Chapter 10: Mechanical Ventilation - Part I of III

from Pulmonary Physiology in Clinical Practice, copyright 1999 by
Lawrence Martin, M.D.


Mechanical Ventilation


OUTLINE OF CHAPTER

Headings in PART I


Introduction (READ THIS FIRST!)
Intubation and mechanical ventilation
Indications for mechanical ventilation
Normal breathing vs. ventilator breathing
Choosing the settings

Headings in PART II


Modes of mechanical ventilation
Controlled ventilation and Assist-Control Ventilation
Intermittent mandatory ventilation
Ventilator settings
Ventilator compliance
Pressure support ventilation
Positive end-expiratory pressure

Headings in PART III


Non-invasive mechanical ventilation>
Choosing the FIO2
Complications of mechanical ventilation
Weaning or Liberating the patient from the Ventilator

Tables and Boxed Information are this color

Clinical Problems are this color

Line figures are surrounded by this color

Terminology Alerts are surrounded by this color


INTRODUCTION

There is probably no more confusing or intimidating subject to the student or health care professional than mechanical ventilation. This situation arises for 3 reasons:

Inadequate teaching is not confined to medical or nursing schools, but extends (sadly) to mainstream textbooks, where chapters on mechanical ventilation seem written more for the author's colleagues than for novices trying to learn the subject. In fairness, it is a difficult subject to teach from a prose textbook, as opposed to a workbook or an interactive tool, like this web site. This is especially so in a general text book that has to cover lots of topics. Yet the truth remains: without some reasonable understanding of the subject to begin with, sections on mechanical ventilation in highly regarded medical texts are usually of little help.

This chapter, then, is designed from the bottom up, to teach, to demystify, to make you feel more comfortable with mechanical ventilators and patients connected to them. It includes the usual figures, tables and text, but also many problems that you should attempt to answer before proceeding further.

For the chapter to be useful to you, two things are required: first, that you have at least some experience caring for patients receiving mechanical ventilation, even if only one or two patients, and even if only as a peripheral caregiver (e.g., student). You should at least know what the machines look and sound like in real life, and what an "intubated patient" looks like. And second, that you have a basic knowledge of gas exchange and how to interpret arterial blood gases, subjects discussed at length in earlier chapters of this book, and in my book on blood gases: All You Really Need to Know to Interpret Arterial Blood Gases

With those two requirements satisfied, I promise that a thorough reading of this chapter (really, studying the chapter), and working on the problems, will provide an adquate basis for understanding mechanical ventilation.


INTUBATION AND MECHANICAL VENTILATION

When the patient's respiratory system can no longer provide adequate oxygenation and/or ventilation, mechanical ventilation with supplemental oxygen is available. To receive mechanical ventilation sufficient for life support, the patient must first be intubated; intubation involves the insertion of a large-bore endotracheal tube into the trachea (Fig. 10-1).

Figure 10-1. Endotracheal tube in the trachea. A, Balloon deflated. B, Balloon inflated. Air that is pushed through the tube enters the lungs since it cannot escape around the tube when the balloon is inflated.


Mechanical ventilation can also be given via a tight fitting face mask, a procedure called non-invasive (i.e., no endotracheal tube) positive pressure ventilation, but this is used only to 'tide over' patients until they improve, and is not sufficient to give full live support. Non-invasive positive pressure ventilation will be discussed later in this chapter.

The route of insertion can be through the nose or mouth or directly into the trachea through a surgical incision (tracheostomy). One end of the endotracheal tube is surrounded by an inflatable rubber balloon; when filled with air, the balloon creates an airtight seal in the trachea. The other end of the tube is connected to two flexible, wide-bore hoses, through a Y connector; one hose is for inspiration and the other, for expiration. The expiratory hose can be connected to a spirometer or to a bellows to measure expiratory volume (Fig. 10-2).

Fig. 10-2. Ventilator connected to a rubber bag, simulating patient's lungs. One hose is part of the inspiratory circuit that delivers air to the patient; the other is part of the expiratory circuit. This ventilator is an example of a conventional volume ventilator, in this case a Puritan-Bennett model 7200.

TERMINOLOGY ALERT. 'Volume Ventilator' is the term used because you set a volume and the machine delivers that volume, at whatever pressure necessary (up to a limit). Old style ventilators were based on setting a pressure limit, and the patient received whatever volume could be delivered up to that pressure. Now, modern ventilators such as the one pictured can deliver either mode of ventilation -- volume-determines pressure or pressure-determines volume. As a practical matter, the latter is called pressure controlled ventilation and is used only for the sickest patients with severe pneumonia and other low-compliance states.


