Sunday, April 11, 2021

Ventilator Management Case File

Posted By: Medical Group - 4/11/2021 Post Author : Medical Group Post Date : Sunday, April 11, 2021 Post Time : 4/11/2021
Ventilator Management Case File
Eugene C. Toy, MD, Manuel Suarez, MD, FACCP, Terrence H. Liu, MD, MPH

Case 9
You are called to recommend the settings of a mechanical ventilator for a  24-year-old man who is a near-drowning victim. The patient was intubated on site by the para­medics with an 8-mm  size endotracheal tube (ETT) ,  and placed on 100% F102 producing an O2 saturation (SaO2)  of 92%. The patient had attended an all-night New Year's Eve party.  He is arousable and responsive to deep stimuli. The chest x-ray shows bilateral  interstitial infiltrates with a normal heart size and clear cos­tophrenic angles. The ETT tip is 3 cm above the carina. His spontaneous respira­tory rate is 12 breaths/minute. His blood pressure is 100/50 mm Hg, heart rate is 150 beats/minute, and temperature is 95°F (35°C).  He weighs 70 kg (154 lb). He is awaiting transfer to the intensive care unit (ICU) for further treatment. 

 What  is the best initial mode of mechanical ventilation and settings for this patient? 
 What are the most common complications of mechanical ventilation? 
 What special problems are presented by near-drowning victims?

ANSWER TO CASE 9:

Ventilator Management

Summary : This is a 24-year-old man near-drowning victim presenting with adult respiratory distress syndrome (ARDS). His HR is 150 beats/minute and blood pressure is normal.
  • Best initial mode and mechanical ventilation settings: The goals in this patient are to achieve an adequate amount of ventilation and oxygenation while decreasing the work of breathing. Assist control (AC) mode following low-volume mechanical ventilation guidelines using 6 to 8 mL/kg as a starting tidal volume (Vt) with a goal of a plateau pressure of <30 cm H20 would achieve these goals. Other initial settings include a rate of at least 16 breaths/minute, an FIO2 of 1 00%, and a positive end expiratory pressure ( PEEP) of 5 cm H2O.
  • Most common complications of MV: Barotrauma, aberrant (esophageal) intubation, and right main stem bronchus intubation.
  • Special problems in drowning victims: Controlling hypothermia and addressing atelectasis.
ANALYSIS

Objectives
  1. To begin with the mechanical ventilator parameters which will best assure an acceptable pH, PACO2, and PAO2 (eg, AC of 16, Vt 6-8 mL/kg, FIO2 100%, PEEP 5 cm H20 ) .
  2. To switch t o pressure support a s soon a s possible t o increase patient comfort and reduce need for sedation.
  3. To keep head of bed elevated at minimum of 45 degree as main key to aspiration precautions.
Considerations
The patient is a 24-year-old man near-drowning victim with ARDS. He needs rapid sequence
intubation (RSI) with MY. Because of the ARDS, a low Vt strategy of 6 to 8 mL/kg with a plateau pressure <30 mm Hg is indicated. The ETT should be sized 8 mm or more to allow fiberoptic bronchoscopy (FOB) to evaluate for any aspirated material while maintaining MV. Hypothermia may alter the patient's neurological status, which cannot be accurately evaluated until core temperature correction. When the chest x-ray shows atelectasis or pulmonary volume loss, especially with high ventilation pressures, an obstruction of the bronchi should be suspected. FOB should be considered to determine if sand or other foreign objects were aspirated.

Approach To:
Mechanical Ventilation

Continuous oxymetry reading is standard in all MV patients. The  goal is to keep O2 saturation equal or exceeding 90%, which equals a PAO2 of 60 mm Hg. This is a time-tested goal of oxygenation and the elbow of the  hemoglobin disassociation curve. An improving or normal neurological status is a secondary predictor of a good outcome. Interestingly, most near-drowning victims suffer more from asphyxiation (pharyngeal spasm from fear of breathing under water) and dry lungs rather than aspiration of water and wet lungs. Increased levels of PEEP and 100% FIO2 may be required to maintain this Osat goal (see Table 9-1). PEEP can prevent the col­lapse of small airways and alveoli by maintaining alveolar recruitment. PEEP further improves oxygenation and matching of ventilation with perfusion (VQ).

