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Brief case discussion

● 64 y/o male presents with pulmonary edema and respiratory

failure. Intubated and on mechanical ventilation.

● Diuresed over 2 days and ready for extubation after a

spontaneous breathing trial.

● 2 hours after extubation the patient is back in respiratory

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Aim

● Refresh basic heart lung interactions and physiology
● Recognize changes in physiology when on mechanical

ventilation

● Discuss parameters and tools for monitoring and their
relevance

● Practical points on measurements
● Role of simple clinical variables

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Definitions

● What is MAP, Cardiac output and SVR?

● What are the various pressures in the lung/respiratory
system?

● What is <b>cardiac transmural pressure</b>?

● What changes happen normally and on mechanical

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When there is flow limitation, flow is

determined by the

<b>difference between </b>

<b>the arterial and critical closing </b>

<b>pressures </b>

and

<b>NOT</b>

the final downstream

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SVR cannot be trusted

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The Pulmonary System

• The pulmonary system (lungs and airways) is everything that
exists between airway opening (Pao) and pleura (Ppl).

<b>Transpulmonary </b>
<b>pressure</b>

P_TP = P_AO – P_pl

<b>P</b>

<b>_A</b>

<b>O</b>

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Pleural pressure

•

Also called Intrathoracic pressure.

•

Pressure outside the lung surface but inside the

chest wall

•

Not constant when measured at different locations

inside the thorax

•

<b>ITP (juxtacardiac) is one of the chief </b>

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<b>Contractility: Force that the heart muscle can </b>

generate at a given length

<b>Preload</b>

Length tension relationship of
cardiac myofibers

<b>Surrogate:</b> end diastolic

<b>volume </b>in chamber

<b>Secondary surrogate:</b>

<b>pressure </b>in chamber (CVP,
Pra, PAOP)

<b>Afterload</b>

Load (wall stress) the heart

muscle needs to eject against

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Changes in mechanical ventilation

<b>Right side</b>

Venous return
RV output

Pulmonary vascular resistance
RV preload (EDV)

RV contractility

<b>Left side</b>

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Right Ventricle

..<b>depends</b> on whether ITP or Palv is affected most by
mechanical ventilation:

When the <b>lungs </b>are <b>stiff and overdistended</b>, <b>RV output falls </b>

due to increase in afterload due to pulmonary hypoxic
vasoconstriction (RV dilates).

When the <b>chest wall is stiff </b>and over distended, <b>RV output falls</b>

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Left Ventricle

LV preload is affected by:

RV preload due to the <b>“series”</b> effect

Increased RV volume may reduce LV compliance

LV is constrained in cardiac fossa by lung volume change

<b>“squeeze”</b>

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NET Effect

●

Despite lower afterload,

<b>↑ITP reduces preload</b>

and this effect predominates in the normal heart.

●

The

<b>failing heart</b>

, however,

<b>benefits from both </b>

<b>reduced preload and afterload.</b>

●

↑ITP may increase pulmonary vascular resistance

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Back to the case

● On mechanical ventilation the <b>reduced preload</b> and

<b>afterload</b> helped cardiac function and improved forward
flow.

● <b>Extubation reversed these benefits </b>suddenly pushing the

patient back into heart failure

● A <b>T-piece trial</b> may have helped reveal this.

● <b>Nitroglycerin </b>could have been used as an adjunct
● <b>Extubation to NIV</b> may be safer rather than to direct

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Tools

● MAP (non invasive may be just as good as invasive

measurement).

● Arterial lines and invasive BP monitoring, CVP
● Invasive Cardiac output

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Clinical parameters

● Skin perfusion (capillary refill)
● Mental status

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Bottomline

Measure values at <b>end expiration.</b>
<b>Zero accurately.</b>

Preload and afterload <b>change </b>with mechanical ventilation and
alter measured pressures.

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References

1. Langer et al. "Awake" extracorporeal membrane oxygenation (ECMO): Pathophysiology, technical considerations, and
clinical pioneering. Critical Care.

2. />

3.
/>

4. Magder S. Invasive intravascular hemodynamic monitoring: technical issues. Crit Care Clin. 2007 Jul;23(3):401-14. PubMed
5. Magder S. Clinical usefulness of respiratory variations in arterial pressure. Am J Respir Crit Care Med. 2004 Jan

15;169(2):151-5. Review.

6. Perel A, Pizov R, Cotev S. Systolic blood pressure variation is a sensitive indicator of hypovelemia in ventilated dogs
subjected to graded hemorrhage. Anesthesiology

7. Magder S. Hemodynamic monitoring in the mechanically ventilated patient. Curr Opin Crit Care. 2011 Feb

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Conversion of: mmHg to

cmH2O

<b>mmHg</b> <b>cmH20</b>

1 1*
2 3
3 4
4 5
5 7
6 8
7 10
8 11

9 12
10 14
11 15
12 16
13 18
14 19
15 20

1 mmHg x 1.36 = cm H2O
*cm H2O reported in

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