Mechanical Ventilation

Mechanical Ventilation Basics:

ABG: arterial blood gas

A/C: Assist/control

EVT: Exhaled VT

PCV: Pressure control ventilation

PIP: Peak inspiratory pressure: Highest pressure encountered

PSV: Pressure support ventilation

SIMV: synchronized intermittent mandatory ventilation

VT: Tidal volume: 5 mL/kg

VC: Volume control

Ve: Minute ventilation

I:E ratio is the duration of time spent in the inspiratory cycle compared with expiration.

NIF: max insp pressure: normal -50 to -100 cm H2O. Must be at least -20 cm H2O

VC: vital capacity, 15 mL/kg or at least 1 liter

RR: <35

pH: >7.25

PaCO2: <55

PaO2/FiO2: normal 475, must be >300

PaO2 >70

Compliance: change in volume/change in pressure

    • In normal lungs I:E ratio is approximately 1:2. One third of the respiratory cycle is spent in inspiration while two third of the respiratory cycle is spent in expiration. In an attempt to mimic spontaneous respiration, the initial I:E is set between 1:2 and 1:4.

    • Longer inspiratory times increase airway and intrathoracic pressures, which may lead to hemodynamic instability.

    • Shorter inspiratory times may cause air trapping and overdistention of the most compliant alveoli while lesser compliant alveoli are suboptimally ventilated or not ventilated at all.

    • The speed at which VT is delivered is determined by flow rate. The inspiratory flow rate is the primary determinant of inspiratory time and of I:E ratio. Although the usual initial flow rate is between 40-60 L/min, flow rate must be customized for each patient based on the desired I:E ratio, respiratory rate, and VT. High flow rates decrease inspiratory time while increasing expiratory time, resulting in turbulent airflow, air trapping, and higher peak inspiratory pressures (PIP). Lower flow rates increase inspiratory time, reduce PIP, and create a more laminar airflow, which improves alveolar gas distribution.

    • "Sensitivity" specifies the inspiratory effort required to trigger an assisted breath in assist/control (A/C) (volume control - VC) mode or to receive gas flow during a spontaneous (SIMV), continuous positive airway pressure (CPAP), and pressure support ventilation (PSV) modes. Sensitivity settings range from 0.5 cm H2O to -30 cm H2O below the baseline pressure with the usual initial setting being -2 cm H2O. If the sensitivity setting is set too low (-30 cm H2O), the patient must generate more inspiratory effort to trigger an assisted breath or the flow of gas. If the sensitivity setting is set too high (-0.5 cm H2O), the ventilator will cycle inappropriately in response to the slightest patient movement.

Initiation of mechanical ventilation.

  • Ventilator type

      • Depends on cycling mechanism. Insp → exp.

      • Timed cycled

      • Volume cycled ventilator are mostly used in ICUs. Has square wave from on flow-time

      • Pressure control: set peak insp pressure. Has descending ramp

  • Mode of ventilation

      • Assist control (AC). Most common initially used mode of ventilation in Pts with resp. failure.

        • Patient gets a ventilator-delivered breath for every Pt initiated inspiratory effort.

        • Controlled ventilator-initiated breaths are delivered automatically when the Pt's spontaneous breathing rate falls below the selected backup rate.

        • For example if the patient breathes at 8 bpm and the AC set rate is 12, the ventilator delivers 12 breaths.

        • What happens if the patient is tachypneic and has a RR of 24, and the AC set rate is 12?

          • The ventilator is compelled to deliver the patient initiated rate of 24. So, respiratory alkalosis is a potential concern when using ACV for patients who are tachypneic.

        • Best initial mode of ventilation to patients with normal respiratory drive but who present with respiratory acidosis, in that the respiratory rate will be determined by metabolic need.

      • Intermittent mandatory ventilation (IMV). Allows Pt to breath at a spontaneous rate and tidal volume without triggering the ventilator, while the ventilator adds additional mechanical breaths at a preset rate and tidal volume.