MECHANICAL VENTILATORS 
The most common reason for mechanical ventilation (MV) is respiratory fail­ure due to sepsis, pneumonia, ARDS, COPD, pulmonary edema, or coma. The objective of MV is to decrease the work of breathing and to reverse life-threatening hypoxemia, hypercarbia, and acidosis. The work of breathing is redistributed back to the systemic circulation (kidneys, heart, brain, gut). MV is delivered via an ETT or tracheostomy tube. The ETT has more dead space than a tracheostomy tube; thus the tracheostomy patient requires lower tidal volumes. The use of fiberoptic-assisted ETT is easier than direct laryngoscopy and has the added benefit of clearly seeing the ETT pass through the vocal cords into the trachea. The IV administration of lidocaine prior to intubation may decrease cardiac arrhythmia and blunt the unde­sired responses induced by ETT insertion into the trachea. 

The MV is a machine,  with adjustable variations in cycling modes between inspiration (inhalation) and expiration (exhalation). Independent variables are set and monitored by microprocessors and displayed on a monitor. MV can control many different means of delivering a positive pressure breath to the patient. This inspiration under positive pressure created by the MV totally reverses the normally negative inspiratory cycle in the spontaneously breathing patient. Some of the more common ventilator modes include: assist control (AC), synchronized intermittent ventilation (SIMV), pressure support ventilation (PSV), controlled mechanical ventilation (CMV), and pressure release ventilation (PRV). 

common mechanical ventilation modes

Tidal volume (Vt), fractional inspired concentration of oxygen (FIO2), respira­tory rate (RR), positive end expiratory pressure (PEEP), peak inspiratory pressure (PIFR), humidification, and warming of inspired air can all be controlled by the MV.

The different MV settings provide a predetermined mixture of patient-initiated (spontaneous) and MY-delivered controlled breaths (see Table 9-1 for common MY modes). The best choice is the  one that delivers and meets the physiologic needs for oxygenation and ventilation while maintaining patient comfort and decreasing the need for sedation.

MVs have sensors that must be activated to deliver an MV breath. Inside the MV tubing, an artificial nose humidifies the respiratory circuit. The artificial nose reduces contamination by respiratory water-borne pathogens by eliminating water reservoirs. Upper airway heat and humidification also is achieved with the patient's own respiratory system. The respiratory circuit tubing should not be changed unless there is a reason (eg, leak). Reduced manipulation of the MV circuitry has decreased patient infection rates and contamination by resistant organisms. MV circuits are equipped with a  built-in reusable suction catheter. This is a clean closed system, in which a collapsible plastic cover built in to the suction catheter allows for its reuse as needed. MV also has the flexibility to allow for the in-line delivery of aerosolized medications without disconnecting the patient from the MV. 

Medications commonly used in MV include β2 agonists, ipratropium bromide, steroids, antibiotics, and mucolytics. Invasive ventilation with MV is needed when noninvasive ventilation (NIV) fails or in situations requiring airway control. Patients intubated for respiratory failure develop respiratory muscle fatigue,  and  muscle retraining is required. Muscular dysfunction must be reversed. Anxiety, which is the most common treatable side effect of MV, can be minimized with pressure support MV and using patient-driven MV modes (SIMV). SIMV is associated with improved synchronization between the patient's natural breathing pattern and the MV. The respiratory demand and the amount of required ventilator support determines the mode of ventilation (see Table 9-2). 

Assist control ventilation is usually the initial MV mode since delivery of a backup respiratory rate and minute ventilation is assured regardless of patient contribution. SIMV or IMV are equivalent since all IMV devices are synchronized. The main goal of MV is to supply needed ventilation and oxygenation by retraining and strengthen­ing the respiratory system and resting the fatigued respiratory muscles. An eventual goal is to exercise the rested muscle to allow successful extubation. Extubation is considered as successful when reintubation is not required within the next 48 hours

Daily portable chest x-rays are advised for all MV patients during the acute course of the disease. This aids not only in evaluating the placement of ETT, the

general principles to mechanical ventilators

recognition of new infiltrates, the development of barotrauma, and the placement of central venous line, but also in detecting abnormalities of NGT or feeding tubes. The extension of the chin away from the chest can move the ETT down and selectively intubate the right main stem bronchus. In contrast, flexion of the chin toward the chest can pull the ETT up and extubate the patient if the ETT is not properly placed. The recommended placement of the tip of the ETT is 3 to 4 cm above the carina (T4 level) to avoid these changes due to chin placement.

