med 610 clinical respiratory diseases & critcare med

Mechanical Ventilation

Case 2 Answers

A 45 year-old. 6-foot tall man presented to the emergency room with a two day history of fever and cough productive of brown sputum. He was hemodynamically stable at the time with a blood pressure of 130/87. His chest x-ray showed a right middle lobe infiltrate and his room air ABG showed: pH 7.32, PCO2 32, PO2 78, HCO3- 18. He was started on antibiotics and admitted to the floor. Four hours later, the nurse calls because she is concerned that he is doing worse. On your arrival in the room, his blood pressure is 85/60, his pulse is120 and his oxygen saturation, which had been 97% on 2L oxygen by nasal cannula is now 78% on a non-rebreather mask. The patient is obviously laboring to breathe with use of accessory muscles and is less responsive than he was on admission. He is diaphoretic and cannot talk in full sentences. On lung exam, he has crackles throughout the bilateral lung fields. You obtain a chest x-ray which shows increasing bilateral, diffuse lung opacities. An ABG is done while he is on the non-rebreather mask and shows: pH 7.17, PCO2 45, PO2 58, HCO3- 14.

What should you do now? Is there a role for CPAP or bi-level positive airway pressure in managing his hypoxemia?

Although there are certain clinical situations in which CPAP or bi-level positive airway pressure are indicated as means of respiratory support, hypoxemic respiratory failure for reasons other than cardiogenic pulmonary edema is not one of them. The data indicates that use of non-invasive ventilation in such situations is associated with worse patient outcomes. This patient is struggling to breathe; his mental status is declining and he is becoming hemodynamically unstable. Finally, he has worsening oxygenation and ventilation as evidenced by a falling PaO2, a rising PaCO2 and a worsening pH. The best option for him is intubation and mechanical ventilation.

A decision is made to intubate the patient and initiate mechanical ventilation for worsening respiratory failure. The intubation proceeds without difficulty. The tube position is confirmed and the anesthesiologist leaves the room. The respiratory therapist has secured the breathing tube. She turns to you and asks what settings you would like to use for the ventilator?

What information do you need to provide to the respiratory therapist?

In most cases, you need to provide the respiratory therapist five pieces of information: the mode of mechanical ventilation, the tidal volume, the respiratory rate, the inspired oxygen concentration (FIO2) and the level of positive end-expiratory pressure (PEEP). In rarer cases you will stipulate a peak inflation pressure, rather than a tidal volume. This is described further in the next question.

The respiratory therapist suggests you use the volume targeted assist Control (AC) mode of mechanical ventilation. How does this work? How does it differ from Synchronized Intermittent Mandatory Ventilation (SIMV) or Pressure Control (PC)?

Which mode is better for your patient?

In Assist Control (AC) ventilation, the clinician determines the tidal volume and the respiratory rate for the patient. The machine guarantees that the patient will receive the set number of breaths at the desired tidal volume each minute. Patients can also initiate their own breaths (i.e. take breaths above the rate set on the ventilator by the clinician); patient-initiated breaths are delivered at the full tidal volume that has been set on the machine.

In SIMV, the clinician also sets the rate and tidal volume and the machine guarantees the patient will receive the set number of breaths at the desired tidal volume each minute. Patients can also initiate their own breaths, but on the extra breaths in SIMV, the patient only gets as much tidal volume as they are capable of taking in on their own; the ventilator does not guarantee a set tidal volume for these breaths. Weak patients may draw small tidal volumes on these extra breaths while strong patients may take in larger tidal volumes.

Pressure control ventilation is quite different than either volume targeted AC or SIMV. In this mode, the clinician sets the peak inflation pressure and respiratory rate but does not specify the tidal volume. Instead, the tidal volume received by the patient varies based on the compliance of the respiratory system and the level of airway resistance. If a patient, for example, has a very compliant respiratory system (emphysema), they will receive a large tidal volume for a set pressure whereas a patient with low respiratory system compliance (ARDS) will only receive a small tidal volume.

At present, there is no evidence that a particular mode of mechanical ventilation is associated with a mortality benefit compared to the other modes. Evidence exists for improvements in short-term physiologic variables with one mode compared to another (which is largely a function of how the comparison is set up), but there are no solid data to support a preference for one mode over another. This is a subject of intense debate and the choice of mode tends to be very physician-, respiratory therapist-, and institution-dependent. In the University of Washington system, the pulmonary and critical care physicians tend to ventilate most of our patients using volume-targeted Assist Control mode and to use pressure control ventilation only in situations in which the patient has a very poor response to standard measures.