        • Synchronized intermittent mandatory ventilation (SIMV). The ventilator allows the patient to initiate own breath. It senses the patient's respiratory efforts at intervals determined by the frequency setting. This allows coordination of the ventilator driven breath with the respiratory cycle of the patient to avoid inadvertent stacking of a mechanical breath on top of spontaneous inspiration. There is less respiratory alkalosis, fewer adverse cardiovascular effects due to lower intrathoracic pressures, less requirement of sedation and paralysis, maintenance of respiratory muscle function, and facilitation of longterm weaning. Some patients have respiratory muscle fatigue and result in failure to wean. This can be avoided by initiating PSV at 4 - 8 cm H20 or addition of flow-by, or both.

      • PCV pressure control mode

      • PSV augments each patient triggered respiratory effort by an operator specified amount of pressure that is usually between 5 and 50 cm H2O. PSV is used to augment spontaneous resp efforts during IMV modes of ventilation during weaning trials. Can also be used as a primary form of ventilation in patients who can trigger the ventilator spontaneously.

      • CPAP is similar to PEEP except that it is used with spontaneously breathing patients. Positive pressure is applied during the entire respiratory cycle rather than just at the end. CPAP is used as a primary ventilatory mode to reexpand atelectatic lungs or as a weaing mode. It increases FRC and PaO2 while decreasing intrapulmonary shunt, work of breathing, and oxygen consumption.

      • PEEP is indicated for any condition associated with widespread alveolar collapse. It aids in gas exchange by increasing FRC. FRC is that volume of gas that remains in the lungs to be available for gas exchange at any phase of the inspiratory cycle. Without it, PaO2 would fluctuate with inspiration and expiration.

        • If PaO2 is already greater than 60 mmHg or FiO2 of 40% or less, the use of PEEP is unnecessary and puts the patient at undue risk of lung injury. However, because there is 3-5 cm H2O of "physiologic PEEP" in the airways of spontaneously breathing patients, an initial PEEP level is usually set at 5 cm H2O. Above 5 cm H2O, PEEP should be applied in 2-3 cm intervals and should be guided by hemodynamic parameters (ie. CO, BP, and HR), and by ABG.

      • IRV. Inspiratory-to-expiratory ratio is greater than the standard 1:2 to 1:3 (i.e., >1:1) to stabilize terminal respiratory units (alveolar recruitment) and to improve gas exchange primarily for patients with ARDS. Goal is to decrease peak airway pressures, to maintain adequate alveolar ventilation, and to improve oxygenation. The use of IRV can be considered in Pts with PaO2 <60 mm Hg despite an FiO2 of >60%, peak airway pressures >40 - 45 cm H2O, or the need for PEEP of >15 cm H2O. However, lung strain can be greater in ALI when IRV is used.

      • Lung-protective, pressure-targeted ventilation (permissive hypercapnia). Controlled hypoventilation with tidal volume (Vt < 6 mL/kg) allows to achieve elevated PaCO2. Used in patients with respiratory failure due to asthma and ARDS. Prevents ventilator-induced lung injury. Additional methods of minimizing lung injury while improving oxygenation during mechanical ventilation in ARDS include prone positioning and the administration of NO. The administration of steroids is controversial but may show benefit. For patients with asthma, the use of helium-oxygen mixture may result in improved lung mechanics compared to the use of oxygen alone.

      • Independent lung ventilation uses a double lumen ETT and two independent ventilator. Used for severe unilateral lung disease, such as unilateral pneumonia, respiratory failure associated with hemoptysis, or a bronchopleural fistula.

      • High-frequency ventilation uses rates that are substantially faster (60 - 300 bpm) and small Vt (2 - 4 mL/kg).

    • Airway pressure release ventilation (APRV) is a form of pressure ventilation, which supports spontaneous breathing on two alternating levels of CPAP "Bi-Phasic Ventilation" (a.k.a.: Bi-Level, BiPAP). APRV switches automatically and regularly between the two operator selected levels of CPAP (pHigh & pLow /PEEP). Cycling between the two levels of CPAP is time triggered. APRV incorporates the "Open Breathing concept/ Floating Exhalation Valve" (the exhalation valve is never occluded completely). This allows the patient to freely inhale or exhale during the entire respiratory cycle (promoting increased patient comfort, patient/ ventilator synchrony, and decreased sedation/ analgesic requirements). It applies CPAP to maintain adequate lung volume and promote alveolar recruitment. The ventilator cycles from high CPAP to low CPAP (high lung volume to lower lung volume), and the patient can breath spontaneously at either level.

