Normal physiologic goals

Targeting normal physiologic goals for the mechanically ventilated patient may not be the best choice. Trying to do so with little regard for the size of the tidal volumes, the pressures applied or the amount of oxygen used may be injurious to the patient. It could cause lung injury or release of inflammatory mediators.

Patients with abnormal lung mechanics are at risk such as the ARDS, asthmatic or COPD patient. Ventilation must be administered with due regard for their abnormal lungs.

Measurement of the patients end-inspiratory plateau pressure is an indirect measurement of their alveolar distending pressure. To measure this a 0.5-2-second end inspiratory breath hold is applied. The plateau pressure should be less than 30cmH20. This usually results in tidal volumes of between 4-8 ml/kg and this has been shown to be lung protective.

Permissive hypercapnia

Indications

The primary goal of ventilation is no longer to normalise blood gas values but rather maintaining adequate gas exchange while minimising the risks of mechanical ventilation. In order to do so sometimes we have to accept that the carbon dioxide levels are going to rise- a hypercapnia.

Hypercapnia may need to be tolerated with the ARDS patient for example. It has been shown that lower tidal volumes are important here as this reduces the stretching forces on the alveoli which can be damaging.

The patient in bronchospasm may be in a situation where the expiratory phase does not complete properly due to the narrowing of the airways, This generates an auto PEEP and the possibility of breath stacking where one breath comes on top of the previous. Adjustments need to be made to the ventilator to allow a complete expiratory cycle such as increasing the inspiratory flow rate, decreasing the tidal volume or decreasing the respiratory rate.

Contraindications

Hypercapnia is associated with cerebral vasodilatation increasing cerebral blood flow. So those patients with a lesion, trauma oedema or any kind of inter cerebral bleed should have their CO2 levels kept within normal boundaries.

Also patients who are hypotensive should not be allowed to become hypercapnic as this will vasodilate them further dropping their blood pressure.

Physiologic effects of hypercapnia

  • Pulmonary vasoconstriction
  • Shift of the oxyheamoglobin dissociation curve to the right
  • Central nervous system depression
  • Decreased alveolar PO2
  • Increased intercranial pressure
  • Decreased renal blood flow
  • Increased ventilatory drive
  • Systemic vasodilatation

During permissive hypercapnia the pH may well worsen as this can be tolerated to as low as 7.2 in previously well patients. To allow some renal compensation to occur it is important to make the rise in CO2 a gradual one where possible. Abrupt changes in the PCO2 are often poorly tolerated.

Oxygen toxicity

Ideally the oxygen should be no higher than 60%. In animal studies 100% oxygen has been shown to cause non-cardiogenic pulmonary oedema within 24-48 hours. Whilst a higher plateau pressure is of greater concern in the ARDS patient where possible high oxygen requirements should also be monitored closely and if necessary there may have to be a tolerance of lower PaO2- permissive hypoxemia.

Targets

  • Oxygenation- 10-13 Kpa however will need to adjust to lower targets in patients with ARDS or COPD
  • Ventilation- normal PaCO2 is 4.5-6Kpa. If the risks of a high plateau pressure or tidal volumes are too great to achieve this then it can be allowed to rise to as high as 13Kpa providing there are no other contraindications as above.
  • Acid base balance- if a rise in PaCO2 is gradual and renal and cardiovascular function are adequate a pH of 7.2-7.3 is usually well tolerated. Respiratory alkalosis should be avoided as this can lead to electrolyte disturbances, seizures and decreased oxygen unloading from heamoglobin.

Permissive hypercapnia–role in protective lung ventilatory strategies. Intensive Care Med. 2004 Mar;30(3):347-56. Epub 2004 Jan 14

Goal-directed therapy for severely hypoxic patients with acute respiratory distress syndrome: permissive hypoxemia. Respir Care. 2010 Nov;55(11):1483-90.

Permissive hypercapnia in ARDS and its effect on tissue oxygenation. Acta Anaesthesiol Scand Suppl. 1995;107:201-8

Ventilation with Lower Tidal Volumes as Compared with Traditional Tidal Volumes for Acute Lung Injury and the Acute Respiratory Distress Syndrome (ARDSNet) The Acute Respiratory Distress Syndrome Network*N Engl J Med 2000; 342:1301-1308

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Intubation
6 Ways To Be Better With a BVM
Mechanical Ventilation- Physiologic Effects
Anatomy of Adult ETT
Mechanical Ventilation- Terminology
Mechanical Ventilation- Modes of Ventilation I
Mechanical Ventilation- Modes of Ventilation II
Mechanical Ventilation- Pressure/Volume/Flow Loops
Mechanical Ventilation- Peak Pressure and Plateau Pressure
PEEP (Positive End Expiratory Pressure)
Increase the rate or the tidal volume
Ventilation/Perfusion
Ventilation Screen- What do some of those numbers mean?
Phase Variables
I:E ratios and total cycle time.
Why do we ventilate?.
Volume controlled ventilation and compliance.
Lung compliance.
How do I describe how my patient is being ventilated?
Pressure/Volume loop
AC versus SIMV
A-a gradient
Pressure Support
Pressure Support Ventilation Curves
Pressure/Volume/Flow Curves
Ventilator Induced Lung Injury
Trigger, Limit and Cycle
Ventilator Associated Pnuemonia

 

Jevon

Jevon

Possibly the book I most highly recommend for nurses to use as their pocket guide. Phil Jevon is a practitioner in Walsall and has produced an easily read, pocket sized tool. You can click on the picture above  to purchase this excellent book.
Talley and O'Conner
Slightly less 'weighty' than Macleods but still with lots of useful detail and information. The latest copies also have a CD with good, well narrated examples of clinical examination.
Macleods
This title is 'Highly Commended' in the 2006 British Medical Association Awards! 'an incredibly thorough book which is very well illustrated - a must in a book explaining how to perform examinations' - ("Medical Student Review"). This book will show you how to: talk with a patient; take the history from the patient; examine a patient; formulate your findings into differential diagnoses and rank these in order of probability; and, use investigations to support or refute your differential diagnosis.