Ventilation and Mechanical Assist Devices

Ventilation and Mechanical Assist Devices

“You don’t need a weatherman to know which way the wind blows.”

Overview

A crucial aspect of providing safe general anesthesia is maintaining normal ventilation. Normal ventilation is defined by the maintenance of normal arterial blood carbon dioxide (PaCO₂) concentrations. Respiratory effort is routinely assessed by observing the movements of the animal’s chest and abdominal wall. Although chest wall movements may be regular and give the appearance of satisfactory gas exchange, they do not ensure adequate movement of air in and out of the lungs. Gas exchange is improved by inflating the lungs to a predetermined pressure or predetermined volume by manually squeezing a rebreathing bag on an anesthetic machine, or by attaching the endotracheal tube to mechanical ventilatory-assist device (ventilator). Controlled (rate and volume) ventilation helps maintain a more stable plane of anesthesia because the lungs are the exchange site for inhalant anesthetic uptake and elimination. High-frequency ventilation is a unique technique based on the principle that diffusion is the primary means by which fresh gases are delivered to peripheral airways for gas exchange.

General Considerations

I Manual or mechanical intermittent positive pressure ventilation (IPPV) when properly applied can be used to maintain stable inhalant anesthesia (the drug delivery system is the lung) and blood gases (PO₂, PCO₂) for prolonged durations with minimal untoward effects

II Positive pressure ventilation requires a thorough familiarity with cardiopulmonary physiology, ventilatory equipment and techniques, and blood gas interpretation

III Mechanical ventilators inflate the lungs to a predetermined volume (volume cycled) or pressure (pressure cycled) or for a preset time at a predetermined gas flow rate (time cycled). Most anesthesia ventilators consist of a compliant pleated compressible bellows connected to an anesthetic breathing circuit. The bellows is contained within an airtight rigid plastic cylinder that can be pressurized to compress the bellows. Driving gas is cyclically introduced into the cylinder (outside the bellows) causing pressure within the cylinder to increase (inspiratory phase). The bellows is compressed and gas contained therein is delivered to the airway. When pressure within the cylinder is released, the process reverses and the elastic recoil of the lung causes the bellows to expand (expiratory phase).

A The breathing circuit must be airtight (no leaks; “closed circuit”) during the inspiratory phase of the ventilation cycle for gas to be directed into the airway and lungs

IV Controlled ventilation may be required in animals that are not breathing adequately or that are difficult to keep anesthetized

V PaCO₂ values and capnometry (ETCO₂) provide continuous assessment of ventilatory adequacy (see Chapter 14)

Reasons for Respiratory Inadequacy

I Depression of respiratory centers

2. Analgesics (opioids)

3. Drug toxicities

1. Acidosis (PaCO₂ > 70 mm Hg; metabolic acidosis)

3. Toxic metabolites (endotoxins)

1. Head trauma (increased intracranial pressure)

II Inability to adequately expand the thorax

1. Neuromuscular blocking drugs

B Pain (splinting of chest)

C Chest wall trauma

D Thoracic surgery

E Abdominal distention

F Muscle weakness

H Bony deformities of the chest wall

1. Weight of abdominal viscera on the diaphragm may impede lung expansion

2. Head-down position

3. Surgical tie downs and abnormal thoracic limb positioning

J Nerve trauma or edema

III Airway obstruction including laryngeal paralysis

A Foreign and bodily materials (e.g., stones, sticks, water, mucus, blood)

B Trauma (nasal, laryngeal, trachea)

C Infection and inflammation

E Congenital anomalies (e.g., collapsed trachea or bronchi)

F Nerve disease or damage

IV Inability to adequately expand the lungs

A Pneumothorax (especially tension pneumothorax)

B Pleural fluid

C Diaphragmatic hernia

V Cardiopulmonary arrest

VI Pulmonary edema or insufficiency

Managing Ventilation during Anesthesia

I Anesthetics are respiratory depressants; positive pressure ventilation may be required to assist or control breathing

A Controlled ventilation helps to ensure adequate alveolar ventilation, aiding in the uptake (inhalants) and elimination of drugs and gases

II Special indications for controlled ventilation during anesthesia

A Thoracic surgery

1. Controlled respirations minimize spontaneous chest wall movements

2. The lungs cannot be inflated when the chest is open (intrathoracic pressure equals atmospheric)

B Neuromuscular blockers (see Chapter 10): neuromuscular blocking drugs paralyze the diaphragm and intercostal muscles producing apnea

C Prolonged surgical anesthesia (more than 60 minutes) reduces respiratory effort, especially in horses, resulting in hypoventilation

D Chest wall or diaphragmatic trauma

a. “Flail chest”: series of adjacent broken ribs resulting in the segment moving independently (chest wall moves in during inspiration instead of out: paradoxical breathing). Usually associated with lung contusion and pneumothorax.

