Tag Archives: pulmo

Venous Blood Gas – VBG-ABG correlation

Venous blood gas can be used toestimate systemic CO2 and pH levels.

Possible sites of VBG:

  1. peripheral venous sample (from venipuncture)
  2. central venous sample (from central venous catheter)
  3. mixed venous sample (from distal port of PAC)

Values from VBG:

  1. PvO2 – venous oxygen tension
  2. PvCO2 – venous carbon dioxide tension
  3. pH
  4. SvO2 – oxyhemoglobin saturation
  5. HCO3 – serum bicarbonate

PvCO2, pH, HCO3 – assess ventilation and/or acid-base status

SvO@ – guides resuscitation

PvO2 – no value

 

Correlations:

  1. Central venous sample
    1. pH – 0.03 to 0.05 pH units lower than arterial pH
    2. PvCO2 – 4-5 mm Hg higher than PaCO2
    3. HCO3 – little or no increase
  2. Mixed venous sample – similar tocentral venous sample
  3. peripheral venous sample
    1. pH – 0.02 to 0.04 pH units lower than arterial pH
    2. HCO3 –  1-2 mEq/L higher
    3. PvCO2 – 3-8 mm Hg higher than PaCO2

*varies with hemodynamic stability

 

Reference:

Uptodate.com. (2018). UpToDate. [online] Available at: https://www.uptodate.com/contents/venous-blood-gases-and-other-alternatives-to-arterial-blood-gases?search=venous%20blood%20gas&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1 [Accessed 22 Oct. 2018].

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Pressure-time and Flow-time Graphs

Idealized pressure–time and flow–time graphs for mechanical ventilation. Note that the plateau pressure can be measured when flow returns to zero.

PIP peak inspiratory pressure, Pplat plateau pressure

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Trigger and cycle variables for each of the most common types of conventional mechanical ventilation

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Idealized pressure–time and flow–time graphs with PEEP set above zero for volume-controlled modes of ventilation

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Idealized pressure–time and flow–time graphs with PEEP set above zero for pressure-controlled modes of ventilation

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Idealized pressure–time graph for controlled mandatory ventilation (CMV) with PEEP set above zero. In this mode of ventilation, each breath is triggered after a specified time has elapsed. The breaths can be delivered in either volume controlled (shown) or pressure controlled (not shown), depending on the ventilator settings

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Idealized pressure–time graph for assist control ventilation (ACV) with PEEP set above zero. In this mode of ventilation, each breath is triggered either due to patient initiation (asterisks) or after a specified time has elapsed (no asterisks). The breaths in ACV can be delivered in either volume controlled (shown) or pressure controlled (not shown), depending on the ventilator settings

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Idealized pressure–time graph for intermittent mandatory ventilation (IMV) with PEEP set above zero. In this mode of ventilation, each set breath is triggered after a specified time has elapsed. In addition, the patient can breathe spontaneously between these machine- triggered breaths. The spontaneous breaths create a small relative negative pressure that are depicted in this graph and noted with asterisks. The machine-triggered breaths in IMV can be delivered in either volume controlled (shown) or pressure controlled (not shown), depending on the ventilator settings

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Idealized pressure–time graph for synchronized intermittent mandatory ventilation (SIMV) with PEEP set above zero. In this mode of ventilation, each set breath (or mandatory breath) is synchronized to a patient trigger after a specified time has elapsed. In addition, the patient can breathe spontaneously between the mandatory breaths. The spontaneous breaths create a small relative negative pressure that are depicted in this graph and noted with asterisks. The mandatory breaths in SIMV can be delivered in either volume controlled (shown) or pressure controlled (not shown), depending on the ventilator settings

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Idealized pressure–time graph for synchronized intermittent mandatory ventilation (SIMV) with PEEP set above zero. In addition, pressure support is being applied to the additional patient-initiated breaths between the mandatory breaths

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Idealized pressure–time graph for pressure support ventilation (PSV) with PEEP set above zero. In this mode of ventilation, each breath is triggered by the patient

 

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Reference:

Layon, A. Joseph, Andrea Gabrielli, and William Friedman. Textbook Of Neurointensive Care. London: Springer London, 2013. Print.

