CASE REPORT
A 31 year old female patient, weighing 70 kg, ASA 2, was
scheduled for total thyroidectomy. Preoperative assessment
revealed acceptable pulmonary function, complete blood count
and renal function. There was no significant tracheal compression
or deviation. Haemodynamic parameters, body temperature and
thyroid function test were within normal limits.
The patient received premedication comprising of pethidine
75mg and phenergan 25mg intramuscularly 1 hour preoperatively.
Induction of anaesthesia was accomplished with intravenous
fentanyl 100 mcg, thiopentone sodium 300 mg and succinylcholine
100 mg; the trachea was intubated with a 7mm reinforced tube
and connected to an Ohmeda anaesthesia machine via a Datex-
Ohmeda specification of Bain's co-axial circuit. Anaesthesia was
maintained with infusions of propofol (2-4mg kg -1), atracurium
besylate (0.4mg kg -1) and 60% Nitrous Oxide in Oxygen.
Vital signs monitored included blood pressure, pulse oximetry,
electrocardiogram and end tidal carbon dioxide (ETCO2).
Initial parameters were normal, with an ETCO2 of 38 mmHg.
After a few minutes, the rise of ETCO2 to 55mmHg necessitated
checking the delivered gas flows, chest expansion and auscultation;
nothing abnormal was detected. The capnograph sensor was
changed; even then the end tidal carbon dioxide tension continued
to rise, with an upward shift of the inspiratory carbon dioxide
(IMCO2) baseline indicating rebreathing. After 30 minutes,
the ETCO2 rose to 75 mmHg. The patient's arterial blood gas
estimation showed severe hypercapnia and respiratory acidosis in
a well oxygenated patient.
(pH 7.080; pCO2 95.3; pO2160.0; HCO3 27.6; O2Sat 98.2%)
The haemodynamic parameters and body temperature
remained normal. After confirming normal ventilator
performance the coaxial circuit was changed. The ETCO2 curve
began a dramatic fall towards the normal range of 40 mmHg and a
return of baseline IMCO2 towards zero. Repeat arterial blood gas
estimation was normal.
The surgery was completed and the patient was extubated with
no adverse post operative event. Further visual examination of
the faulty co-axial circuit did not reveal any defect in both outer
transparent and inner coloured tubings, the attachments were well
fixed at both the machine and patient ends. In order to explore
the inner tube, the outer tube's distal connection was detached.
A 2-3cm circumferential laceration of the inner tube was detected
about 10 cm from the patient end of the circuit. This crack was
otherwise invisible with normal assemblage of the circuit.
DISCUSSION
Bain's coaxial circuit was conceived and introduced into
anaesthesia practice in 1972 by Bain and Spoerel.1 The inner,
coloured tube which carries inspiratory gases may become faulty.
Any disconnection or breech of its integrity increases the dead
space breathing profoundly, followed by hypercapnia and its
complications.
The circuits, usually supplied sterile, are reused after washing
and drip dried. Due to cost considerations, it is common practice in
most units to reuse these circuits. Early reports of malfunctioning of the inner tube soon came after its introduction into practice.
Hanallah and Rosales in 1974 reported avulsion of the inner tube
of a reused Bain's circuit with significant dead space breathing
and hypercapnia, however, overt signs of hypercapnia were absent
except for suspicious increase in blood loss.2 There are several other
reports implicating detachment of the inner tube and consequent
hypercarbia within the first hour of anaesthesia.3,4,5
Obstruction to flow through the inner tube of a coaxial circuit,
due to compression, twisting or kinking is reported as gas flow
failure to the patient end before induction of anaesthesia.6,7,8 Berner
reported of a disconnection in the metallic part of the anaesthesia
machine.9 His patient developed profound hypercapnia into the
postoperative period. However, this present report, rare in the
literature, is that of a malfunction of the inner tube resulting
from a laceration of its wall and consequent dead space breathing
evident in the first half hour of anaesthesia.
