1. MECHANICAL VENTILATION
Things “I” wish I knew when I was an Intern
Amit Gupta, MD
Internal Medicine
North Mississippi Medical Center

2. Mechanical Ventilation
Indications for Intubation and Ventilation
Principles of Mechanical Ventilation
Patterns of Assisted Ventilation
Ventilator Dependence: Complications
Liberation from Mechanical Ventilation: Weaning
Troubleshooting
Arterial Blood Gases

3. Indications for Mechanical Ventilation
“….An opening must be attempted in the trunk of the trachea, into which a tube or cane should be put; You will then blow into this so that lung may rise again….And the heart becomes strong….”
-Andreas Vesalius (1555)

4. Indications for Mechanical Ventilation
1. “Thinking” of Intubation: elective v/s emergent
2. “Act of weakness?”
3. Endotracheal tubes are not a disease and ventilators are not an addiction
4. And the usual elective and emergent indications that you all know!
Instead of the usual list, follow these rules:
There is a tendency to delay intubation as long as possible in the hopes that it will be unnecessary. Elective intubation carries fewer dangers than emergent intubation. So, if the patient’s condition is severe enough that intubation is considered, then proceed without delay
Remember you will never be faulted for establishing the control of airways
ETT and ventilators do not create the need for mechanical ventilation: cardiopulmonary and neuromuscular disease do

5. Objectives of Mechanical Ventilation
Improve pulmonary gas exchange
Reverse hypoxemia and Relieve acute respiratory acidosis
Relieve respiratory Distress
Decrease oxygen cost of breathing and reverse respiratory muscle fatigue
Alter pressure-volume relations
Prevent and reverse atelectasis
Improve Compliance
Prevent further injury
Permit lung and airway healing
Avoid complications

6. Strategies for Mechanical Ventilation
Ventilatory Parameter
Traditional
Lung-Protective
Inflation Volume
10-15 ml/kg
5-10 ml/kg
End-insp. pressure
Peak Pr<50cm water
Plateau Pr<35
PEEP
PRN to keep FiO2<0.6
5-15 cm of water
ABG
Normal, pH
Hypercapnia allowed, pH

7. Monitoring Lung Mechanics
Proximal Airway Pressures (end-inspiratory)
1. Peak Pressure Pk
Function of: Inflation volume, recoil force of
lungs and chest wall, airway resistance
2. Plateau Pressure Pl
Occlude expiratory tubing at end-inspiration
Function of elastance alone
At constant inflation volume the peak pressure is directly related to the airflow resistance and to the elastic recoil force of the lungs and the chest wall.
Plateau pressure is measured by occluding the expiratory tubing at the end of inspiration.
Because no airflow is present when the plateau pressure is created, the pressure is not a function of flow resistance. Instead Plateau pressure is proportional directly to the elastance of the lungs and the chest walls.

8. Use of Airway Pressures
Pk increased Pl unchanged:
Tracheal tube obstruction
Airway obstruction from secretions
Acute bronchospasm
Rx: Suctioning and Bronchodilators
Very useful for determining the cause of sudden deterioration of a patient on a ventilator.

9. Use of Airway Pressures
Pk and Pl are both increased:
Pneumothorax
Lobar atelectasis
Acute pulmonary edema
Worsening pneumonia
ARDS
COPD with tachypnea and Auto-PEEP
Increased abdominal pressure
Asynchronous breathing

10. Use of Airway Pressures
Decreased Pk:
System air leak: Tubing disconnection, cuff leak
Rx: Manual inflation, listen for leak
Hyperventilation: Enough negative intrathoracic
pressure to pull air into lungs may drop Pk.

11. Compliance Static Compliance (Cstat):
Distensibility of Lungs and Chest wall
Cstat = Vt/Pl
Normal C stat: ml/cm of water
Provides objective measure of severity of illness in a
pulmonary disorder
Dynamic Compliance:
Cdyn: Vt/Pk
*Subtract PEEP from Pl or Pk for compliance
measurement
Use Exhaled tidal volume for calculations
Connector tubing between the ventilator and the patient expands during lung inflations, some of the TV is lost in this tubing.
Connector tubing compliance is 3ml/cm of water
Meaning that 3ml of volume is lost for every 1cm increase in inflation pressure.
Example: Inflation vol from vent is 700cc at Pk of 40cm water, then volume lost to expansion of ventilator tubing is 40X3=120ml. Therefor actually only =580ml reaches the patient, and this should be used as the Vt for compliance measurement.

