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LETTER TO THE EDITOR
Year : 2011  |  Volume : 27  |  Issue : 2  |  Page : 291-292

Abnormal function of a normal ventilator


Department of Neuroanaesthesiolgy, All India Institute of Medical Sciences, New Delhi, India

Date of Web Publication3-Jun-2011

Correspondence Address:
Rakesh Garg
58-E, Kavita Colony, Nangloi, Delhi - 110 041
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9185.81851

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How to cite this article:
Garg R, Rath GP. Abnormal function of a normal ventilator. J Anaesthesiol Clin Pharmacol 2011;27:291-2

How to cite this URL:
Garg R, Rath GP. Abnormal function of a normal ventilator. J Anaesthesiol Clin Pharmacol [serial online] 2011 [cited 2019 May 27];27:291-2. Available from: http://www.joacp.org/text.asp?2011/27/2/291/81851

Sir,

Any alteration in delivered tidal volume (TV) during positive pressure ventilation in children under anesthesia may lead to volutrauma or barotrauma. [1] Usually, a lower than the preset TV is delivered due to either an inadvertent leak or as a result of loss of compliance in the breathing circuit. [2],[3] We encountered an unusual scenario in which the delivered TV was higher than the preset TV on the ventilator.

A 2-year-old, 13 kg child was scheduled for revision of ventriculo-peritoneal shunt. In the operating room, routine monitors like electrocardiogram, noninvasive blood pressure, and arterial oxygen saturation (SpO 2 ) were attached. Anesthesia was induced with fentanyl 25 μg and propofol 25mg, and neuromuscular blockade was achieved with vecuronium 1.2 mg. Airway was secured with an uncuffed endotracheal tube (ETT) and capnography monitoring (EtCO 2 ) was initiated. After ensuring the optimal position of the tracheal tube by bilateral chest auscultation, the child was connected to the ventilator (Datex Ohmeda 7000 Ventilator) with volume control mode of ventilation. The respiratory rate (RR) was adjusted to 15 breaths/min and minute ventilation (MV)-2 L/min. The patient was hemodynamically stable and the airway pressure was 13mmHg and the EtCO 2 36 mmHg. After the patient was positioned for the surgery, the breathing circuit was readjusted after disconnection and was secured with a tube stand. To refill the bellows of the ventilator, fresh gas flow (FGF) was increased. An increase in airway pressure up to 22-24 mmHg was noticed along with a gradual fall of EtCO 2 reading. The correct position of ETT was verified. The air entry was equal on both the lungs, without any added sounds. The SpO 2 and hemodynamic parameters were within normal limit. The volume displacement of the bellows was noticed to be 250 mL. The ventilator settings were rechecked and found to be unchanged. The FGF was decreased to 1 L/min, after which the airway pressure was settled down to the normal values. Rest of the anesthetic course was uneventful. At the end of the procedure the machine and circuit were rechecked, but no significant leak was found.

A discrepancy between preset and delivered TV readings can arise due to change in system compliance, FGF, leak in the breathing system, erroneous location of volume sensors in the breathing circuit, and change in airway resistance. [1],[2] Delivery of a high TV can mislead the attending anesthesiologist and force him to change settings in order to prevent pulmonary trauma, as in our case. Such a ventilator malfunction can be of concern in patients with lung pathology or certain procedures like craniotomy or thoracic surgeries, where high airway pressures need to be avoided.

A ventilator delivers TV at the value preset. The mechanism to compensate for FGF may or may not be present. Anesthesiologists should be aware of the type of ventilator being used and whether it is capable of FGF compensation or not. The ventilators available with the newer generation anesthesia machines have the mechanism for compensation of circuit compliance. However, the same may not be true for the ventilators of older version anesthesia machines. [1] The ventilator used in this case did not have compensatory mechanism for FGF (as mentioned in the manual of the ventilator). During inspiration, any amount of FGF introduced to the breathing system by the anesthesia machine is delivered to the patient, in addition to the gas delivered by the ventilator.[2] A higher FGF results in higher inspiratory volume. In the case reported, the FGF was increased to fill the bellows, but not reduced after the bellows were filled. The total flow was 7 L/min and thus the child was receiving a TV of about 250 mL (130 mL from the ventilator and approx. 150 mL by the FGF during the inspiratory phase), with the ventilator set at MV of 2 L/min and a RR of 15 breaths/min. An increase in airway pressure and a gradual fall in EtCO 2 were seen due to an increase in minute ventilation and tidal volume delivered rather than any respiratory problem. Whenever such ventilators are used it is recommended that one should readjust the ventilator settings whenever the FGF is changed.

We have highlighted a shortcoming in the function of some ventilators. The normal functioning of the "specific ventilator" should be well understood by the user. In ventilators with the absence of compensatory mechanism, the FGF should be cautiously adjusted to prevent delivery of high tidal volumes.

 
   References Top

1.Bachiller PR, McDonough JM, Feldman JM. Do new anesthesia ventilators deliver small tidal volumes accurately during volume-controlled ventilation? Anesth Analg 2008;106:1392-1400.  Back to cited text no. 1
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2.Rothschiller JL, Uejima T, Dsida RM, Cote CJ. Evaluation of a new operating room ventilator with volume-controlled ventilation: The Ohmeda 7900. Anesth Analg 1999;88:39-42.  Back to cited text no. 2
    
3.Tokumine J, Sugahara K, Gushiken K, Ohta M, Matsuyama T, Saikawa S. Non-zero basal oxygen flow a hazard to anesthesia breathing leak test. Anesth Analg 2005;100:1056-8.  Back to cited text no. 3
[PUBMED]  [FULLTEXT]  




 

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