      • Indications:

        • PaO2/ FiO2 ratio < 300. ALI

        • PaO2/FiO2 ratio <200. ARDS

        • Bilateral infiltrates (consistent w. edema-patchy, diffuse).

        • No evidence of left atrial hypertension.

      • Relative Contra-Indications:

        • Serve COPD, emphysema

        • Pneumothorax

        • Blood pressure: < 90 systolic or < 60 mean

        • Unilateral lung disease

      • The expiratory time is the key variable - it should be short enough to prevent derecruitment and long enough to obtain a suitable tidal volume. The expiratory time is set between 0.4 to 0.6 seconds - the tidal volume is your target (between 4 and 6ml/kg). If the tidal volume is inadequate, the expiratory time is lengthened; if it is too high (>6ml/kg) the the expiratory time is shortened. Maintains expiratory lung volume & prevents alveolar closure during the release phase.

      • The high CPAP (PEEP) level is set at the mean airway pressure level from the previous mode (pressure control, volume control etc). If you are starting off with APRV then start high (28cmH2O of less) and work your way down. Higher transalveolar pressures recruit the lungs.

      • Low PEEP is set at 0 cmH2O. The large pressure ramp allows for tidal ventilation in very short expiratory times. Allows for a rapid peak expiratory phase, augmenting CO2 removal.

      • The inspiratory time is set at 4-6 seconds (the respiratory rate should be 8 to 12 breaths per minute - never more). Allows for alveolar recruitment.

      • Neuromuscular blockade should be avoided: the patient should be allowed to breath spontaneously (this is beneficial). The breaths can be supported with pressure support - but the plateau pressure should not exceed 30cmH2O

      • Rise time, trigger and FiO2: Set FiO2 per. Patients oxygen needs: PO2 60-80/ spO2 90-92%, unless Hb < 7 gm (then 95% or physician order). Note- spO2 88-90% ok if FiO2 > 60%, with Hb > 6, & mean BP > 50.

      • There are two different ways to wean patients from APRV. If lung mechanics rapidly return to normal, the patient should be weaned to pressure support. If ARDs is prolonged, then the high CPAP level is gradually weaned down to 10cmH2O, and then the patient is converted to a standard vent wean

      • Mechanical ventilation with inhaled NO has been demonstrated to improve gas exchange in adults and children with respiratory failure, including patients with ARDS, primary pulmonary HTN, cor pulmonale secondary to congenital heart disease, and after cardiac surgery or heart or lung transplantation. IH NO acts selectively at the PA, causes PA vasodilation, deceases PA pressures without reducing BP or CO, and improving oxygenation by reducing intrapulmonary shunt. Generally, 5 - 20 ppm NO is administered, and the level of methemoglobin is monitored periodically.

Ventilator support criteria

Nasal cannula:

1 L/min: 21% - 24%

2 L/min: 25% - 28%

3L/min: 29% - 32%

4L/min: 33% - 36%

5L/min: 37% - 40%

Nonrebreather mask:

6L/min: 60%

7L/min: 70%

8L/min: 80%

9L/min: 90%

10 - 15L/min: 95% - 100%

Venturi mask for Pt with chronic hypercarbia and moderate to severe hypoxemia. Allow precise administration of O2. Used mostly in patients with COPD with hypercarbia, because O2 can be titrated to minimize CO2 retention.

24%, 28%, 31%, 35%, 40%, and 50%

NIPPV:

    • CPAP: can be used if PaO2 < 60 - 65 mm Hg with NRM and pt is conscious, cooperative, able to protect airway, and hemodynamically stable. Delivered by a tight-fitting mask equipped with pressure limiting valves. Pt. who do not tolerate CPAP because of claustrophobia or aerophagia, hypoxemia, hemodynamic instability should have endotracheal intubation. Initially 3 - 5 cms of H2O of CPAP should be applied and if the PaO2 is still < 60 mm Hg (SaO2 <90%), the level of CPAP should be increased in steps of 3 - 5 cm H2O up to a level of 10 - 15 cm H20).

