Positive pressure ventilation: what is the real cost?

1
Positive pressure ventilationwhat is the real
cost?

• Br J Anaesth 2008 101 446-57
• R4 ???

2
Positive pressure ventilation

• Copenhagen polio epidemic
• Reduction in mortality 87 ? 40
• considerable deviation from the normal
  physiological mechanism of respiration
• by Mushin in 1st edition of Ventilation of the
  Lungs
• Pathophysiological price to pay
• Not all complications are obvious or immediate

3
Oxygenation and ventilation

• Monitoring of oxygenation
• Focuses on inspired oxygen concentration,
  arterial blood gases
• Arterial values of oxygenation not ideal
  parameters
• Circulation
• Tissue and cell transport
• Mitochondrial function between lung and cell
• In the critically ill
• Relatively low arterial saturation ? but,
  adequate tissue oxygenation
• Jeopardous tissue oxygenation ? not related
  directly to lungs
• Low saturation PaO2 poorly correlate with
  tissue oxygenation
• Difficult to oxygenate -gt tolerance develops
  rapidly -gt no markers of tissue hypoxia
• few patients with lung injury die of hypoxemia
• Reconsider about methods of assessing adequacy of
  oxygenation

4
Breathing, ventilation, and intrathoracic
pressures

• In spontaneous breathing
• Inspiration
• Small negative intrapleural, interstitial,
  alveolar pressures
• Expiration
• Intrapleural pr returns atmospheric, but
  remains negative
• Interstitial alveolar pr atmospheric or
  slightly positive
• In positive pressure ventilation
• Inspiration
• High intrathoracic pr
• Expiration
• Return towards atmospheric pressure

5
Ventilation, alveolar ventilation, and recruitment

• Surfactant
• Modifies the effects of Laplaces law
• Small alveoli are easy to inflate less tendency
  to collapse
• In positive pressure ventilation
• Individual time constants of lung regions or
  alveoli
• Airway resistance alveolar compliance
• Determine the effect of pressure in different
  regions of lung
• Positive pressure -gt preferentially aerate high
  compliance areas
• Collapsed alveoli may require high sustained
  pressure

6
Ventilation, alveolar ventilation, and
recruitment - continued

• Recruitment
• Opening maintaining open potentially under
  ventilated areas
• To hence alveolar surface area involved in gas
  exchange
• Initial sustained high pressure with subsequent
  PEEP at various levels
• In ARDS
• Only achieved a mean recruitment of 13
• High levels of PEEP (15 mmHg) are more effective
• Both in maintaining alveolar patency improving
  oxygenation
• Effective in preventing collapse and derecruiment
• ? Sustained increase in intrathoracic pressure

7
Ventilation, alveolar ventilation, and
recruitment - continued

• Result of recruitment is unpredictable
• Increase gas distribution not to abNL areas
• Over-inflating funcional areas
• ? potentially impairing their function
• Recruitment in damaged lungs
• ? may exacerbate problems

8
Ventilation pressure and stretch

• Lower peak pressure ventilation
• Reduce mortality
• Shear forces occur particularly in initiating
  inflation
• May cause injury
• Stress shear forces
• Cytokine production increased
• White cell sequestration
• May predispose to injury infection
• In recruitment
• Lowering of the peak pressure
• Whereas higher PEEP
• ? maintains alveolar patency reduces the shear
  forces

9
Ventilation and surfactant

• Distortion of surfactant spread
• With positive pressure ventilation
• Forced air ? pressure waveform
• Wave formation in surfactant layers
• Altering the uniformity of spread
• Influences the production function of
  surfactant
• In lung injury (inflammation)
• Reduced type II pneumocytes ? reduced surfactant
  production
• Release protein other materials
• Affects ability of surfactant to form surface
  structures
• Membrane permeability changes ? Fluid dilution of
  surfactant
• Polymerizing fibrin ? Adsorbs surface active
  compounds
• Role in immune defence of surfactant
• Influence inflammatory response
• Substantial role in mucosal immunity
• ? Vicious cycle to cause further injury