13. Patterns of Assisted Ventilation
Assist Control
Intermittent Mandatory Ventilation
Pressure Controlled Ventilation
Pressure Support Ventilation
Positive end-expiratory ventilation
Continuous Positive Airway Pressure

14. Assist Control Ventilation
Volume-cycled lung inflation
Patient can initiate each mechanical breath or Ventilator
provides machine breaths at a preselected rate
Maintain I:E ratio to 1:2 to 1:4. An increase in Peak flow
decreases the time for lung inflation and increases the I:E
Ratio
I:E ratio of <1:2 can cause hyperinflation by air trapping
Diaphragmatic contraction continues during ACV and
increases the work of breathing.
Volume cycled means that each machine breath delivers a preselected lung inflation volume

15. Assist Control Ventilation
Adverse effects:
In a tachypneic patient>>Lead to overventilation and
severe respiratory alkalosis>> Hyperinflation and
Auto-PEEP>> Lead to Electromechanical
dissociation

16. Intermittent Mandatory Ventilation
Delivers volume cycled breaths at a preselected rate with spontaneous breathing between machine breaths
Less Alkalosis and Hyperinflation
Synchronized IMV
IMV delivers periodic volume cycled breaths at a preselected rate but allows spontaneous breathing between machine breaths.
Because each spontaneous breath does not trigger a machine breath, there is a reduced risk of respiratory alkalosis and hyperinflation
Synchronized IMV: Machine breaths are synchronized to coincide with spontaneous lung inflations.

17. Intermittent Mandatory Ventilation
Disadvantages:
Increased work of Breathing:
Spontaneous breathing through a high resistance circuit
Solution: Add Pressure support
Cardiac Output Changes:
C O decreased by decreasing ventricular filling
C O increased by reducing ventricular afterload
More significant decrease in patients with LV dysfunction
Increased work of breathing could lead to respiratory muscle fatigue and ventilator dependency

18. IMV vs. ACV Switch to IMV for:
Rapid breathers with alkalosis and over-
Inflation
Switch to ACV for:
Patients with respiratory muscle weakness and
LV dysfunction

19. Pressure Controlled Ventilation
Pressure cycled breathing, fully ventilator controlled
Inspiratory flow rate decreases exponentially during lung inflation
(+)Reduces peak airway pressure and improves gas exchange
(-)Inflation volume varies with changes in mechanical properties of the lungs.
Suited for patients with neuromuscular diseases and normal lung mechanics

20. Inverse ratio Ventilation
PCV combined with prolonged inflation time
Inspiratory flow rate is decreased
I:E ratio reversed to 2:1
Helps prevent alveolar collapse
(-) Hyperinflation, Auto-PEEP and decreased cardiac output
Use: ARDS with refractory hypoxemia or hypercapnia ?mortality benefit

21. Pressure Support Ventilation
Pressure augmented breathing
Allows patient to determine the inflation volume and respiratory cycle duration
Uses: augment inflation during spontaneous breathing or overcome resistance of breathing through ventilator circuits (during weaning)
Popular an a non-invasive mode of ventilation via nasal or face masks
At the onset of each spontaneous breath, the negative pressure generated by the patient opens a valve that delivers the inspired gas at a preselected pressure (5-15cm water) The patient’s inspiratory flow rate is adjusted by ventilator as needed to keep inflation pressure constant.
When inspiratory flow rate falls below 25% of the peak inspiratory flow, the augmented breath is terminated

22. Positive end-expiratory pressure
Alveolar pressure at end-expiration is above atmospheric pressure : PEEP
Extrinsic PEEP
Auto PEEP
Normally the alveolar pressure at the end of expiration is equal to the atmospheric pressure, called the zero reference point.

23. Positive end-expiratory pressure
EXTRINSIC PEEP
Applied by placing pressure limiting valve in the expiratory limb of ventilator circuit
Prevents end-expiratory alveolar collapse and recruits collapsed alveoli
This decreases intrapulmonary shunting, improves gas exchange and improves lung compliance, allowing the FiO2 to be reduced to less toxic levels
Valve exerts back pressure and exhalation proceeds until this back pressure is reached where upon, flow ceases. Something like placing the distal end of the expiratory tubing under water. Back pressure would be equal to the distance the tube is submerged.