    • BiPAP: Both inspiratory and expiratory pressures can be applied. The IPAP decreases the patient's work of breathing and the CPAP improves gas exchange by preventing alveolar collapse. In using BiPAP, an IPAP which is a.k.a PSV (pressure support vent) + CPAP 10 - 15 cm H20, and EPAP = CPAP of 3 - 5 cm H20 are reasonable starting points. PSV can be increased incrementally 3- 5 cm H20 using the Pt's respiratory rate as a guide. O2: 4 - 10 L or 50 - 100%. RR: 14 - 20 bpm. If CO2 is high, ▲ IPAP (Vt) or ▲ RR. If O2 is low, ▲ PEEP or ▲ O2.

VENTILATOR MANAGEMENT

    • FiO2. Hypoxemia is more dangerous than brief exposure to high inspired levels of oxygen. Initial level of oxygen should be 100%. Adjustments in the FiO2 can be made to achieve a PaO2 of >60% or an SaO2 of >90%.

    • Minute Ventilation is determined by the respiratory rate and tidal volume. In general, a RR of 10 - 15 breaths/min is an appropriate rate with which to begin. In COPD and CO retainers, the minute ventilation is adjusted to achieve the patient's baseline PaCO2 and not necessarily a normal PaCO2. Inadvertent hyperventilation with resultant respiratory alkalosis in these patients may be associated with serious serum electrolyte shifts and arrhythmias. Initial tidal volumes can be set at 10 - 12 mL/kg. Pts with decreased lung compliance (ARDS) often needs smaller lung volumes (6 - 8 mL/kg) to minimize peak airway pressures and iatrogenic morbidity.

    • PEEP is defined as the maintenance of positive airway pressure at the end of expiration. It can be applied to spontaneous breathing Pt in the form of CPAP or to the Pt who is receiving mechanical ventilation. PEEP improves lung compliance and oxygenation while decreasing the shunt fraction and the work of breathing. PEEP increases peak and mean airway pressures, which can increase the likelihood of barotrauma and cardiovascular compromise. PEEP is used primarily in Pts with hypoxic respiratory failure (e.g., ARDS, cardiogenic pulmonary edema). Low levels of PEEP (3 - 5 cm of H2O) may be useful in COPD, to prevent dynamic airway collapse during expiration. The main goal of PEEP is to achieve a PaO2 of >55 to 60 mm Hg with an FiO2 of <60% while avoiding significant cardiovascular sequelae. Usually, PEEP is applied in 3 - 5 cm H2O increments during monitoring of oxygenation, organ perfusion, and hemodynamic parameters. Pts who receive significant levels of PEEP (i.e., >10 H2O) should not have have their PEEPs removed abruptly, because removal can result in collapse of distal lung units, the worsening of shunt, and potentially life-threatening hypoxemia. PEEP should be weaned in 3 to 5 cm H2O increments while oxygenation is monitored closely.

    • Inspiratory Flow Rate. Flow rates set inappropriately low can be associated with prolonged inspiratory times leading to development of auto-PEEP. Resultant stacking of breaths and lung inflation can adversely impair venous return to the heart. COPD patients are at greatest risk. Increasing the inspiratory flow rate and prolonging the expiratory phase (I:E) time helps to mitigate this process.

  • Trigger sensitivity. Most mechanical vent use pressure triggering either to initiate a machine-assisted breath or to permit spontaneous breathing between IMV breaths, or during trials of CPAP. The patient must generate a decrease in he airway circuit pressure equal to the selected pressure sensitivity. Most patients do not tolerate a trigger sensitivity of < -2 cm because of autocycling of the ventilator. The smallest triggers sensitivity should be detected, allowing the patient to initiate mechanical or spontaneous breaths without causing the ventilator to autocycle.

    • Flow-by. Refers to triggering of the ventilator by changes in the airflow as opposed to changes in airway pressures.

The following suggests the need for mechanical ventilation:

  • VC <10-15 mL/kg

  • NIF weaker than -20 cm H2O

  • Vt (tidal vol) <5 mL/kg

  • RR >35 breath/min

  • Ve: Minute vent (Vt x RR): <10 L/min

  • Rise in PaCO2 >10 mm Hg

  • Alveolar-arterial gradient (FiO2 - 1): >450

  • PaO2 with supplemental O2: <55 mm Hg

  • PaO2/FiO2 <150

  • Physical appearance: labored breathing, retracting, nasal flaring, paradoxical abdominal breathing

  • Mental status: comatose or not protecting the airway.