2. Space-occupying lesions

a. Diaphragmatic hernia

b. Blood, fluid, neoplasia

E Maintain a more stable plane of anesthesia

F Obesity and special positioning

G Control of intracranial pressure

H Convenience: eliminate concerns about hypoventilation and poor gas exchange (low oxygen [O₂]; high carbon dioxide [CO₂])

Considerations for and Consequences of Ventilation

I Pulmonary system

A Normal lungs are well ventilated during spontaneous breathing (Fig. 13-1)

FIG. 13-1 Anatomic drawing of various components of the lung and its blood supply. Cyclical changes or constant increases in lung volume and airway pressure can impede blood flow through the lung. The intrathoracic space is a potential space that can expand (e.g., pneumothorax, hemothorax).

1. Inspiration is active and dependent upon the integrated function of the nervous and musculoskeletal systems

2. Expansion of the rib cage and flattening of the diaphragm increase the volume of the thoracic cavity and the negative pressure in the thoracic cavity, drawing air into the lung

3. Expiration is passive returning lung volume to its original state

a. The balance between the ribs’ tendency to spring outward and the lungs’ elastic tendency to collapse determines the initial lung volume

B The portions of the lung in closest contact with moving surfaces (i.e., the peripheral lung field, diaphragm) undergo the greatest volume changes during spontaneous ventilation

C Decreased breathing of hypoventilation causes increases in CO₂ (PaCO₂ > 45 to 50 mm Hg)

1. Results in respiratory acidosis

2. Activates the sympathetic nervous system

3. Predisposes to cardiac arrhythmias

a. Caused by sympathetic stimulation and hypoxemia

4. Initially causes peripheral vasoconstriction (sympathetic effect) followed by peripheral vasodilation as a direct effect of CO₂ on smooth muscle tone

a. Increases cerebral blood flow and intracranial pressure

5. Increased PaCO₂ can produce narcosis

a. PaCO₂ levels above 80 to 100 mm Hg have an anesthetic effect

6. Hyperpnea and an increased work of breathing

D Positive pressure ventilation inflates the peribronchial and mediastinal areas of the lung; the peripheral segments are relatively less ventilated compared with normal spontaneous breathing

1. Pressure or volume ventilation increases airway diameter and thus anatomic dead space, reducing alveolar ventilation

E Positive pressure ventilation results in a significant reduction in lung compliance; the lung becomes stiffer and predisposes to atelectasis and hypoxemia

1. Small-airway closure can occur

2. The distribution of ventilation is altered, leading to ventilation-perfusion mismatching

3. Ventilators that deliver a preset volume compensate for stiffer lungs by ensuring delivery of a constant volume independent of airway pressure

II Cardiovascular system (Table 13-1)

TABLE 13-1

Cardiovascular System: Effects of Breathing and Intermittent Positive Pressure Ventilation

Phase of Cycle	Intrathoracic Total Pressure	Total Thoracic Blood Volume	Left Ventricle Cardiac Output
NORMAL BREATHING
Inspiration active	(−5 cm of H₂O)	↑	↓
Expiration passive	↑(−2 cm H₂O)	↓	↑
IPPV
Inspiration passive	↑(10-20 cm H₂O)	↓	— (↓ if IPP prolonged)
Expiration generally passive	↓To atmospheric pressure	↑	↓

IPP, Intermittent positive pressure; IPPV, intermittent positive pressure ventilation.

A The subatmospheric pressure within the thorax augments venous return during inspiration; this subatmospheric pressure is made more negative during inspiration by contraction of the diaphragm

B The pressure in the airway and lungs is transmitted to the thoracic cavity during controlled ventilation, impeding venous return and potentially decreasing cardiac output (see Fig. 13-1; Fig. 13-2)

FIG. 13-2 Manual lung inflation produces a positive pressure in the lungs, which is transmitted to the intrathoracic space, reducing lung blood flow and lowering cardiac output. Pressure values reflect changes relative to atmospheric pressure.

C Controlled ventilation decreases arterial blood pressure and cardiac output when:

1. Average airway pressure is consistently more than 10 mm Hg

2. Circulating blood volume is low, caused by dehydration and blood loss

3. Sympathetic nervous system activity is impaired, caused by anesthesia, local anesthetics, and shock

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Tags: Handbook of Veterinary Anesthesia

Sep 6, 2016 | Posted by admin in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off

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