ASSIST CONTROL VENTILATION
In AC, MV breaths are delivered at a preset rate and tidal volume. If a spontaneous breath is not generated within a specified time, a mechanical breath will be delivered at a scheduled time period depending on the rate set. For example, the MV will cycle a breath every 3 seconds for set rate of 20 breaths/minute, even if no spontaneous breath occurs within that minute. The patient can only breathe and receive MV breaths above the set rate, but never below it. Lack of coordination of the patient's breathing with the MV breaths may cause significant patient discomfort and an increase in the work of breathing (see Figure 9-1 for waveform representation of different MV modes ).

The goals of MV are to provide adequate minute ventilation (Vm = rate X Vt) and minimize the risk of barotrauma. In an AC mode, if the patient breathes above the set MV respiratory rate, the machine will deliver another full MV breath which can lead to an acute respiratory alkalosis. Tachypnea on the AC MV mode can lead to the stacking of MV breaths with trapping of air as the expiratory time decreases.
This results in auto PEEP. Intrinsic and auto PEEP are factors of fast respiratory rates and shorter exhalation times (reverse I-E ratios ) . If during exhalation the pressure does not return to baseline (0) , this intrinsic PEEP increases the inspiratory effort; the patient's next inspiratory effort must overcome this new baseline pressure to initiate the next spontaneous breath. Lengthening the expiratory time by decreasing the volume or rate alleviates this problem and decreases the intrinsic PEEP/auto PEEP. Disconnecting a patient from the MV circuit loses the PEEP and the alveoli that had been recruited. These alveoli will now collapse and are difficult to re expand. Should this auto PEEP be the cause of hemodynamic deterioration, the disconnecting maneuver can be life saving. Intravascular volume should be increased with fluids and PEEP levels should be decreased.

SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION
SIMV is similar to AC, except that breaths that are spontaneously generated by the patient occur without activating an MV breath. Breaths initiated by the patient are only in the Vt amount generated by the patient and not by the MV. Both ventilator breaths and spontaneous breaths generated by the patient can be assisted with pressure support ventilation (PSV) . SIMV should not be used alone without PSY since this actually increases the work of breathing. SIMV of at least 5 cm H20 of PS should be applied. All IMV are synchronized to keep a ventilator breath from pushing against a patient's natural or forced exhalation. This will avoid possible barotrauma and pneumothorax (PTX) secondary to increased intra bronchial pressures.

Ventilator Management ECG
Figure 9-1. Mechanical ventilator modes. Waveforms of various mechanical ventilator modes dem­onstrating volume versus time. (Reproduced, with permission, from Gomella LG,  Haist SA. Clinician's Pocket Reference. 11th  ed.  New Yo rk, NY: McG raw-Hill Education; 2007; Figure  20-20.)

PRESSURE SUPPORT/CONTROLLED VENTILATION
Pressure support ventilation ( PSV) and pressure-controlled ventilation (PCV) were originally designed for weaning or liberating the patient from MV. These modalities should be used in combination when SIMV is used, since SIMV needs a support mode. In PSV, the patient is spontaneously breathing and each patient breath is assisted by a preset amount of "pressure support" measured in centimeter H2O. Pressures can be set for inhalation and exhalation, as well as for continuous or intermittent application. With increased inhalation pressure, Vt increases. This effect usually reaches a maximum at pressures of 25 to 30 cm H2O. PSV acts as an assist device that allows the patient to establish the Vt and respiratory rate at levels which are most comfortable. This mode does not require much sedation.

PCV is the same as PSV except there is a preset pressure that must be reached with every patient-initiated breath. PCV activates itself only if the preset pressure is not reached by the patient's own efforts. It is technically referred to as a series of ventilations that are time triggered, time cycled, and pressure limited. PSV and PCV are ideally used when low airway pressure is required as in patients with a pneumothorax, and when there are concerns of barotraumas.

CONTINUOUS POSITIVE AIRWAY PRESSURE
CPAP is the most commonly used MV support mode to reduce the need of or liberate a patient from MV. In CPAP, there is a continuous pressure so that each inspiration is assisted by a preset amount of pressure. Since CPAP is continuous, it acts as PEEP during exhalation. Ventilation on CPAP occurs on spontaneous breaths by the patient. No preset mechanical breaths occur, which leads to more patient comfort and a decreased need for sedation.

