PEEP

Answer 12

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

Question 13

Typical values for mean airway pressure:

1. Normal lungs
2. Obstruction
3. Non-compliant lungs

Answer 13

1. 5-10 cmH2O
2. 10-20
3. 20-30

Question 14

Describe thoracic compliance during VCV

Answer 14

C = Tv / [Pplateau - PEEP]

Normal = 50-80 ml/cmH2O
Pul edema, ARDS = <25 ml/cmH2O

Question 15

Describe thoracic compliance during PCV

Answer 15

C = Exhaled Tv / [P(airway end-insp) - PEEP]

Question 16

Formula for inspiratory resistance

Answer 16

R = (Ppeak - Pplateau) / V (insp flow rate)

Pressure needed to overcome resistive forces of lungs and chest wall

Question 17

Formula for expiratory resistance

Answer 17

R = [Palveoli - PEEP] / PEFR

Question 18

Describe Volutrauma

Answer 18

Infiltrative lung diseases - PNA, ARDS

• inflation dist to normal regions of lung
• these high volumes given to lungs with dec functional volumes

Question 19

Describe lung protective ventilation

Answer 19

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

Question 20

Describe Atelectrauma

Answer 20

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

Question 21

Describe Biotrauma

Answer 21

PPV –> proinflammatory cytokine release –> SIRS (even during low lung volume ventilation)

Question 22

Describe Barotrauma

Answer 22

PPV –> air leaks from any rupture in the airways –> pneumothorax, pneumomediastinum, subcutaneous emphysema, pneumoperitoneum

Question 23

How does PPV effect preload?

Answer 23

Reduces Preload

1. Dec VR: + intrathoracic pressure dec pressure gradient for venous return (VR) into thorax
2. Dec ventricular filling: + pressure outside heart dec transmural pressure during diastole
3. Inc pulm vac resistance: dec vent filling

Question 24

How does PPV effect afterload?

Answer 24

Dec LV afterload

acts like a hand squeezing the LV

Question 25

How does PPV effect cardiac output?

Answer 25

Balance b/w dec vent preload & dec afterload –> determined by:

1. Cardiac function
2. Low intravascular volumes
3. Intrathoracic pressure

Question 26

How does cardiac function (normal vs. CHF) effect CO?

Answer 26

1. Cardiac function
   - Normally dec preload > dec afterload but preload minimally effected d/t inc intrabdominal pressure = dec, same or Inc during PPV
   - Dec afterload&raquo_space;> dec preload = INC CO

Question 27

How does low intravascular volume effect CO?

Answer 27

Preload red effect of PPV predominates in both normal and CHF –> CO DEC

Especially when intrathoracic pressures are high