Initial Mechanical ventilator settings:

PROTOCOL for LUNG PROTECTIVE VENTILATION

  • Mode of ventilation: ACV

  • Tidal volume: Set initial Vt at 8 mL/kg using the patient's predicted body weight (PBW):

    • Males: PBW = 50 + [2.3 x (Ht in inches - 60)]

    • Females: PBW = 45.5 + [2.3 x (Ht in inches - 60)

  • RR (minute ventilation): 16 - 24 bpm. Do not exceed rate of 35/min

  • FiO2: 100% (1)

  • PEEP: 5 - 7 cm H20

  • Reduce by 1 mL/kg q2hr until Vt = 6 mL/kg

  • Adjust FiO2 and PEEP to keep PaO2 >55 mm Hg or SaO2 >88%

  • When VT is down to 6 mL/kg, measure:

    • Plateau pressure (Ppl): actual pressure in the alveoli themselves at the end of insp.

    • Arterial PCO2 and pH

    • If Ppl >30 cm H20 or pH: <7.3, then follow protocol for low volume ventilation in ARDS

Respiratory Failure: ▼ RV preload, ▼ LV afterload

Hypoxic Pt.

  • Mode of ventilation: ACV or SIMV

  • Tidal volume: (6 ml/kg). Set initial Vt at 8 mL/kg using the patient's predicted body weight (PBW):

    • Males: PBW = 50 + [2.3 x (Ht in inches - 60)]

    • Females: PBW = 45.5 + [2.3 x (Ht in inches - 60)

  • RR (minute ventilation): 16 - 24 bpm. Do not exceed rate of 35/min

  • FiO2: 100% (1)

  • PEEP: (variable)

Status Asthmatics: Pt is male 5' 10', 65 kg

AC10/

COPD

  • Mode of ventilation: ACV

  • Tidal volume: 5 - 7 mL/kg

  • RR: <24 mins

  • FiO2: 40 - 50% (0.4 - 0.5)

  • PEEP: dependent

Restrictive Lung condition

  • Mode of ventilation: ACV or SIMV

  • Tidal volume: 5 - 7 mL/kg

  • RR: 16 - 24 mins

  • FiO2: 40 - 50% (0.4 - 0.5)

  • PEEP: FiO2/O2 saturation-dependent

Complications with ventilator support

  • Barotrauma

  • VILI. Overdistention and rupture of distal air spaces during mechanical ventilation as a volume-related rather than pressure-related injury. Excessive inflammation volumes produce stress fractures in the alveolar capillary interface, and this leads to infiltration of distal air spaces with inflammation 3 cells and proteinaceous material. The resulting clinical condition known as ventilator-induced lung injury (VILI) is similar to ARDS. He intermittently cytokines from neutrophils infiltrate the lungs and travel to distant organs to produce widespread inflammatory injury and multiorgan failure.

  • Alveolar overdistention

  • Hypotension

  • Pneumonia

  • Atelectasis

  • DVT

  • GIB

  • Neuropathy

  • Acute sinusitis

  • Hypotension

Initial Ventilator Settings

  • Check CXR after placing Pt. on ventilator.

  • Check ABG in 30 min - 1 hr after any change in any parameters.

  • Hypotension → Give IVF

  • DVT and GI prophylaxis: Heparin 5000 units sc q12hr or Lovenox 40 mg sc daily; Protonix 40 mg IV daily (bid for GIB)

  • If ABG shows high PaCO2 → increase tidal volume to blow off CO2 or ↑ RR.

  • PEEP is useful in hypoxic respiratory failure such as ARDS or CPE

  • Low PEEP levels can be used in COPD to keep airways open.

  • Increasing PEEP decreased venous return to the heart and might lead to drop in BP.

  • High levels of tidal volume and PEEP might predispose patients to barotrauma (vent-induced lung injury)

Management of problems and complications:

  • Airway malpositioned and occluded:

  • Worsening of respiratory distress and arterial oxygen desaturation:

    • Deterioration of patient's cardiopulmonary status

    • Problem with mechanical ventilation.