AIRWAY PRESSURE RELEASE VENTILATION
Airway pressure release ventilation (APRV) is another form of MV that allows patients to breathe spontaneously over intermittent and variable levels of CPAP. APRV may be thought of as alternating levels of CPAP with or without pressure support. APRV allows patients to breathe spontaneously over intermittent and alternating levels of CPAP. These alternating levels of CPAP are termed, at the higher
level (Phigh) , which is a recruiting maneuver, and with an alternating lower set pressure (Plow) or PEEP, which maintains patency of the recruited airways. In an APRV,  the inspiratory cycle is set by the length of the inspiratory time. For example, cycle time of 6 seconds = RR of 10 breaths/minute (60 sec/6 sec = 10 cycles). APVR works best at respiratory rates of < 15 breaths/minute, preferably near 12 breaths/minute or less. Alveoli are recruited, preventing their collapse, by the continuous pressure set and maintained by APVR.

JET VENTILATION AND HIGH-FREQUENCY VENTILATION
Jet ventilation is rarely used in routine practice. Bronchopleural fistula is one condition in which high-frequency ventilation (HFOV) may assist healing by having low inflation pressures. For HFOV, the patient must be temporarily paralyzed since oxygenation is controlled by diffusion and CO2 by ventilation. The use of paralytic agents has significant neurological sequelae in patients who eventually recovered. A small Vt is delivered at very high respiratory rates in the range of 180 to 600 breaths/ minute. The HFOV or jet ventilation is an alternative mode of MY that protects the lungs. Most clinical trials on HFOV have been performed on neonates. Awareness of the injurious effects of mechanical ventilation has led to a renewed interest and advances in using HFOV in adult ALI/ARDS patients. HFOV is characterized by rapid oscillations of a diaphragm ( at frequencies of 3-10 Hz, ie, 180-600 breaths/ minute) driven by a piston pump.

The pressure swings become more attenuated as they move distally from the airways to the alveoli, resulting in a very small Vt. Use of HFOV in inhalation injuries has been an effective treatment. Alsaghir and colleagues demonstrated that prone positioning may improve oxygenation and reduce mortality in patients with ARDS or more severe illnesses. The prone position makes nursing care much more difficult.

VOLUMETRIC DIFFUSIVE RESPIRATION
Volumetric diffusive respiration (VDR), a form of high-frequency ventilation, is very effective in cystic fibrosis and smoke inhalation patients due to the copious secretions. YDR requires a high degree of respiratory therapy education and time. YDR acts as a high-frequency percussive ventilator, enabling the ability to clear copious secretions.

SEDATION AND OTHER RELATED MV ISSUES
The major goal of sedation in MY is to control anxiety and provide better coordination between the patients' own breathing efforts and the MV. Propofol (recently made famous by the death of singer Michael Jackson) is a frequently used agent for this purpose. Propofol is a short-acting, intravenously administered hypnotic agent. Its uses include induction and maintenance of general anesthesia, sedation for mechanically ventilated adults, and sedation for procedures. It is an extremely short-acting agent, which often causes vasodilation with hypotension; the hypotension usually responds to increased fluids or discontinuation of the drug. In critically ill patients, propofol is superior to lorazepam both in effectiveness and overall cost. A favorite agent of neurologists and neurosurgeons, it decreases intracranial pressure and is rapid cleared enables quick evaluation of the patient's mental status merely by discontinuing infusion. It has no analgesic properties.

A propofol syndrome has been described about 1% of patients, consisting of rhabdomyolysis and metabolic acidosis. Propofol produces sedation without causing respiratory depression which makes MY easier to deliver. Paralytic agents should be avoided unless absolutely necessary because of its frequent neurological sequelae.

Acid suppression with a proton pump inhibitor (PPI) or H2 antagonist is recommended to prevent GI bleeding from gastric sources; the main drawback is increased bacterial overgrowth because of the acid suppression, leading to aspiration pneumonia with potentially antibiotic-resistant pathogens. H2 antagonists may have an advantage over PPI in this instance. The dose and delivery of aerosolized medications such as β2 agonist and ipratropium bromide should be doubled above the standard dose (2 U dose vials) for MV patients with an ETT. The ETT increases the area of aerosol deposition thus requiring larger volume of medication to reach the airways. Ninety percent of the volume of aerosolized medications remains in the tubing of the aerosol delivery equipment.

CLINICAL CASE CORRELATION
  • See also Case 8 (Airway Management) , Case 11 (Asthmatic Management), and Case 12 (Non,invasive Methods of Ventilatory Support).