The endotracheal tube, with its balloon inflated, allows the machine (mechanical ventilator) to push air into the patient's lungs under positive (above atmospheric) pressure. Since air cannot escape around the tube, it enters the lungs and takes part in gas exchange. The machine then allows air to be passively exhaled through the endotracheal tube; after exhalation the patient is ready to receive the next ventilatory cycle. In practice, a variety of machines and types of endotracheal tubes is used, but the principle for their use is the same. They are all based on the following principles:

CRITERIA FOR INTUBATION &/or AND MECHANICAL VENTILATION

Intubation only
(oxygenation and ventilation are adequate but the patient needs a secure airway)

  • Threatened airways obstruction;e.g., deep coma
  • Definite airways obstruction;e.g., laryngeal edema, tracheal tumor

    Either indication must be based solely on the clinical examination, although arterial blood gas measurement is often helpful to assure that mechanical ventilation is not necessary.

    Intubation and mechanical ventilation
    (oxygenation and/or ventilation are not adequate)

  • Apnea

  • Impaired alveolar ventilation (as assessed by PaCO2) when accompanied by one or more of the following:
    a. Depressed mental status
    b. Increasing fatigue
    c. Reduced PaO2 that cannot otherwise be corrected
    d. Severely deranged pH that cannot otherwise be corrected
    e. Compromise of upper airways (e.g., by secretions)
  • Low PaO2 (e.g., less than 60 mm Hg): a. that cannot be improved with an FIO2 less than 0.50, and b. that is causing symptoms or seriously impairing bodily function

Probably the most common reason for 'intubation only' (without mechanical ventilation) is deep coma -- the patient is totally unresponsive, except perhaps to deep pain, but has adequate oxygenation and ventilation. An endotracheal tube is placed in such a patient to assure a tent airway since airways occlusion is always possible from the patient's tongue receding into the oropharynx or from inspissated secretions in the upper airways. An endotracheal tube is not placed the patient (as is sometimes thought) to prevent aspiration. There is no evidence that using a soft-cuffed tube prevents aspiration; indeed, clinical experience suggests that the act of intubation itself can lead to aspiration pneumonia. Therefore the assessment of deep coma should be based on the entire physical examination. Absence of the gag reflex, per se, is not a reliable guide for the need for intubation.

The other common indication for intubation only is definite upper airways obstruction. Patients with laryngeal edema, tracheal tumor, macroglossia, and other upper airway problems may initially be seen with stridor. When such obstruction is assessed as life-threatening (e.g., based on arterial blood gas analysis, or presence of cyanosis, extreme degree of muscular effort to breathe, or shock) a secure airway is mandatory. Of course it may be difficult to pass an endotracheal tube, in which case an emergency tracheostomy or cricothyroidotomy may be necessary.


INDICATIONS FOR MECHANICAL VENTILATION

Earlier, mechanical ventilation was discussed in relation to alveolar ventilation (Chapter 4) and to

oxygenation (Chapter 6). Impairment of alveolar ventilation (assessed by PaCO2) and/or oxygenation (assessed by PaO2) are the only physiologic reasons for instituting mechanical ventilation. Although mechanical ventilation can lead to better cardiac, renal, or cerebral function, the basic goal for its use must be to improve the PaO2 and/or the PaCO2 or to reduce the FIO2 or the amount of work needed to maintain blood gas values at an acceptable level. Criteria for instituting mechanical ventilation, listed in the box above, can be applied in all cases. Some examples are in the box below.

  • Patients undergoing cardiopulmonary resuscitation are intubated and mechanically ventilated to reverse the inevitable hypercapnia and hypoxemia.
  • During general anesthesia patients are routinely ventilated to prevent the hypercapnia and hypoxemia that would otherwise result.
  • Patients are sometimes intubated even though their PaCO2 is in the normal range. This may be justified if the work of breathing is at the patient's limit and if decompensation is feared. In such cases the goal of mechanical ventilation is to relieve the patient of some or all of the work of breathing and to maintain PaCO2 at an acceptable level.
  • Patients are occasionally intubated to improve oxygenation even though the PaO2 may be above 50 mm Hg. This is justified if the FIO2 is dangerously high and if the patient is in respiratory distress and not improving. Again the goal is to maintain an adequate PaO2.
  • Intubation may be warranted if excessive, uncontrolled hyperventilation results in a dangerously high pH. For example, a patient with central neurogenic hyperventilation (e.g., from a brainstem tumor) may have a PaCO2 of 6 mm Hg, a pH of 7.67, and a PaO2 of 120 mm Hg while breathing room air. Mechanical ventilation with the addition of drug-induced muscle paralysis may be the only way to control the severe alkalemia.