      • Check Airway to ensure patency and correct positioning of the the patient's airway so that adequate oxygenation and ventilation can be administered during the ensuing evaluation.

        • Check ventilator alarms, airway pressures, and tidal volume. Low pressure alarms with decreased Vt may suggest a leak in the ventilator circuit.

        • Disconnect the patient from the ventilator and manually ventilate with an anesthesia bag using 100% O2. Use a PEEP valve on the ambu bag for patients who were on PEEP.

        • If manual vent is difficult: Check airway patency by passing suctioning cath through the endotracheal tube or tracheostomy. Listen for prolonged expiration continuing to the point of the next maunal breath. This suggests the presence of gas trapping and auto-PEEP.

        • Check for VS and perform a rapid physical exam with attention to the patient's cardiopulmonary status. Check for symmetry of breath sounds or tracheal deviation suggesting tension PTx. Note other parameters, including cardiac rhythm and hemodynamics.

        • Treat appropriately on the basis of the foregoing evaluation. If autoPEEP and gas trapping is suspected, reduce minute ventilation with periods of hypoventilation 4 - 6 breaths/min or even apnea for 30 - 60 secs.

        • Return patient to the vent only after checking its function.

    • Acute increase in peak airway pressure:

    • Loss of tidal volume

    • Asynchronous breathing ("fighting" or "bucking" the vent)

    • Organ hypoperfusion or hypotension

    • AutoPEEP

    • Barotrauma or volutrauma

    • Positive fluid balance and hyponatremia

    • Cardiac arrhythmias

    • Aspiration

    • VAP

    • Upper GIB

    • Acid-base complications

      • NAGMA

      • Metabolic alkalosis

      • Respiratory alkalosis

    • Oxygen toxicity

Initial Mechanical ventilator settings: MTRIP

  • Mode of ventilation: ACV or SIMV

  • Tidal volume: 450 - 500 mL (6 - 12 ml/kg)

  • RR (minute ventilation): 10 - 16 bpm

  • FiO2: 80%-100% (1) (0.5 - 1), then decreased by 10% until it is 50% or less based on ABG and SpO2.

    • Incidence of pulmonary fibrosis increases greatly at higher O2 concentrations. An FiO2 of 50% is considered toxic if it is applied for extended periods, whereas an FiO2 less than or equal to 50% is considered safe. If it not possible for the physician to decrease the FiO2 out of the toxic range, the physician may consider adding PEEP

  • Pressure support (3 - 8 cm H20)

  • PEEP: +5 cm H20

  • Flow-by 5 - 20 L/min

AC/ 500/12/70/10/PEEP 5

Example.

A normal spontaneous VT is 5-6 mL/kg IBW. However, a ventilator VT should be calculated as 6-8 mL/kg IBW to ensure adequate lung inflation but to prevent overdistension and barotrauma. The initial machine rate is set between 8-12 bpm. However, the Pt's minute ventilation (Ve) which is RR x VT prior to the initiation of mechanical ventilation. For example, prior to mechanical ventilation, our 50-kg patient was breathing 50s time per minute with a VT of approximately 200 mL. Her Ve 10 L/min. Initial VT = 400 mL (8 mL/kg) and rate, 10 bpm would only yield a 4-L Ve that could be inadequate for this patient. The patient is put on A/C-10-500-40%-5. 2 hours after intubation his ABGs are as follows: 7.30/58/70/SaO2: 95%, Sr. HCO3 = 24. The problem is with ventilation as the ABG reveal respiratory acidosis.

She is having problems with CO2 elimination or ventilation. To improve ventilation, either respiratory rate or VT should be increased. Because her VT is already set at 8 mL/kg IBW, an increase in VT would increase airway pressures, putting her at risk for barotrauma and/or overdistention of the most compliant alveoli. Increasing the RR from 10-14 would improve ventilation without significantly increasing airway pressure.

Neurologically stable patients.

Virtually all patients are well served with an initial ventilator order that includes intermittent

mandatory ventilation mode. The positive pressure breaths that are delivered by the mechanical ventilator are triggered by the patient, usually by generating a small pressure difference in the patient-ventilator circuit. The patient is able to breathe in between and have entirely unsupported breaths.