COMPREHENSION QUESTIONS

9.1 Low-volume ventilation is needed for a septic patient with ARDS and severe hypoxia on 90% FIO2• The chest x-ray shows bilateral infiltrates with a normal heart size, a typical x-ray presentation ARDS. The patient weighs 80 kg. What is the correct amount of tidal volume to begin with for this patient on a mechanical ventilator?
A. 750 mL tidal volume
B. 480 mL tidal volume
C. 300 mL tidal volume
D. 550 mL tidal volume
E. 250 mL tidal volume

9.2 You are called to evaluate a mechanically ventilated patient for new onset hypotension. The patient has a blood pressure of 100/60 mm Hg, with a 20 mm Hg of pulsus paradoxicus and increased JVD at 45 degree of HOB elevation. The patient has wheezing throughout both lungs and is breathing at 35 times/minute on mechanical ventilator settings of SIMV 20 breaths/minute, Vt of 800 mL, PS of 10 mm Hg, a PEEP of 10 mm Hg, and FIO2 of 40% . ABO results on these setting are pH 7.36, PACO2 45mm Hg, PAO2 77 mm Hg. Which of the following would you advise to do next to relieve the hypotension?
A. Decrease the PEEP and auto PEEP by decreasing rate and tidal volume.
B. Start vasopressors to reverse hypotensive effect of PEEP.
C. Increase PEEP to improve hemodynamics
D. Change to assist control mode and keep PEEP the same.
E. Do not make any changes.


ANSWERS TO QUESTIONS

9.1 B. Previously, it was thought that tidal volumes (Vt) of 10 to 15 mL/kg were required to prevent atelectasis during MV; however, these higher volumes are no longer used. The Surviving Sepsis Guidelines recommend a strategy of low volume ventilation using a Vt of 6 to 8 mL/kg with a plateau pressure <30 mm Hg H20. This method is referred to as low-volume ventilation and is very effective
in supporting patients with sepsis and ARDS. The best initial Vt is approximately 480 mL using 6 mL/kg. The arterial pH should be kept at 7.20 or higher. Bicarbonate treatment should only be considered when the pH is below 7.20. This low-volume ventilation method is effective in preventing and treating ARDS. Sedation will be needed for patient comfort with this method.

9.2 A. Increased PEEP or auto PEEP increases the intrathoracic pressure ( ITP ) , and decreases venous return t o the heart. This reduces right ventricular filling and can decrease cardiac output, leading to hypotension. Attempts should be made to increase expiratory time to decrease auto PEEP. With severe hypotension due to auto PEEP, disconnecting the patient from the MV and allowing a long expiration will generally correct the hypotension. Lowering the auto PEEP levels can be achieved by decreasing the respiratory rate, decreasing the tidal volume, and increasing the expiratory time. Giving IV fluids and expanding the intravascular space helps reverse the decreased preload and assists in restoring the blood pressure by restoring intravascular and right ventricular filling volume.

CLINICAL PEARLS
 Low-volume  ventilation is important in patients with ARDS to  prevent alveolar barotrauma. 
 Pressure support ventilation (PSV) helps reduce the need for sedation. 
 Acid suppression with proton-pump inhibitors or histamine-2 (H2) blockers  heightens the risk for resistant bacterial pneumonia in hospitalized patients, PPI more than H2 blockers. 
 PSV increases patient comfort and resistance to spontaneous respiration by patient. 
 Avoid neuromuscular blockade since  this is associated with long-term neurological defects. 
 Do not change the plastic circuitry of MV unless needed.  Unnecessary changing of tubing increases the rate of infection. 
 The use of artificial noses for humidification in MV circuitry reduces the risk of waterborne respiratory infections compared with the use of in line humidifiers. 
 Volumetric diffusive percussive  respiration (VDR)  enables clearance of copious secretions resulting from smoke inhalation and patients with cystic fibrosis. 

References

Alsaghir Ah, Martin CM. Effect of prone positioning in patients with acute respiratory distress syndrome: a meta-analysis. Crit Care Med . 2008;3 6 ( 2 ) :603 -609 . 

Loscalzo J. Harrison's Pulmonary and Critical Care Medicine. New York, NY: McGraw-Hill; 201 1 . 

Malhotra A . Clinical therapeutics, low-tidal-volume ventilation in the acute respiratory distress syndrome . N Engl ] Med. 200 7 ;3 5 7 : 1 1 1 3 - 1 1 20. 

Tobin MJ . Advances in mechanical ventilation. N Eng! ] Med. 200 1 ;344: 1 9 86 - 1 996. 

Toy EC, Simon B, Takenaka K, Liu T, Rosh A. Case Files Emergency Medicine . 2nd ed. New York: McGraw-Hill; 2009.

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