 

 

 

 

 

 

Colonization vs. Infection

The cutoff for significant number of colony forming units to differentiate between colonization and infection depends on the diagnostic test:

  • tracheobronchial secretion, 10–5  CFU/ ml;
  • BAL, 10–4 CFU/ml
  • protected specimen brush, 10–3 CFU/ml

 

Reference

Bein, Thomas et al. “The Standard Of Care Of Patients With ARDS: Ventilatory Settings And Rescue Therapies For Refractory Hypoxemia”. Intensive Care Med 42.5 (2016): 699-711. Web. 14 May 2016.

ARDS: Standard of Care

Life-threatening hypoxemia defined as:

  1. ABG PO2 <60mm Hg
  2. SPO2 <88%
  3. PF ratio <100

“Simple” and global parameters (PaO2, SaO2, SvO2, lactate) are imprecise surrogates for hypoxia in ARDS patients. However an individualized, organ-specific approach for monitoring of hypoxemia is currently not available. Therefore a target for conservative arterial oxygenation is recommended (PaO2 =  65–75  mmHg, SaO2 = 90–95 %), which should be bundled in a general “organ failure prevention” strategy.

Standard of Care inclues:

  1. Mechanical ventilator settings
    1. limited TV (6ml/Kg BW)
    2. high PEEP (>12 cmH20)
    3. recruit maneuvers
    4. balanced respiratory rate (20-30/min)
  2. prone positioning
    1. early (</=48h after onset)
    2. prolonged (repetition of 16-hour sessions)
  3. advanced infection management / control
    1. early diagnosis (blood culture, BAL) and infection source (CT scan)
    2. administration of broad spectrum anti-inefctives
  4. neuromuscular blockage (cisatracurium <=48h after onset) and adequate sedation (score-guided)
  5. negative fluid balance

ARDS Network Protocol (Marino, 2014)

Reference

Bein, Thomas et al. “The Standard Of Care Of Patients With ARDS: Ventilatory Settings And Rescue Therapies For Refractory Hypoxemia”. Intensive Care Med 42.5 (2016): 699-711. Web. 14 May 2016.

The DahLIA Trial

DahLIA stands for the Dexmedetomidine to Lessen ICU Agitation study, which was a double-blind, placebo-controlled, RCT conducted at 15 ICUs in Australia and New Zealand from 2011 to 2013.  The study aimed to determine how effective dexmedetomidine is in vented patients with delirium.

METHODOLOGY:

  • dexmedetomidine (or placebo) was started at 0.5 ug/kg/h and titrated to rates between 0 to 1.5 to achieve sedation goals; drug was continued until no longer required or up to 7 days
  • Outcomes include ventilator-free hours in the 7 days following randomization plus 21 secondary outcomes (see table below)

RESULTS:

  • 75 patients were enrolled, with 3 removed from study group – leaving 39 in treatment and 32 in placebo group
  • dexmedetomidine significantly increased ventilator-free hours (144.8h vs 127.5h)
  • dexmedetomidine significantly reduced time to extubation (21.9h vs 44.3h)
  • dexmedetomidine significantly accelerated resolution of delirium (23.3h vs 40h)

 

 

Primary and Secondary Study Outcomes

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Kaplan-Meier Analysis of the Proportion of Patients Remaining Intubated During the First 7 Days of the Study

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References

Reade, Michael C. et al. “Effect Of Dexmedetomidine Added To Standard Care On Ventilator-Free Time In Patients With Agitated Delirium”. JAMA (2016): n. page.