10
Effects on the cardiovascular system

• In normal breathing
• In inspiration, assists venous return, pulm
  capillary flow
• With positive pressure ventilation
• During inspiration increased intrathoracic
  pressure
• Decrease venous return, RV output, pulm blood
  flow
• On expiration intrathoracic pressure returns to
  zero
• PEEP ? positive pressure continued ? inhibit
  venous return
• Fluid administration improves venous return
  cardiac output
• Increase CVP, increase end-capillary pressures in
  lungs other organ
• Salt water retention
• Classically d/t increased secretion of
  anti-diuretic hormone
• More recently, atrial natriuretic peptide
  implicated
• Correction by IV fluid ? further fluid retention

11
The pulmonary capillary and blood flow

• Mean capillary pressure 7-10 mmHg
• Normal breathing
• Interstitium alveoli pressure lower than
  capillary perfusion pressure
• Pressure within lung lower than capillary
  pressure
• In COPD with hyperinflation
• High intrathoracic pressure on expiration
• ? increasing capillary resistance
• Positive pressure ventilation
• Peak inspiratory pressure limited to 30 mmHg
• Compress capillary, impede flow
• In expiration, PEEP of 15 mmHg
• Prevents recovery of normal flow
• Inflatable lung region pressure transmitted
• Damaged or infected lung better perfusion
• ? impeded capillary flow (ineffective hypoxic
  vasoconstriction)

12
The pulmonary capillary and blood flow -
continued

• High venous pressure secondary to positive
  pressure ventilation
• ? interstitial fluid retention
• Capillary stress failure in extreme exercise
  in racehorses
• Pulmonary artery pressure, inflation pressure,
  venous pressure high
• ? damage the integrity of capillary endothelium
• Probably also seen in humans
• Compensatory mechanisms ineffective
• May aggravate ventilation-perfusion mismatch

13
Lymphatics

• Functions drainage defence
• Lung lymphatics
• Within interstitium, thin, single cell conduits
  with valves
• Inspiration negative pressure ? drain into
  lymphatics
• Pressure in pph lymphatics max 4 mmHg
• Hydrostatic gradient between lymphatics central
  veins
• During inspiration
• Flow easily impeded by
• External pressure on lymphatic walls
• Outflow resistance

14
Lymphatics -continued

• Positive pressure ventilation
• During inspiration
• Push fluid from alveolus to interstitium
  lymphatics
• May compress thin-walled vessels
• High CVPs
• Significant hydrostatic barrier to flow
• During expiration
• Allow resumption of flow
• In PEEP
• Helps remove fluid from alveoli, but reduction in
  thoracic duct drainage
• ? fluid retention in interstitium
• PEEP in injured lung
• Increases lymph production, but impairs lymph
  flow
• Impaired drainage
• ? fluid accumulation in lung pleural spaces
• ? increased susceptibility to lung infection

15
Organ systemsand positive pressure ventilation

• Effects on kidney
• Decreased cardiac output
• Decrease in GFR
• Diversion of intra-renal blood flow
• Effects of PEEP on hepato-splanchnic circulation
• Venous congestion
• Reduced portal blood flow
• Increased hepatic blood volume
• Reduced hepatosplanchnic lymphatic drainage
• Sustained increases in CVP lead to
• Venous congestion
• Increased end-capillary pressure
• Altered fluid dynamics within organs
• Affects lymphatic function
• Raised intraabdominal pressure
• Impairs lymphatic drainage
• High thoracic duct pressure
• Increases interstitial fluid in liver kidneys

16
Conclusions

• Ventilation with PEEP
• Proven, effective modality in anesthesia ICU
• But, cause physiological derangements
• Redistribution of alveolar ventilation
• Altered capillary perfusion
• Functional changes in surfactant
• Transcapillary fluid shifts
• Impaired lymphatic drainage
• Impeded venous return
• Prolonged ventilation
• Lung injury
• Infection
• Multi-organ system dysfunction
• Real cost of ventilation or oxygenation
• ? higher than realized

17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
Conclusions - continued

• Are there potential alternatives?
• Tank or cuirasses
• Generate a negative inspiratory pressure
• Development of bedside extracorporeal oxygenation
• Using the heart as the pump
• Lung assist devices
• In their infancy
• For future progress
• Recognition of physiological derangement
• Accepting the physical physiological
  constraints
• In further evolution of positive pressure
  ventilation
• Technology of positive pressure ventilation
• Now more than 50 yr old
• Time to consider alternatives