A typical ventilator order for neurologically stable patients is:

SIMV (synchronized intermittent mandatory ventilation) mode of FIO2 (fraction of inspired

oxygen) of 0.4–1.0, respiratory rate at 8–12 breaths/minute, tidal volume of 10–15 mL/kg,

PEEP (positive end – expiratory pressure) of 2–5 cm of H2O, and an inspiration/expiration ratio of 1–3

Weaning from Mechanical Ventilation. Weaning is a gradual process of withdrawal from mechanical Ventilatory support. Successful weaning depends on the condition of the patient and on the status of the cardiovascular and respiratory systems.

Weaning Parameters

  • Respiratory Criteria:

    • PaO2 >60 mm Hg or more with an FiO2 <40 -50%, and PEEP <5 cm H20 or less

    • PaCO2 and pH acceptable

    • Spontaneous tidal volume > 5 mL/kg

    • Vital capacity > 10 mL/kg

    • MV < 10 L/min

    • MVV = 2 x MV

    • Max NIP - >25 cm H20

    • RR <30 breath/min

    • Static compliance >30 mL/cm H2O

      • Static compliance is compliance when there no movt of air.

    • RSBI (RR/Vt in liters) <100 breaths/min/L

    • No significant respiratory acidosis

  • Cardiovascular Criteria:

    • No evidence of myocardial ischemia

    • HR <140 bpm or less

    • BP normal without vasopressors or with minimum vasopressor support (e.g., dopamine <5 mcg/kg/min).

    • Stable vital signs at a 1 - 2 hours of spontaneous breathing trial.

  • Adequate mental status:

    • Pt's mental status: awake, alert, cooperative. GCS >13

  • Absence of Correctable Comorbid Conditions:

    • Pt. is afebrile, T <38C

    • Hb: 8-10 mg/dL

    • No significant electrolyte abnormalities

    • No significant metabolic acidosis

    • Resolution of acute phase of disease

Measurements Used to Identify Patients who will tolerate a Spontaneous Breathing Trial (SBT)

The Spontaneous Breathing Trial

    • Done when the patient is still connected to the ventilator circuit - breathing through the vent.

      • Allows monitoring of Vt and RR during spontaneous breathing to detect rapid and shallow breathing that often signals failure to sustain spontaneous breathing.

      • Drawbacks:

        • Increases the work of breathing due to:

          • negative pressure that must be generated to open an actuator valve in the vent and receive the inhaled oxygen mixture, and

          • resistance created by the vent tubing between the Pt. and the vent.

        • To counteract this increased work of breathing, PSV is used routinely during spontaneous breathing trials.

    • T- piece trial: Pt can be removed completely from the vent and allowed to breathe from an independent source of oxygen.

    • PSV: It adds enough inspiratory pressure to reduce the work of breathing through the ETT and ventilator circuit without augmenting the spontaneous tidal volume. 5 - 7 cm H20 is routinely used for this purpose.

    • Protocol is to allow 30 - 120 min for the initial trial of spontaneous breathing. Success or failure is judged by how the patient tolerates, clinically (comfortable vs labored breathing), breathing pattern (the presence or absence of rapid, shallow breathing) and gas exchange (e.g., ability to maintain SaO2 >90% and end-tidal PCO2 normal or consant throughout the trial).

    • Patients who have been ventilator dependent >1 wk will need at least 8 hours (and sometimes up to 24 hours) of spontaneous breathing before considering removing them off the vent.

    • An increase in Vt during spontaneous breathing trials suggests anxiety, whereas a decrease in Vt suggest continued need for ventilatory support.

    • When the Vt is unchanged or increased:

      • Check PaCO2. A ▼ PaCO2 indicates that ventilation is adequate and suggests that anxiety is the problem. Sedation is helpful in such cases.

      • If the PaCO2 is normal or increasing, the patient is returned immediately to ventilatory support (a normal PaCO2 in the face of a high minute ventilation is a sign of ventilatory failure).

Failure to ventilate:

    • Neurological: ischemic stroke, metabolic encephalopathy, sepsis, seizures, opioids, benzos, hypocapnic metabolic alkalosis, cervical cord injury, phrenic nerve injury, neuromuscular process, myopathy, NMBA, aminoglycosides, steroids,

    • Respiratory: airway obstruction, flail chest, swollen tongue, pleural effusion, bronchospasm, auto-PEEP, increased intra-abdominal pressure.

