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Ventilation How to Initiate Mechanical Ventilation

The ventilation strategy is determined by whether the patient has failure to ventilate or failure to oxygenate. The first problem is managed by increasing the patients minute ventilation, the second by recruiting collapsed lung units and controlling mean airway pressure.

I do not know of two doctors who agree on an initial ventilatory strategy. Every patient who is intubated is in need of a rest, and I always start my patients on controlled modes. If failure to ventilate or protect the airway was the problem, I use volume ventilation, to correct the respiratory acidosis, being careful not to damage the lung. If failure to oxygenate is the problem, I use pressure modes of ventilation, and carefully titrate the CPAP and the pressure control levels to set targets. While the choice of control mode is probably irrelevant (assist control (AC) or intermittent mandatory ventilation (IMV)), it is important that the patient’s spontaneous breaths are supported, which means adding pressure support to (S)IMV.

It is important to be aware that patients often wake up from the short neuromuscular blockers/hypnotic agents and start thrashing around and bucking the ventilator. You must sedate your patient until (at least) you are in full control of the cardiorespiratory situation. A bolus of fentanyl or morphine along with lorazepam, midazolam or propofol is usually required. For profoundly hypoxemic patients, the addition of a neuromuscular blocking agent (ensuring that the patient is deeply sedated) is often required.

The figure above describes an initial ventilatory strategy, designed around the cause of respiratory failure. Every intensivist must develop their own ideas about ventilation strategy, and this is one such. For patients with ventilatory failure, controlled minute ventilation is required, with reversal of the precipitating cause and rapid weaning to extubation. For hypoxemic respiratory failure, a more malignant course is to be expected, and I would advocate a pressure controlled strategy. Pressure controlled ventilation has several advantages over volume control, principally unlimited flow in assisted breaths, better distribution of gas, avoidance of overdistension of more compliant lung units, and the ability to tightly control mean airway pressure (by varying the inspiratory time).

How do I set PEEP for my patient?

Remember that the objective of using PEEP is to 1) restore functional residual capacity, & 2) to increase mean airway pressure and improve oxygenation by reducing ventilation-perfusion mismatch. The required PEEP depends on:

1) The extent of lung injury: determined by the alveolar-arterial oxygen gradient (or the PaO2/FiO2 ratio).

2) The patient's chest wall compliance. As stated previously (click here), additional weight to the chest reduces FRC, by reducing the tendency of the chest wall to spring outwards; the chest wall compliance. This is examplified in obese patients, and explains their chronic respiratory failure. Low chest wall compliance can be acquired in critical illness due to circumferential chest dressings, extensive edema, and, in particular, raised abdominal pressure. Patients who have had large volume fluid resuscitation develop extensive tissue edema, bowel distension, ascites and abdominal hypertension. The diaphragmatic excursion is limited, and dependent atelectasis results. Moreover, the heart increases in size and weight and compresses the left lower lobe.
The result of this is that patients with low chest wall compliance, such as surgical critically ill patients, require higher trans-alveolar pressure to achieve the same tidal volumes, which means higher PEEP to restore FRC. Elderly patients, particularly those with COPD, may have very high chest wall compliance, and require relatively low levels of pressure to generate target tidal volumes.

What is the extent of the
patient's lung injury?
What is the patient' chest
wall (Cw) compliance?

Target PaO2>60-80
What FiO2 is required to
achieve this?

NORMAL
 

LOW
-obesity
-edema
-abdominal hypertension

FiO2 Normal Cw Compliance
PEEP in cmH2O
Low Cw Compliance
PEEP in cmH2O
0.3 5 10
0.4 8 12
0.5 10 14
0.6* 12 16
0.7* 14 18
0.75* 16 20
0.8* 18 22
0.9* 20 22
1* 22 24

*Consider alternative methods of increasing mean airway pressure such as prolonging the duration of inspiration - by increasing inspiratory time in pressure control or adding an inspiratory pause in volume control.



http://www.ccmtutorials.com/rs/mv/strategy/index.htm