How to choose the setting?

Professor Barry Baker
MB BS (Qld) DPhil (Oxon) FANZCA FRCA FJFICM DHMSA
Nuffield Professor of Anaesthetics
University of Sydney and Royal Prince Alfred Hospital,
Sydney, Australia.

The ventilation goals for controlled ventilation during anaesthesia are to:

• sustain and/or improve gas exchange
• prevent or reverse atelectasis,

and where the patient is not completely paralysed to:

• enhance coordination between ventilatory assistance and any respiratory efforts which the patient may make.

When the patient is subjected to long term controlled ventilation as often occurs in intensive care there are further goals to:

• prevent muscle deconditioning
• foster lung healing.

Initially there are a number of very basic considerations:

• whether or not there is a need for neuromuscular paralysis
• if neuromuscular paralysis is necessary whether total paralysis is necessary or only partial paralysis
• levels of PCO2 and PO2 required
• one lung or two lung ventilation
• whether the patient has normal or abnormal lungs
• whether respiratory or cardiovascular variables are paramount.

In general what is good for respiration is inversely proportional to what is good for circulation.
alveolar gas molecular equilibrium = volume x pressure x time
This may be bad for circulation because:

• venous return is inhibited
• alveolar distension may occur
• surfactant production may be decreased and function impaired
• pulmonary vascular resistance may increase
• left ventricular compliance of the heart may be compromised by septal shift and distended tense lungs

What ventilator settings may be set?

• tidal volume
• frequency
• minute ventilation
• inspiratory time
• expiratory time
• I:E ratio
• inspiratory flow rate
• inspiratory flow waveform
• volume and pressure limitation to inspiration
• inspiratory hold / breath hold / plateau pressure / sighs / PEEP / CPAP
• inspired oxygen FIO2

There are a number of potential adverse effects on the lung when controlled ventilation is instituted (Malarkkan et al. Anaesthesia 2003; 58: 647-667):

• volutrauma (Webb & Tierney Am Rev Resp Dis 1974; 110: 556-565)
• atelectrauma (Robertson, Robertson, Van Golde Pulmonary Surfactant 1984)
• barotrauma
• bronchiolectasis (Slavin et al. BMJ 1982; 285: 931-934)
• atelectasis
• biotrauma cytokine release (Tremblay & Slutsky Proc Assoc Am Phys 1998; 110: 482-488)

There has been some suggestion that protective ventilation may occur when the upper (expiratory) and lower (inspiratory) inflection points of the pressure – volume curve are used to control ventilator settings (Brower & Rubenfeld Crit Care Med 2003; 31: S312-S316; Bensenor et al. Anesth Anal 2003; 97: 145-150; De Chazal & Hubmayr BJA 2003; 91: 81-91; Stenqvist BJA 2003; 91: 92-105).  Very short inspiratory times are bad for both respiratory and circulatory variables, and very long inspiratory times are better for respiratory variables and worse for circulation variables.
Inspiratory flow waveforms have also been studied with similar conflicting benefits (Baker Int.Anaesth.Clinics 1971; 9: 39-64; Baker & Hahn Resp Physiol 1974; 22: 227-239; Baker et al. BJA 1977; 49: 1207-1220; Baker et al. BJA 1982; 54: 539-546; Whiteley et al. Resp Physiol Neurobiol 2002; 131:269-284; Ehrhardt et al. Crit Care Med 2001; 29: 1207-1214; Yang & Yang Chest 2002; 122: 2096-2104).  For respiratory variables a decreasing inspiratory flow waveform is better than constant and sine inspiratory flows which are better than an increasing inspiratory flow waveform; whereas for cardiovascular variables increasing and sine inspiratory flow waveforms are better than a decreasing flow which in turn is better than a constant inspiratory flow waveform.
Intermittent inspiratory sighs, positive end-expiratory pressure (PEEP), continuous positive airway pressure (CPAP) and plateau pressure all tend to limit atelectasis and improve oxygenation, but may compromise circulation or may lead to alveolar shearing stress and lung damage (Mead et al. JAP 1970; 28: 596-603; Gattinoni et al. Am J Resp Crit Care Med 1995; 151: 1807-1814; Lichtwarck-Aschoff Am J Crit Care Med 2000; 162: 2125-2133; Rothen et al. BJA 1999; 82: 551-556).
Recently there has been emphasis on optimal ventilation, and other ventilatory patterns which allow synchronisation with the patient’s respiratory efforts, and some studies show improved variables with such modes of ventilation (Yu et al Anesth submitted 2005).
Additional oxygen may be necessary to maintain arterial oxygenation particularly for patients with diseased lungs, but increasing the inspired oxygen concentration may increase the likelihood of alveolar collapse or reduction in functional residual capacity (FRC) (Baker et al. BJA 1993; 70: 259-266; Joyce, Baker et al. BJA 1996; 76: 292-296; Joyce & Williams JAP 1999; 86: 1116-1125).
In summary the ventilator settings for “healthy” lungs in controlled ventilation during anaesthesia should be:

For comparison when there is respiratory or cardiovascular compromise the ventilator settings should be:

In general what is good for respiration is inversely proportional to what is good for circulation.

Copy from:http://www.anesthesia.org.cn/2005china/eng/eng06.doc

External links :

• Brief Introduction of Anaesthetic Machine
• The Anesthesia Gas Machine
• Anaesthetic Technician
• Controlled Ventilation during Anaesthesia
• Educational simulation of the anesthesia machine pre-use check
• Volatile Anaesthetics protect the heart
• The CO₂ Absorption Capacity of Fully and Partially Desiccated Soda Lime