|
Table 1 . Suggested breathing system circuit factors for fresh gas flow calculation [ 12 ] . MV = minute volume .
Breathing system
Circuit factor
Lack ( Mapleson A ) MV × 1 – 2
|
ETCO ² ( mmHg )
40
|
Alpha
Phase II
|
Phase III |
Beta |
ETCO ²
Phase 0
|
|
Bain ( Mapleson D MV × 2 – 4
T-piece ( Mapleson F ) MV × 2 – 4
|
0 |
Phase I |
Expiration
Time
|
Inspiration |
Circle None required
Figure 2 . A normal capnogram .
The normal capnogram ( Figure 2 ) consists of : [ 2 , 4 , 5 , 14 ]
Example calculation : A 12 kg dog is to be anaesthetised and a Lack will be used as the breathing system . The calculation would run as follows , without specific parameters gained from a specific patient or the approximation of an RR from a panting patient :
FGF = MV × CF MV = 100 – 400 ml / kg / min = 1,200 – 4,800 ml / min CF for Lack = × 1 – 2 1,200 – 4,800 ml × 1 – 2 = 1,200 – 9,600 ml / minute
This results in a massive range of potential FGF rates for the 12 kg dog . To make this a simpler calculation without needing to have the patient present , MV can be approximated as 200 ml / kg / min [ 12 ] . This would enable the above FGF calculation to run as follows :
MV = 200 ml / kg / min = 2,400 ml / min 2,400 × 1 – 2 = 2,400 – 4,800 ml / min
Therefore , the patient could be started on an FGF of 3,600 ml / min , with the knowledge that they may require more or less than the starting flow rate . Once the patient is deemed to be stable and in the maintenance phase of the anaesthetic , assessments can be made as to whether a lower FGF rate could be applied [ 8 ] .
Different texts cite slightly different values for CF . The factors suggested here make the standard calculation for FGF provision simple and easy to work out in the clinical setting [ 3 ] . Great accuracy in the calculation becomes unnecessary when FGF is titrated down to the patient ' s requirements .
Capnograms , ETCO 2 and FiCO 2
When capnography is added to the applied monitoring , the anaesthetist can then use the capnogram , ETCO 2 and FiCO 2 to further monitor the appropriateness of the selected FGF .
Phase I – the baseline
This is the beginning of exhalation and represents the exhalation from the larger airways and equipment . The
CO 2 value during this phase should be 0 . Phase II – an expiratory upstroke
This is the representation of the exhalation of mixed gases from the alveoli and anatomical dead space .
Phase III – the plateau
This is the representation of the endpoint of the exhalation of alveolar air . Capillaries are continuously diffusing CO 2 into the alveoli , giving the plateau a slight upward slope .
Phase 0 – the inspiratory downstroke This is the representation of the inhalation of CO 2
-free gases , so the CO 2 concentration rapidly declines to 0 .
The ETCO 2 is measured at the end of the plateau and normal values are 35 – 45 mmHg . The highest permissible ETCO 2 value is 60 mmHg . If this is exceeded , blood pH becomes more acidic , which can affect myocardial function and increase the likelihood of cardiac arrhythmias [ 2 , 4 , 6 ] . Acceptable levels for FiCO 2 are up to 7 mmHg [ 5 ] .
If the capnogram does not return to baseline and there is a positive number displayed as FiCO 2
, this shows that the patient is inspiring CO 2 ( Figure 3 ).
ETCO ² ( mmHg )
40
0 Normal time base
Figure 3 . Capnogram displaying inspired CO 2
[ 5 ]
.
36 Veterinary Nursing Journal