Unlike spontaneous breathing or conventional ventilation, the time spent at maximal inflation is typically much longer than time deflated. Time low is minimized to prevent collapse of alveoli and barotrauma. The video below shows shows treated pig lungs being ventilated with typical APRV settings.
Using the Murray score as a guide we can assess whether a patient with ARDS would potentially benefit from venovenous extracorporeal oxygenation (vv-ECMO).
Once on vv-ECMO however, there are two factors that are most crucial in determining adequate blood gas exchange.
Percent ECMO blood flow / cardiac output
EBF/CO is the ratio of ECMO circuit blood flow rate as percent of cardiac output. Both are measured in L/min, and therefore you end up with a dimensionless percentage. Since the membrane lung is providing the oxygenation and CO2 removal, the larger portion of the total cardiac output that is passed through the oxygenator the more gas exchange that can occur.
If EBF/CO can be consistently maintained >60%, then oxygenation saturation (SaO2) is almost always >90%.
vv-ECMO circuits can usually support blood flows up to about 5 Lpm before cavitation/chatter limit further increases. So for typical patients, EBF/CO is not usually a problem unless the patient has a very high cardiac output as seen in liver failure, septic shock, hyperthyroidism, or severe anemia.
Maintaining a patient’s hemoglobin level above 10 g/dL with red blood cell transfusions can improve O2 delivery and achieve adequate SaO2, while allowing for lower EBF/CO ratios and lower total circuit blood flow rates.
Recirculation Blood Flow
Recirculation in ECMO circuits occur when oxygenated blood is returned to the ECMO circuit without passing through the peripheral vasculature. This represents wasted ECMO blood flow, and in can be thought of as reducing the effective EBF/CO.
Recirculation blood flow (RBF measured in Lpm) can be assessed by comparing pre-oxygenator pO2 with a peripheral pO2 (not ScvO2 from the pulmonary artery catheter which should be the same as post-pump pO2). Measuring a true mixed peripheral pO2 is difficult in vv-ECMO because pulmonary artery catheter has received oxygenated blood from the ECMO circuit. A venous sample from the internal/external jugular vein or other peripheral vein can be used as a substitute, but reflect local circulation that may not be reflective of the overall system. A theoretical, but often not practical method is to turn off the sweep gas and measure the gas sample from the pre-oxygenator.
Recirculation occurs because of the inherent geometry of using vv-ECMO cannulas that are both drawing and returning blood to the sample venous blood pool. Cannulas can also be misplaced or misdirected, which is critically important in the double lumen venovenous (DLVV) cannulas (eg Avalon) where recirculation can range from 20-50%.
Once an adequate effective ECMO blood flow has been established, (EBF-RBF)/CO, by ensuring that there is that ECMO flow is >60% of the cardiac output, and that recirculation is minimal, we can optimize the oxygen and CO2 gas exchange. Fortunately, these can generally be set independently on the vv-ECMO circuit using the air/oxygen mixer (blender) and sweep gas flow regulator.
Sweep gas flow rate
Carbon dioxide elimination depends on sweep gas flow rate (set on the the sweep gas regular). The rate of sweep gas flow through the membrane lung determines blood decarboxylation. Carbon dioxide tension (pCO2) is mostly unaffected by FDO2 or ECMO blood flow once it is adequate.
Fraction Delivered O2 Gas
The fraction of delivered O2 gas (FDO2) is set on the blender, and directly affects oxygenation of the blood. The sweep gas flow rate has little effect on blood pO2/SO2.
1. Intensive Care Med 2013. Blood oxygenation and decarboxylation determinants during venovenous ECMO for respiratory failure in adults.
In the last step of a percutaneous tracheostomy, there are cases where it has been very difficult to cannulate the tracheostomytube into the trachea.
In these cases, it is important to first verify that the tracheotomy has been adequately dilated. If they has been done, I have found that the problem is with the Shiley tube and trocar. Even if you are meticulously careful aligning the bevel of the tracheostomy tube with the trocar, the edge of the Shiley tracheostomy tube can catch of tracheal rings. It then becomes very difficult to cannulate it into the trachea because any additional pressure or manipulation simply compresses the trachea rather and forcing the Shiley into the trachea.
In these circumstances, what I have found is that the edge of the Shiley tube has been distorted by catching on the tracheal rings. If this has occurred, I have replaced the Shiley tube with a new one and the procedure has proceeded smoothly from there.
Have you ever seen this or had this problem during your perc trachs? Share your experience below.
Fiberoptic oral intubation with flexible fiberoptic scope is useful technique for intubating a patient in respiratory distress which it is desirable to maintain spontaneously breathing during the intubation. Unlike nasotracheal intubation, it can be slightly more challenging because the scope must make a sharp turn in the posterior oropharynx before entering the hypopharynx. During nasotracheal intubation, the scope can make the turn in the nasopharynx and take a straight path to the trachea.
It is key that patient is adequately anesthetized. Sedation with medications that allow spontaneous breathing and allow the patient to protected the airway are useful, avoiding deep sedation and paralytics as used in RSI. These can include ketamine, droperidol, dexmedetomidine, etc.
Oral Airways for Intubation
Since the patient is generally in an upright position, the tongue, mandible soft tissues can fall backwards obstructing the airway. This can make passage of the fiberoptic scope difficult. This can be improved with a jaw thrust, but is facilitated greatly by the use of a specialized intubating oral airway.
