Page 455 - WSAVA2018
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Respiration
The rate, rhythm and nature of breathing effort should be consciously observed. See previous notes in ‘Summary of Central Nervous Monitoring’ table. Changes in the respiratory rate and effort may indicate a change in areas such as
Anaesthetic depth
· Increased respiratory rate - becoming lighter
· Decreased respiratory rate – becoming deeper
· Increased or ‘jerky’ respiration may indicate that the patient is feeling pain stimuli
It is also important to monitor both the rebreathing bag and the patient’s thorax to detect:
· Anaesthetic circuit disconnection;
· Inconsistency between breathing excursions of the chest and the amount of air moving in and out of the bag;
· Airway obstruction problems (kinked endotracheal tube).
End tidal CO2 monitor
Capnometry is the measurement of carbon dioxide in exhaled gas of a patient. The basic physiology behind capnometry is that tissues generate carbon dioxide that is delivered to the lungs, via the blood and then exhaled. The exhaled carbon dioxide is sampled from the distal end of the endotracheal tube, analysed by a capnometer and displayed on a capnograph.
rebreathing system
Capnographs generally work on the principle that C02 absorbs infra-red radiation; a beam of infra-red light is passed across the gas sample to fall onto the sensor, the presence of C02 leads to a reduction in the amount of light falling onto the sensor which in turn changes the voltage in the circuit and the reading is portrayed.
Phases of a Capnogram
There are 4 distinct phases of the capnogram:
Phase I
The initial flat portion. The animal is inspiring and because inspired gas is 100% O2 the CO2 reading is 0.
Phase II
Ascending portion of the graph that represents the first appearance of carbon dioxide during exhalation. As the animal exhales, the first air out represents what was in the trachea. This is relatively low in CO2. Next the bronchial and bronchiolar air comes out with increasing levels of CO2.
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Phase III
A plateau representing the exhalation of alveolar gas which contains the highest level of CO2. The very last part of phase III is the air that was deep in the alveoli and is referred to end tidal CO2.
Phase IV
The downward slope that corresponds to the initial inspiration. Because there is some CO2 present in the mechanical dead space of the anaesthetic machine the phase initially shows some CO2. This rapidly falls to 0 once dead space gas is inhaled only pure O2 is passing the sampling port.
Normal End tidal C02 = 35 – 45mmHg
Analysing alteration of the normal Graph
An elevated baseline Phase I (inhalation) is caused by the patient rebreathing C02.
Causes include:
· Exhausted Soda lime
· Incompetent one-way valves in circle system · Inadequate fresh gas flow rates in a non-
A slanted or prolonged expiratory upstroke Phase II indicates an obstruction of airflow. The exhalation of the anatomical dead space air is prolonged giving a gentler slope to that part of the graph.
Causes include:
· Kinked endotracheal tube
· Secretions that may be partially blocking endotracheal tube or patient airways
· Patient bronchospasm
Lack of expiratory plateau Phase III indicates lack of good alveolar sample. The animal is not completely exhaling the alveolar air.
Causes include:
· Patients taking small shallow breaths
· High fresh gas flow rates in non-rebreathing systems
Terminology
Hypercapnia
From the Greek hyper = ‘above’ and kapnos = ‘smoke’, a condition where there is too much carbon dioxide in the blood. Hypercapnia will generally trigger reflexes that increase breathing and access to oxygen.
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