The last example is an unusual situation, but it does reinforce the main point -- mechanical ventilation is indicated only when there is a need to improve or control the PaO2 and/or PaCO2. Obviously, good clinical judgment is necessary when deciding about intubation, especially when the PaCO2 is in the normal range or the PaO2 is above 50 mm Hg.

Clinical problem 1

Based on criteria presented in the box, which of the following cases would warrant immediate intubation and mechanical ventilation?

a. A 50-year-old man is comatose from drug overdose. PaCO2 is 51 mm Hg, PaO2 is 76 mm Hg, and pH is 7.31 while breathing room air.

b. A 29-year-old man is alert but in respiratory distress; he is breathing 42 times/min. PaCO2 is 38 mm Hg. pH is 7.42, and PaO2 is 47 mm Hg while breathing 60% oxygen through a face mask.

c. A 61-year-old woman who has severe emphysema is alert but is in moderate respiratory distress; her respiratory rate is 24/min. PaO2 is 75 mm Hg while breathing nasal oxygen at 2 L/min, PaCO2 is 59 mm Hg, and the pH is 7.37. Her chest x-ray is clear.

d. A 29-year-old woman is suffering from diabetic ketoacidosis. Her pH is 7.10, PaCO2 is 26 mm Hg and PaO2 is 110 mm Hg while breathing room air.

e. A 31-year-old drug addict responds briefly to the administration of Narcan (a narcotic antagonist) by opening her eyes and crying out and then lapses back into a state of semi-stupor. PaCO2 is 31 mm Hg. pH is 7.38, and PaO2 is 89 mm Hg while breathing nasal oxygen at 3 L/min.


NORMAL BREATHING VS. VENTILATOR BREATHING

Understanding ventilators is not possible without first understanding the changes in airway pressure during normal breathing. Fig. 10-3, top, shows the changes in airway pressure in the mouth during spontaneous, quiet breathing. Airway pressure during inspiration is negative, allowing air to enter the lungs, and is positive during expiration. Also, there is normally a brief expiratory pause, when airway pressure remains at atmospheric pressure.

Figure 10-3. Top. Normal, spontaneous breathing. Bottom. Pressure curve showing upper airway pressures (measured at mouth) during mechanical ventilation.


A basic difference between normal breathing and mechanical ventilation can be seen in the changes in airway pressure with each breath (Fig. 10-3, top). As used today, mechanical ventilation is always positive pressure, i.e., air goes in because airway pressure is above atmospheric. Since it is also delivered intermittently, i.e. with each breath, it is therefore called intermittent positive pressure ventilation or IPPV.

TERMINOLOGY ALERT. 'IPPV' is not to be confused with IPPB, which stands for intermittent positive pressure breathing. IPPB is a once-popular method for delivering aerosol medication to the patient under positive pressure, usually up to about 15 cm H20. The patient usually held a mouthpiece in place in his mouth, while the machine pushed in air with the medication under postive pressure (the method is still used, but not as widely as years ago). IPPB was never designed as a life support technique, although under certain cirumstances it could work that way, if the pressures are high enough. In marked contrast, IPPV is the generic term for all life-support mechanical ventilation that delivers breaths under postive airway pressure. Invariably the pressures with IPPV are higher than are genearated with IPPB, and there are a host of other differences as well. They are really two different concepts with similar initials.

Thus, IPPV is simply the general or generic term for all forms of mechanical ventilation used in hospitals today. During the 1950s polio epidemic, negative pressure ventilators or "iron lungs" were widely used, but no longer. These iron boxes created a negative atmospheric pressure around the patient, sucking out the thorax and thus creating a negative pressure inside the airways, allowing air to enter. They are no longer used. Today all mechanical ventilation is "IPPV."

It is important to remember that IPPV fundamentally alters the normal airway pressures. This fact accounts for most of its benefits and complications.


Choosing the settings

A continual source of confusion to students, interns, housestaff is choosing the appropriate ventilator mode and settings. In many cases, the settings are chosen by a respiratory therapist and merely seconded by the physician. At a minimum, the physician should be able to comfortably order the following settings:

1. Mode of ventilation: CMV, IMV, Spontaneous are the 3 most common

2. Respiratory Rate: Varies widely, usually 10-14 to start

3. Tidal Volume: Controversial; a safe level is about 7-10 cc/kg lean body weight

4. FIO2: Varies from .21 to 1.00, depending on clinical circumstances

Additional settings that are usually determined by the respiratory therapist include:

There are other ventilator settings and alarm limits, but the 8 listed above are, in my opinion, the main ones that non-respiratory therapists need to be aware of. Particularly, an understanding of #1-4 are crucial in managing any patient on mechanical ventilation.


Continue with Part II of Chapter 10: Mechanical Ventilation

From Pulmonary Physiology in Clinical Practice, copyright 1999 by
Lawrence Martin, M.D.