 

Understanding Airway Pressure Release Ventilation

APRV or Airway pressure release ventilation
by Stock and Downs in 1987

Provides CPAP for a prolonged time with a time-cycled release phase to a lower set of pressure for short period of time

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  1. P high – this is the CPAP pressure
  2. T high – this is the time period on the higher pressure
  3. P low – this is the lower set of pressure
  4. T low – this is the time period on the lower pressure

Time on P high maintains a certain lung volume for alveolar recruitment
Time on P low allows ventilation and CO2 removal
Patient able to breath spontaneously at any time regardless of the ventilator cycle
If there is no spontaneous respiratory effort, APRV becomes inverse ratio ventilation

  • How does APRV work in ARDS?
    1. Mean airway pressure increased for lung recruitment
    2. Avoids repetitive inflation and deflation thereby preventing VILI or the need for recruitment maneuvers

Mean airway pressure on APRV formula:
(P High × T High) + (P Low × T Low)(T High + T Low)

Other Notes:

  • Spontaneous breathing allowed, not confined to an arbitrary I:E ratio, improves patient comfort and synchrony
  • Adding PSV to P high is feasible but contradicts limiting the airway pressure and may cause lung distention; alters normal sinusoidal flow of spontaneous breathing – ultimately PSV+APRV defeats its purpose and is not recommended
  • APRV + automatic tube compensation (ATC) – helps overcome artificial airway resistance during SBP without causing lung distension and preserving sinusoidal flow pattern

Advantages of APRV:

  1. Improved oxygenation parameters (PF ratio, lung compliance), better VQ matching,
  2. Improved oxygenation, better V/Q match, lesser dead space
  3. Improved hemodynamics: Decrease RA pressure, increased venous return, improves preload, increases CO; cf inverse ratio PCV in ARDS – higher CI, O2 delivery, SVO2%, UO and lower vasopressor/inotrope usage, lactate, CVP
  4. Effects on regional blood flow: improves blood flow to respiratory muscle, GI tract, UO, GFR
  5. Decreases need for NM blockade and sedation
  • Indications
    1. ARDS
    2. Atelectasis after major surgery
  • Contraindications
    1. Requiring deep sedation (cerebral edema with inc ICP, status epilepticus)
    2. No data on asthma in exacerbation or COPD (short release time not beneficial if prolonged expiration required)
    3. No data on patients with neuromuscular disease

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  • Set up
    1. pressures and TV
      1. P high should be below the high inflection point (HIP) on static volume-pressure curve
      2. P low should be above the low inflection point on the curve
    2. time
      1. T high should allow complete inflation (indicated by end-resp phase of no flow when not breathing)
      2. T low should allow complete exhalation with no gas flow at its end
    3. Recommend setting ATC to 100%, avoid over sedation

Initial set up

  1. P high 20-30cm H20
  2. P low 0-5 cm H20
  3. T high 4-6s
  4. T low 0.2-0.8s

Troubleshooting

  1. Poor oxygenation
    1. inc P high, T high or both to increase mean airway pressure
    2. prone position
  2. Poor ventilation
    1. inc P high and decrease T high to increase minute ventilation
    2. increase T low by 0.05-0.10 s increments
    3. Decrease sedation to increase patients contribution to minute ventilation

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Below is a 14-minute youtube video by Allan Prost that explains the basics of APRV.

Marino has a graph that illustrates the differences between CPAP, BiPAP and APRV

And these are the suggested initial settings for HFOV and APRV in Marino’s textbook.

References

Ann Thorac Med. 2007 Oct-Dec; 2(4): 176–179. doi:  10.4103/1817-1737.36556 PMCID: PMC2732103 Airway pressure release ventilation by Ehab G. Daoud

YouTube,. “Airway Pressure Release Mode Of Mechanical Ventilation.Avi”. N.p., 2016. Web. 19 Jan. 2016.

Marino, 2014. The ICU Book.

Brainstem Levels and Patterns of Respiration

Brainstem Levels and Patterns of Respiration