Failure to oxygenate:

    • Edema, fibrosis, consolidation, alveolar dead space, bronchial dead space, airway collapse

    • Hyponatremia, hypernatremia, hypokalemia, hypomagnesemia, hypophosphatemia, hypothyroidism, steroids, AI, aspiration pneumonitis, ascites, abdominal packs, CHF, MI, volume overload, malnutrition, pain, anemia,

Failure of Spontaneous Breathing:

  • Low cardiac output

    • Check prior to spontaneous breathing trial: O2 Extraction (SaO2 - SvO2). The SaO2 is monitored by a pulse oximeter, and the SvO2 can be measured with a CVC in the SVC. The SaO2 - SvO2 is normally about 25%, and it will increase to 50% in low output states

    • Check the arterial-end tidal PCO2 Gradient (PaCO2 - PetCO2): O in healthy subjects. ▼ CO → ▼ PetCO2 relative to arterial PaCO2 and this is reflected in an increase in the PaCO2 - PetCO2 difference. An increase in the dead-space ventilation from lung disease will also increase the PaCO2 - PetCO2 gradient.

    • Check for MI. Do an ECG

    • Overfeeding

    • Respiratory muscle weakness

Factors to be considered in the weaning process:

WEANS NOW

  • Weaning parameters

  • ETT: use of largest ETT possible. Consider supplemental PSV. Suction secretions

  • ABGs, check acid-base status - avoid and treat metabolic alkalosis. Maintain PaO2 at 60 - 65 mm Hg to avoid blunting of respiratory drive. For patients with CO retention, keep PaCO2 at or above the baseline level.

    • Oxygenation: PaO2 >60, FiO2 0.4, PEEP <5cm H2 O

  • Nutrition. Ensure adequate nutritional support. Avoid electrolyte deficiencies. Avoid excessive calories.

  • Secretions. Clear regularly. Avoid excessive dehydration.

  • Neuromuscular factors. Avoid neuromuscular depressing drugs. Avoid unnecessary corticosteroids.

  • Obstruction of airways. Use bronchodilators when appropriate. Exclude FB within the airway.

  • Check for cuff leak after dropping the balloon.

  • Wakefulness. Avoid oversedation. Wean in morning or when patient is most awake. Make sure enough ancillary staff is available.

  • Explain process of extubation to patient and check for cooperation for liberation from vent.

  • Keep crash cart by side.

RSBI = Respiratory rate/Tidal volume in liters. (RR/Vt). Normally 40 - 50/L. It if often >100/L in Pts who do not tolerate spontaneous breathing.

RSBI of <100 after 1 hr of CPAP with PSV 5 and PEEP 5, the patient is weanable.

Initiation set up for mechanical ventilation: 6 steps.

    1. Set up equipment, perform SST (short self-test) on ventilator, vent will also do an automatic test for tubing compliance.

    2. Decide on type of ventilation cycling: insp → exp. Set initial as volume control.

    3. Mode of ven; support mode.

    4. Settting, volume, pressure, FiO2, RR, PEEP, I:E ratio

    5. Alarms - high pressure alarm, apnea alarm

    6. Attach and monitor the patient

Vent setting: Phase I - initial set up:

Frequency: 8-12

Vt: 8-12 mL/kg "IBW"

IBW females: 105 + (5 x number of inches more than 60 inches) = wt in lbs. Wt. in lbs/2.2 = kg

IBW males: 106 + (6 x number of inches over 60 inches) = wt in lbs. Wt. in lbs/2.2 = kg

e.g, female. 5 feet 2 inches = 62. 105 + (5 x 2) = 115 lbs = 115/2.2 = 52 kg

e.g. male. 5 feet, 2 inches = 106 + (6 x 2) = 118 lbs = 118/2.2 = 54 kg.

FiO2: prior to intubation, whatever FiO2 Pt. was on (e.g - BVM → FiO2: 1)

PEEP 5 cm H2O

Sensitivity: Pressure sensitivity - 2 cm H2O; Flow sensitivity: 2 L (flow sucking by the pt)