Suppose you put the patient on a volume-targeted Assist Control mode of mechanical ventilation. How do you choose the tidal volume?

The tidal volume is chosen based on the patient’s weight. It is critical, however, to make sure you use the correct weight in the calculations. The size of the lungs is largely a function of a person’s height. Therefore, rather than using the patient’s actual weight to determine the tidal volume, you should, instead, use the patient’s ideal body weight, the value of which is derived from the patient’s height. To calculate the ideal body weight in kilograms, you can use the following formulas:

Men:          [(height in inches – 60) X 2.2] + 50
Women:     [(height in inches – 60) X 2.2] + 45

Failure to use the correct weight can lead to disastrous consequences for the patient. If you were to put a 150 kg person on a tidal volume of 10 ml/kg of their actual body weight, you would end up delivering tidal volumes of 1.5L and, as a result, would significantly increase the risk of barotrauma and ventilator-induced lung injury.

Once you know the ideal body weight, you can choose the tidal volume for the patient. The majority of patients are placed on a tidal volume corresponding to 8-10 ml/kg of their ideal body weight. In order to decrease the risk of air-trapping and barotrauma, patients intubated for COPD or asthma exacerbations are often placed on 6-8 ml/kg of their ideal body weight. Patients who develop ARDS are placed on 4-6 ml/kg of their ideal body weight but are not typically started at these low levels. Instead, they are started on 8-10 ml/kg and the tidal volume is gradually decreased over a period of time. Finally, many ventilator-dependent patients with spinal cord injuries are maintained on higher tidal volumes ranging from 12 ml/kg to as high as 20 ml/kg of their ideal body weight. Proponents of this practice argue that it improves patient comfort and prevents mucous plugging and atelectasis which predispose to pulmonary complications in this patient population. The data supporting this practice is actually quite limited and there is considerable debate as to the safety and efficacy of such high tidal volumes in these patients.

The patient in this case is 6-foot tall (72 inches). Using the formula above, his ideal body weight is about 76kg. Therefore, you would place him on a tidal volume between 600 and 750 ml, which corresponds to between 8 and 10 ml/kg.

What respiratory rate should you choose for the patient?

Contrary to the popular, but erroneous, practice of choosing the ubiquitous respiratory rate of “12” because “that is what I’ve always seen other people do,” the respiratory rate should be selected based on an assessment of the patient’s minute ventilation requirements. For example, a patient with a normal bicarbonate of 24 who was intubated for a procedure may only need 6-8 liters/minute of ventilation whereas a patient in severe sepsis with a bicarbonate of 10 may require upwards of 20-25 liters/minute of ventilation to maintain an adequate acid-base status. Once you derive an estimate of the minute ventilation (VE) needs of the patient, you can use your previously determined tidal volume and simple division to calculate an appropriate respiratory rate (RR = VE/tidal volume)

This practice is particularly important in the period immediately following intubation. Most patients receive paralytic agents for intubation and, as a result, have no ability to mount respiratory efforts for 15 to 60 minutes following the procedure. They are dependent on you to choose the correct rate and give them an adequate amount of minute ventilation. Failure to do so will lead to increasing respiratory acidosis and worsening pH. As the paralytic agent wears off and sedative needs decrease, the patients will often set the respiratory rate on their own in an effort to match their minute ventilation needs.

What should the FIO2 and PEEP be set at?

In the majority of cases patients are initially placed on an FIO2 of 1.0 and a PEEP of 5 cm H2O.If the initial ABG drawn 30 to 60 minutes after intubation reveals an adequate PaO2 (above 65 mm Hg) the FIO2 can be turned down to a lower level. Further adjustments are made in the FIO2 based on the patient’s oxygen saturation and blood gases are not necessary for every change in this parameter. It is not uncommon to hear comments about maintaining patients on too high an inspired oxygen concentration for too long a period of time, as there is concern about provoking oxygen toxicity in the lungs. This concern is largely based on the results of laboratory studies in animal and normal human volunteers and concrete evidence of its occurrence in critically ill ICU patients is lacking. If a patient requires a high FIO2 in order to maintain adequate oxygenation then they should receive those high levels for as long as necessary.

The PEEP is typically not decreased below 5 cm H2O but can be increased as necessary (up to 15 to 20 cm 2H0) to support oxygenation in patients with severe hypoxemia (discussed further below). It tends to be a more effective tool in diffuse, rather than focal, lung processes such as ARDS.

Case Index>>

Top of Page