24. Positive end-expiratory pressure
Cardiac Performance:
Greater reduction in cardiac filling and cardiac output (Q),
irrespective of level of PEEP!
It is a function of PEEP induced increase in mean
intrathoracic pressure
Oxygen transport Do2:
Do2 = Q X 1.3 X Hb X SaO2
Systemic O2 delivery may vary with the effect of PEEP on
the Cardiac Output.
Low levels of PEEP are as deleterious for cardiac output if the mean intrathoracic pressure is high
Effects of PEEP on systemic oxygenation are determined by the change in cardiac output not by change in the arterial oxygenation
Best PEEP curve?

25. Positive end-expiratory pressure
Best PEEP: Monitor Cardiac Output
Another measure: Venous Oxygen Saturation
If VOS decreases after PEEP applied= Drop CO
Swan-Ganz catheter may be indicated in most patients on PEEP

26. Positive end-expiratory pressure
CLINICAL USES:
Reduce toxic levels of FiO2 (ARDS not pneumonia)
Low-volume ventilation
Obstructive lung disease (Extrinsic=Occult PEEP)
In localized lobar pneumonia, PEEP may overdistend the normal alveoli and redirect blood back to the diseased areas
Prevents the repeated opening and closing of small airways which is a source of further lung injury
Small airways collapse in obstructive lung disease, PEEP can keep these open at the end of expiration. Level of peep should be enough to counterbalance the closing pressure of the small airways, but not be more than the intrinsic-PEEP, so as to not impair the inspiratory flow. The level of Extrinsic peep which first causes an increase in peak pressures is taken as the level of occult PEEP.

27. Positive end-expiratory pressure
CLINICAL MISUSES:
Reducing Lung Edema
Routine PEEP
Mediastinal Bleeding after CABG
PEEP does not reduce lung edema in ARDS, but can increase water accumulation in the lungs by impaired lymphatic drainage from lungs
Glottic closure at end exp causes low level of physiologic PEEP. No documented benefit in intubated patients
PEEP is transmitted across the walls of the blood vessels, it may not reduce transmural pressure. No benefit.

28. Continuous positive Airway Pressure
Spontaneous breathing
Patient does not need to generate negative pressure to receive inhaled gas
CPAP replaced spontaneous PEEP
Use: Non-intubated patients (OSA, COPD)
In spontaneous PEEP, a negative pressure is required for inhalation. CPAP thus reduces work of breathing

29. Occult PEEP Intrinsic or Auto-PEEP or Hyperinflation
Incomplete alveolar emptying during expiration
Ventilator Factors: High inflation volumes, rapid rate, low exhalation time
Disease factors: Asthma, COPD
Consequences: Decreased CO/EMD, Alveolar rupture, Underestimation of thoracic compliance, increased work of breathing.
If extrinsic PEEP does not increase Pk, then occult PEEP is present
Factors predisposing to auto peep fall in 2 categories. Occult peep increases work of breathing: hyperinflation places lungs in the flatter portion of the pressure volume curve. Take a deep breath and then try to breathe in further, higher pressures are needed to inhale the tidal volume.
Level of Extrinsic peep that first causes a rise in peak inspiratory airway pressures is a quantitative measure of occult PEEP.

30. Complications of Mechanical Ventilation
Toxic effects of Oxygen
Decreased cardiac output
Pneumonia and sepsis
Psychological problems
Ventilator dependence

31. Complications of Mechanical Ventilation
Purulent sinusitis
Laryngeal Damage
Aspiration :Value of routine tracheal suctioning
Tracheal Necrosis (pressure below 20mm water)
Alveolar rupture: Pneumothorax, pneumomediastinum, subQ emphysema, pneumoperitoneum
Basilar and sub-pulmonic air collections in the supine position, as seen on X-ray
Excluding the complications if intubation, sedation etc.
One liter of saliva a day with a billion bugs per microliter.

32. Liberation from Mechanical Ventilation: Weaning
Weaning: Gradual withdrawal of mechanical ventilation
Misconceptions:
Duration- longer duration, harder to wean
Method of weaning determines ability to wean
Diaphragm weakness is a common cause of failed weaning
Aggressive nutrition support improves ability to wean
Removal of ET tube reduces work of breathing

34. Bedside Weaning Parameters
Normal Adult range
Threshold for weaning
PaO2/FiO2
>400
200
Tidal Volume
5-7ml/kg
5ml/kg
Resp. Rate
14-18/min
<40/min
Minute Ventl.
5-7L/min
<10L/min
Vital capacity
65-75ml/kg
10ml/kg

35. Bedside Weaning Parameters
Maximal Inspiratory Pressure
>-90 cm Water (F)
>-120 cm water (M)
-25cm of water
Rate/Tidal Volume
<50/min/L
<100/min/L
These two have a greater predictive value than the ones mentioned before which have a very poor predictive value.