There are three available oral airways including the Williams airway (shown below), Berman airway, and Ovassapian airway. They are all specifically designed for oral fiberoptic intubation to guide the fiberoptic scope and endotracheal tube around the oropharynx and keep the tongue and hypopharyngeal tissue from obstructing the trachea. The airways also protect the scope from being damaged if the patient bites down.
- Sedative medication
- Fiberoptic scope (eg bronchoscope)
- Lidocaine 4% cream and tongue blade
- Lidocaine nebulization
- Appropriately sized endotracheal tube
- Williams airway
- 10 mL syringe
- Securing device for endotracheal tube
- Capnography monitor / Colorimetric device
Once you have gathered all of the equipment and the patient has been adequately preoxygenated, you can begin the actual intubation steps.
- Remove 15mm adaptor form endotracheal tube and on fiberoptic scope.
- Place patient in comfortable sitting or near sitting position.
- Lidocaine nebulization.
- Lidocaine 4% cream placed on the back of the patient’s tongue. Have the patient gargle the cream as it runs down the back of their mouth.
- Ketamine dissociative medication.
- Insert the Williams airway.
- Advance the scope to the end of the Williams airway.
- Assistant can make minor adjustments to the position of the Williams airway that scope it is directed at the vocal cords.
- Inject lidocaine 4% liquid on cords.
- Advance the scope through the vocal cords and into the trachea.
- Advance the endotracheal tube over bronchoscope through the vocal cords.
- Remove the fiberoptic scope and the Williams Airway.
- Replace the 15 mm adaptor
- Inflate the endotracheal tube balloon and confirm placement.
- Provide patient extra sedation if required.
There are critical supplies that must be immediately for every ECMO patient, both for routine maintenance and in case of emergencies. We keep these supplies in a dedicated cart outside the patient’s room. The supplies are checked and restocked once per shift. The cart is portable and very convenient. We have a dedicated cart for each patient and they are prepared and stored ahead of time, ready for a new patient to arrive.
The contents have changed over the years, but this is the current configuration and how the drawers are organized.
For patients requiring prolonged mechanical ventilation a cuffed tracheostomy tube is required in place of the endotracheal tube. Traditionally this has been done with an open surgical procedure. However, a bedside procedure has been developed that allows the tracheostomy to be placed using the Seldinger technique with dilation of the dilation rather than dissection. It is formally known as a Percutaneous Dilational Tracheostomy (PDT) and can be done either with or with bronchoscopic guidance.
This is considered a minimally invasive bedside procedure that may be easily performed in the intensive care unit or at the patient’s bedside – with continuous monitoring of the patient’s vital signs.
Evaluation for Perc Trach
Two critically important preoperative criteria for PDT are:
- The ability to hyperextend the neck
- Presence of at least 1 cm distance between cricoid cartilage and suprasternal notch ensuring that the patient will be able to be reintubated in case of accidental extubation
Patients should not be considered for this procedure if they are:
- Children (younger than 12 years of age)
- Patients with severe coagulopathies
- Patients with unidentifiable landmarks
Perc Trach Techniques
There are several different systems and approaches for PDT, but the one in most widespread use is the Ciaglia. With this technique, there is no sharp dissection involved beyond the skin incision. The patient is positioned and prepped in the same way as for the standard operative tracheostomy. General anesthesia is administered and all steps are done under bronchoscopic vision.
Advantages of Perc Trach
The procedure itself is fairly easy to learn, especially in proctored settings. Time required for performing bedside PDT is considerably shorter than that for an open tracheostomy. Elimination of scheduling difficulty associated with operating room and anesthesiology teams for critical care patients. PDT expedites the performance of the procedure because critically ill patients who would require intensive monitoring to and from the operating room need not be transported. Cost of performing PDT is roughly half that of performing open surgical tracheostomy due to the savings in operating room charges and anesthesia fees.
The neck should be carefully palpated and all of the anatomy carefully identified (thyroid cartilage, cricoid cartilage, and 2-3 tracheal rings). The ideal location of the tracheostomy would be between the 1st and 3rd tracheal ring. Once you have identified your location, a horizontal skin incision made about 2-3 cm in length.
The pretracheal tissue is cleared by blunt dissection, until the trachea is clearly palpable. It need not be fully visualized.
The bronchoscope is feed through the endotracheal tube but kept with the tube itself. The endotracheal tube is withdrawn until the Kelly clamp can be seen bouncing between the tracheal rings. The cuff should not be higher than the level of the glottis. A laser can be used through the surgical would to also help guide the withdrawal of the endotracheal tube.
The introducer needle is then used to puncture the anterior wall of the trachea under direct bronchoscopic visualization.
The needle is withdrawn leaving the catheter.
A guidewire is fed through the catheter. On the bronchoscope, it should be seen going distally down the trachea towards the carina.
The catheter is removed, and the first small dilator is used to dilate the track.
The large progressive dilator is then used to further dilate the track over the extended catheter.
A tracheostomy tube with inner trocar is cannulated into the trachea over the extended catheter.
The tracheostomy tube trocar, wire, and extended catheter can then be removed. The tracheostomy tube should be inflated and the inner cannula insert. You should inspect the site for any cuff leak. The bronchoscope should be removed from the endotracheal tube, and placed down the tracheostomy tube to visualize the carina. Only then should the endotracheal tube be removed.
The tracheostomy tube is secured to the skin with sutures and the tracheostomy tape.
Dont forget to document your perc trach procedure well.