Icu Book Ch. 24: Principles Of Mechanical Ventilation

Front 1

How were the first mechanical ventilators designed to work?

Back 1

-through pressure-cycled ventilation, where the lungs were inflated until a preset pressure was reached

Front 2

Why did pressure-cycled ventilation fall out of favor?

Back 2

Because the inflation volume varied with changed in the mechanical properties of the lungs

Front 3

So what has become the standard method of positive-pressure ventilation, and why?

Back 3

-volume-cycled ventilation: which inflates the lungs to a predetermined volume, and delivers a constant volume despite any changes in lung compliance

Front 4

Volume-cycled ventilation waveforms

Back 4

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-the lungs are inflated at a constant flow rate, producing a steady increase in lung volume
-the tracings are uniquely different for the proximal airways (Pprox) vs. the alveoli (Palv)

Front 5

Pprox waveform

Back 5

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-proximal airway pressure shows an abrupt initial rise, then a gradual rise the rest of lung inflation

Front 6

What is the initial abrupt rise a reflection of?

Back 6

Flow resistance in the airways

Front 7

Palv waveform

Back 7

-the pressure in the alveoli shows only a gradual rise throughout inflation

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Front 8

Changes in the tracings when there is an increase in airway resistence?

Back 8

-it will magnify the initial rise in proximal airway pressure, but the alveolar pressure will remain unchanged, meaning higher inflation pressures are needed to deliver the set volume, but the alveoli are not exposed to higher pressures

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Front 9

How does this compare to if the compliance of the lungs is reduced?

Back 9

-there's an increase in both the proximal airway and alveolar pressure, thus the higher pressures needed to deliver the set volume is transmitted to the alveoli

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Front 10

Why is this significant?

Back 10

The increase in alveolar pressure in noncompliant lungs can lead to pressure-induced lung injury

Front 11

Influence of PPV on cardiac performance

Back 11

-is complex, but involves changes in preload and afterload, and is explained by exploring the changes to transmural pressure created by PPV

Front 12

What is the significance of transmural pressure?

Back 12

It's the pressure that determines the ventricular filling (preload), and the resistance to ventricular emptying (afterload)

Front 13

influence of lung mechanics on transmural pressure transmission

Back 13

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-the left shows what happens in a normal lung when inflated to 700 ml, the alveolar pressure is completely transmitted into the pulmonary capillaries, and there is no change in transmural pressure across the capillaries
-on the right is inflation in less compliant lungs, where the increased pressure is not transmitted fully to the capillaries, and results in compression of the vessels

Front 14

what is the significance of this?

Back 14

-in conditions which cause decreased lung compliance, such as pulmonary edema or pneumonia, PPV compresses the heart and intrathoracic blood vessels (which is sometimes detrimental, sometimes beneficial)

Front 15

ways PPV decreases preload

Back 15

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does so in multiple ways:
-decreases the pressure gradient for venous inflow into the thorax (decreases the difference between pressure in and out of the thorax, which is driving venous return into the thorax)
-pressure on the outer surface of the heart reduces cardiac distensibility, reducing filling during diastole
-compression of pulmonary vessels increases pulmonary vascular resistance, which can impede RV output, which causes dilation of the right ventricle and pushes the intraventricular septum toward the left ventricle, reducing LV size and filling (ventricular interdependence)

Front 16

positive intrathoracic pressure and afterload

Back 16

-compression facilitates emptying during systole, like a hand squeezing the ventricle during ejection
-afterload, or the impedence to ventricular emptying, is a function of the peak systolic transmural wall pressure, and incomplete transmission of positive intrathoracic pressure into the ventricular chambers will decreases transmural pressure across the wall during systole, which decreases afterload

Front 17

overall effect of PPV on cardiac output

Back 17

-since it reduces vetricular filling during diastole, but also reduces afterload during systole, the effect on CO depends on which predominates

Front 18

what factor determines which predominates?

Back 18

-volume status

Front 19

what happens in euvolemia?

Back 19

-the effect on afterload predominates, and PPV increases CO, which causes an increase in BP during inflation, known as reverse pulsus paradoxus

(this may explain how chest compression increase CO)

Front 20

PPV effect on CO during hypovolemia

Back 20

-the predominant effect of positive intrathoracic pressure is to reduce preload and decrease CO, thus avoiding hypovolemia during PPV is important

Front 21

some rules about the indications for mechanical ventilation

Back 21

1. the indication for mechanical ventilation is thinking of it: we tend to delay it as long as possible hoping it wont be necessary, but this only creates the potential need to intubate emergently rather than in an elective controlled setting, putting the patient at risk

2. intubation is not an act of weakness, but rather shows strength of conviction in making a difficult decision

3. initiating mechanical ventilation is not a kiss of death, and the fallacy that once on a vent always on a vent should never influence the decision to intubate

Front 22

old model of mechanical ventilation

Back 22

-they used to recommend large tidal volumes to prevent alveolar collapse giving 10-15 ml/kg with PPV even though during spontaneous respiration ppl only take 5-7 ml/kg, and the volumes are even greater if using a mechanical sigh

Front 23

new model of mechanical ventilation

Back 23

-the conventional large volumes can damage the lungs, and promote injury to distant organs through release of inflammatory cytokines
-ventilator induced lung injury is a new concept changing the way mechanical ventilation is done

Front 24

ventilator-induced lung injury

Back 24

-in the lung diseases that most often require mechanical ventilation, like pneumonia or ARDS, the pathology is not evenly distributed to the lungs (even in ARDS where it looks like it is on CXR), so there are areas of diseased lung and areas of normal lung
-inflation volumes delivered by the ventilator are distributed preferentially to the normal lung, which gets hyperinflated, causing stress fractures at the alveolar-capillary interface due either to barotrauma or volutrauma

Front 25

3 adverse consequences of alveolar rupture

Back 25

1. accumulation of alveolar gas in either the lung parenchyma (pulmonary interstitial emphysema), mediastinum (pneumomediastinum), or pleural cavity (pneumothorax)
2. an inflammatory lung condition similar to ARDS
3. multiorgan injury from the release of inflammatory mediators into the bloodstream- called biotrauma

Front 26

lung-protective ventilation

Back 26

-a large study of ARDS patients showed tidal volumes of 6 ml/kg of predicted body weight to have reduced mortality compared to 12 ml/kg, so this is now recommended for all pts with acute respiratory failure