36. Maximal Inspiratory Pressure
Pmax: Excellent negative predictive value if less than –20 (in one study 100% failure to wean at this value)
An acceptable Pmax however has a poor positive predictive value (40% failure to wean in this study with a Pmax more than –20)

37. Frequency/Volume ratio
Index of rapid and shallow breathing RR/Vt
Single study results:
RR/Vt> % wean attempts unsuccessful
RR/Vt< % successful
One of the most predictive bedside parameters.

38. T-Piece Weaning
On-off toggle switch that circulates between on and off the ventilator
Inhaled gas is delivered at a high flow rate
Varied protocols: like 30min-2hr on and off, or keep as long as possible and if tolerated for >2-4hr…. Deemed successsful (RR, TV, HR, diaphoresis, sat)
Failed T piece: Resume Vent support till comfortable, 24h
Show figure
vent
Airflow with CPAP
patient

39. T-Piece with Ventilator
Drawback: increased resistance due to vent tubing and actuator valve in circuit
Provide minimum pressure support (PSV) :Pmin
Pmin= PIFR X R
PIFR is during spontaneous breathing
R is airflow resistance during mech ventilation
R= Pk-Pl/Vinsp
(Vinsp:inspiratory flow rate delivered by the vent)
PSV to overcome resistance of the circuit and valve

40. IMV Weaning
Gradual decrease in no of machine breaths in between the spontaneous breaths
False security: It does not adjust to patient’s ventilatory demands to maintain constant MV
End point in IMV weaning is the T-piece trial
Most important to recognize when a patient is capable of spontaneous unassisted breathing
T-piece more rapid than IMV
False sense of security, patient often left unattended more than those with T-piece weaning

41. Complicating Factors DYSPNEA
Anxiety and dyspnea are detrimental (low dose haloperidol or morphine)
CARDIAC OUTPUT
Increased LV afterload can reduce CO, impair diaphragm function, promote pulmonary edema
(Use Swan to monitor CO, may use dobutamine)
ELECTROLYTE DEPLETION
OVERFEEDING
ELECTROLYTES:phosphorus, K, Mg OVERFEEDING: Excess calories causes excess CO2 production, can impair ability to wean

42. The Problem Wean RAPID BREATHING: Check TV
Low TV>> Resume vent support
TV not low…….. Check arterial pCO2
Arterial pCO2 decreased>sedate (anxiety)
Arterial pCO2 not decreased> Resume vent
Rapid breathing could be the result of anxiety, which leads to hyperventilation and high tidal volume. Where as wean failure muscle fatigue and cardiopulmonary disease usually causes rapid shallow breathing.

43. The Problem Wean ABDOMINAL PARADOX
Inward displacement of the diaphragm during inspiration is a sign of diaphragmatic muscle fatigue
HYPOXEMIA
May be due to low CO and MVO2
HYPERCAPNIA
Increase in PaCO2-PetCO2: increase dead space ventilation
Unchanged gradient: Respiratory muscle fatigue or enhanced CO2 production

44. Tracheal Decannulation
Successful weaning is not synonymous with tracheal decannulation
If weaned and not fully awake or unable to clear secretions, leave ETT in place
Contrary to popular belief, tracheal decannulation increases the work of breathing due to laryngeal edema and secretions
Do not perform tracheal decannulation to reduce work of breathing
CSA of tracheal tube is 50 cu. mm and that of the glottis is 66 cu. mm

45. Inspiratory Stridor
Post extubation inspiratory stridor is a sign of severe obstruction and should prompt reintubation
Laryngeal edema (post-ext) may respond to aerosolized epinephrine in children
Steroids have no role
Most need reintubation followed by tracheostomy

47. ARDS and Low Volume Ventilation
ARDS Network trial : NEJM May 4, 2000 p
Traditional: TV 10-15ml/kg, keep plateau<50cm water
Low TV ventilation: TV 6ml/kg, keep plateau<30cm water
Need high RR in Low TV group to prevent acidosis
Permissive hypercapnia tolerated well, if needed, use IV bicarb to maintain pH
May add PEEP in addition to the low TV group to prevent atelectrauma (open-close alveoli>> alveolar fracture)
Results: Lower mortality in the Low TV group (31% v/s 39.8% p<0.007); Higher days without vent use and lower average plateau pressures in low TV group.