ARDS. Ventilator Strategies

Question 1

A 45 years old alcoholic has massive aspiration and develops severe ARDS. He is admitted in ICU. Vent settings are as follows: A/C-VC, TV 720ml, (12ml/kg, IBW), f 12, +8cmH2O, FiO2 40%. Ppressure 36 cmH2O, ABG: 7.38/44/68mmHg. An ARDS protocol sets for TV 6ml/Kg IBW, while maintaining an acceptable PCO2 and pH. (No increase in respirators rate and maintaining a low minute ventilation).
According to recent clinical trials results, these changes for 48 to 72 hours are likely to lead to which of the following:
A)Improving lung compliance and oxygenation

Answer 1

Exercising ARDS protocols such as low tv and decreasing Ppress is of common use in ICU’s strategies to prevent excessive lung stretches to reduce or prevent acute lung injuries, termed as ventilator induced lung injuries (VILI) But these strategies have also adverse effects on lung physiology such as a decrease tidal volume without an increase on resp rate will reduce the minute ventilation and consequently decreased CO2 clearance leading to respiratory acidosis. Thus, the permitted “permissive hypercapnia” is seen as acceptable trade-off instead of a pending VILI. Furthermore, a minimal TV, as well as a minimal resp rate represents no lung recruitment which in turns leads to atelectasis and worsening lung compliance and ventilation -perfusion mismatch- and finally Hypoxemia. This strategy of permitting “permissive hypercapnia” (- low minute volume-),(over big minute volumes) has proven to be of better survival rates when preventing over stretches lung injuries. However, when using low mv
strategy, the first two days in mechanical ventilation are the worst with acute respiratory acidosis and hypoxemia, but in most cases, these differences disappear after the third day translating in abetter survival rate. In conclusion, inducing “permissive atelectasis/hypoxemia” may be a reasonable trade-off if overstretch induced lung injury can be prevented.

Question 2

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Answer 2

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Question 3

what determines clearance of carbon dioxide?

Answer 3

it is determined by alveolar ventilation and physiologic dead space.

Question 4

how do you measure physiological dead space?

Answer 4

it is the addition of anatomical dead space plus alveolar dead space.

Question 5

how do you measure anatomical dead space and what is the normal measure?

Answer 5

the difference between alveolar ventilation (which is difficult to measure) and minute ventilation (easy to calculate) determines the anatomical dead space which is approximately 150 ml in an adult male