Monday, January 9, 2012

900 ML are you Serious?

Puffer Fish, by Karen Garrett de luna

When was the last time an anesthesiologist has tried to give you ventilator settings?

In my practice it is rare to nonexistence that an anesthesiologist orders ventilator settings. This is primarily due to respiratory therapy ventilator management protocols in place at facilities I have worked at.
Even though these protocols are well established, occasionally a physician will order their own settings.

Recently, an anesthesiologist wanted me to change the tidal volume from 500 to 900 ml on their post-operative open heart patient.
As any proactive respiratory therapist would have done I questioned why?

Because they weigh greater than 100 kg”.

“He needs the volume to adequately ventilate & decrease atelectasis”.

Wait he may weigh 200 pounds however, he is only 5’-6” tall, 900ml would be greater than ~13 ml/kg/ideal body weight”.

“With the current tidal volume of 500 his plateau pressures are 25 cmH2O & increasing the tidal volume will increase this significantly”.

“He is ventilating fine on his current setting of 500 ml, his PaCo2 on the most recent ABG was 42, and the minute ventilation has not changed significantly since then”.

“Also he is oxygenating fine, he is on a FIO2 of 45% with a minimal PEEP of 5 cmH2O”.

Ok, keep him on the current settings.

Has this happened to you?

Have you asked “why do they order such large tidal volumes?”

Are anesthesiologists unaware of lung protective ventilation strategies?

Yes, anesthesiologists are well aware of lung protective strategies, however due to various situations in the operating room one may have to use larger tidal volumes to adequately ventilate the patient.

Conditions Requiring Larger Vt

Large Leaks- related to chest trauma, this is obvious so I won’t elaborate.

Patient Positioning- There are a variety of procedures that place the patient a supine or trendelenburg position that directly impairs pulmonary gas exchange.
Examples include gynecological & abdominal surgeries.
An extreme example is the “steep trendelenburg” position most commonly associated with “robot-assistedlaparoscopic prostectomy”. During this procedure the patient is placed at a 30°-to-45° for many hours.

Pneumoperitoneum-is the intentional insufflation of CO2 into the peritoneal cavity during laparoscopic surgery. The additional CO2 decreases pulmonary compliance by the increase of abdominal pressure and additionaly increases the PaCO2.

Note- many gynecological, abdominal, & prostate surgeries are performed laparoscopically, so this coupled with adverse positioning further compromises V/Q matching. 

Apparatus- In my facility it was practically a standing order ‘If the patient is on ≥ 10 cmH20 of PEEP, then an ICU ventilator will be used during the procedure’. Anesthesia machines do not deliver gas the same as an ICU ventilator & many anesthesia machines are outdated.

  1. Older generation anesthesia machines do not compensate for the volume lost from the compressible patient breathing circuit.

  1. In some anesthesia delivery systems the fresh gas flow (FGF) and tidal volume setting are dependent/additive. FGF is often deceased after induction to save on anesthetic agent usage. This reduction in FGF reduces the delivered Vt unless there is a compensatory increase in the set Vt.

  1. System volume- an anesthesia ventilator has on the average ~ three times the system volume of an ICU ventilator. This additional system volume decreases the capabilities of total flow & pressure delivery generated by the device. Dr. Katz & colleagues exposed that older generation anesthesia ventilators would not adequately meet the minute ventilation & airway pressure demands of ~ 34% of their sampled patients with acute respiratory failure [1].

In summary anesthesiologist may order larger tidal volumes in the intensive care unit, which may put the patient at risk for volume & barotrauma. Nevertheless, be patient; consider the dynamics of their environment and the equipment they have to work with. Be engaged, an educator, explain and show differences of the two technologies, let an unbiased test lung do the talking.

"Have Test Lung, Will Travel". 

1.      Katz, J. et. al. (2000). ImprovedFlow Capabilities of the Datex-Ohmeda Smartvent Anesthesia Ventilator. Journal of Clinical Anesthesia. 12 (2): 40-47.