Hypercapnia induces catecholamine release. The patient
may present with tachycardia, arrhythmias, excessive sweating,
hypertension and peripheral vasodilatation which may
result in excessive intraoperative blood loss. Berner reported
haemodynamic changes, pupillary dilatation, hyperkalaemia and
severe respiratory acidosis.9 Whereas Ghai et al. reported two cases
of intraoperative arrhythmias.4 The patient in our report did not
manifest these catecholamine responses of hypercapnia, probably
due to masking by the depth of anaesthesia, or due to a delayed
response. Neural and respiratory consequences of hypercapnia
may be masked by anaesthesia, and cardiovascular complications
are erroneously associated with light planes of anaesthesia. Other
causes of intraoperative hypercapnia, including hypoventilation,
poor soda lime function and malignant hyperpyrexia must be
excluded.
Detection of any co-axial circuit malfunction is recommended
in several tests:
several tests:
-Visual inspection of tubings for obvious disruption and
detection of gas flow obstruction.
This tests the low pressure system and the integrity of the
inner tube. With the coaxial circuit connected to the anaesthetic
machine and the patient end open, fresh gas flow of oxygen is set at
1L min-1 and the adjustable pressure limiting (APL) valve closed.
Oxygen is then flushed via bypass through the circuit. Collapse of
reservoir bag, by the creation of a Venturi effect, indicates an intact
inner tube. Any defect in line will produce a negative result.
Foex-Crampton Smith Manouever 11
This occlusion test, which is more comprehensive, assesses the
gas flow line from the flow meters of the machine to the patient end
of the circuit. With oxygen flow at 2 liters min -1 the patient end of the inner tube is occluded briefly for 2 to 3 seconds using the
forefinger.12 A positive test is indicated by descent of the rotameter
bobbin due to back pressure; with removal of the finger, the bobbin
ascends to its original position. Any breech of integrity gives a
negative result. The possible accidental occlusion of the outer tube
of the circuit may nullify the test with false results. Hence, Ghani
suggested the use of the plunger of a 3 ml syringe to occlude the
inner tube more precisely.13 With Ghani's method the movement
of the rotameter bobbin is accompanied by a hissing sound on
removal of the plunger. Intersurgical coaxial circuits now come
with dedicated occlusion devices.
In our tests we found the 3 ml plunger is suitable for the
Intersurgical Bain's circuit but inadequate for Ohmeda coaxial
circuit, for which the plunger of a 5 ml syringe was more
fitting, because of its larger tube diameter. Szypula et al. in their
comparison of the two tests, found the occlusion test more sensitive
than the Pethick test.14
Even though the reused circuits are washed without dismantling
any components, the potential for damage from poor handling
exists, especially as deterioration of plastic material is known to
occur with constant exposure to anaesthetic gases. However, we
could not explain the selective laceration of the inner tube in this
case. Traumatic damage, if it is, would be expected to have affected
the outer tube preferentially. However, it is important to note that
even a new unused circuit has been implicated in a report of inner
tube malfunction.7
CONCLUSION
This report highlighted a case of severe hypercapnia due to dead
space rebreathing complicating an unusual damage to the inner
tube of a co-axial circuit.
The hidden potential for catastrophic complications which may
result from a malfunction of the inner tube mandates a thorough
check, test of patency and integrity of the inner tube of any coaxial
circuit before use. The occlusion test is now recommended
over the Pethick test by some associations.15
The reliance on cardiovascular manifestations of hypercapnia
may delay correctional intervention as catecholamine response
may be obtunded in deep anaesthesia. End tidal carbon dioxide
monitoring is vital for early detection of hypercapnia. Jouan et al.
suggested that arterial blood gas should be checked in a patient
with unexplained tachycardia and noncompliance with the
ventilator in the presence of adequate and that liberal use of muscle
relaxants is recommended.3
Overall, careful handling of circuits by experienced staff may
prevent damage that may be costly to life. The rotational use of
these circuits is recommended, with intervening periods of rest.
Furthermore, the manufacturers should consider using more
resilient material for the inner tube of co-axial circuits.
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