INTRODUCTION:

Mechanical ventilator was first used by a Roman physician Galen in the second century by blowing air into the larynx of a dead animal using a reed. Author George Poe also used a mechanical respirator for an asphyxiated dog. In 1929 The Drinker and Shaw tank-type ventilator was one of the first negative-pressure machines widely used for mechanical ventilation. It was also known as the iron lung, this metal cylinder completely covers the patient up to the neck.

In this ventilator a vacuum pump created a negative pressure in the chamber, which resulted in expansion of the patient's chest. Due to this, there was a change in the

chest geometry which in turn reduced the intrapulmonary pressure and allowed ambient air to flow into the patient's lungs. When the vacuum was terminated, the negative pressure that was applied to the chest decreased to zero, and the elastic recoil of the chest and lungs allowed passive exhalation.⁽¹⁰⁾

Respiratory system:

Respiratory system is a very vital system for human body as no human can survive outside the womb without it. Organs of respiratory system help to take in the oxygen and expel out the carbon di oxide. Respiratory system starts from the nose till lungs and is divided into upper respiratory tract and lower respiratory tract. The upper respiratory tract consists of Nostrils, Nasal Cavities, Pharynx, Epiglottis, and the Larynx and the lower respiratory tract consists of Trachea, Bronchi, Bronchioles, and the Lungs.⁽⁹⁾

Basics of Breathing:

In a process of normal respiration, negative pressure generates in the lungs in order to draw the environmental air into the lungs. Opposite to this mechanical ventilation needs positive pressure to force air into the lungs. Positive pressure ventilation can be delivered either by invasive technique or non-invasive technique. In invasive technique a tube called endotracheal tube in inserted into patient’s trachea via mouth and is then connected to a mechanical ventilator. In non-invasive technique, positive pressure ventilation is given through a mask or BiPAP machine (Bimanual positive airway pressure).⁽¹¹⁾

Ventilator modes and settings:

Mode and settings of a ventilator depends upon the patient’s condition. Ventilator has modes like AC (Assist Control), SIMV (Synchronised Intermittent Mandatory Ventilation) and CPAP (Continuous Positive Airway Pressure). Which mode to set depends upon the patient’s respiratory rate and pattern.⁽¹¹⁾

Assist-Control Ventilation (ACV):

It is also known as continuous mandatory ventilation (CMV). As the term suggests, in this each breath is either an assist or control breath, and all are of the same volume. If the volume is larger, the more expiratory time is required. If the I:E ratio is less than 1:2, then patient will have progressive hyperinflation. This mode is not suitable for the patient who breathes rapidly because it may result to both hyperinflation and respiratory alkalosis. One thing to remember is that mechanical ventilation does not eliminate the work of breathing, because the diaphragm may still be very active.⁽¹¹⁾

Synchronized Intermittent-Mandatory Ventilation (SIMV):

This mode guarantees a certain number of breaths, but unlike ACV, patient breaths are partially their own, and this reduces the risk of hyperinflation or alkalosis. Mandatory breaths are synchronized to coincide with spontaneous respirations. One of the major disadvantages of this mode is that there is increased work of breathing and a tendency to reduce cardiac output, which may prolong ventilator dependency. If pressure support is also added on top of spontaneous breaths then it can reduce some of the work of breathing. SIMV mode is not suitable for patients with left-ventricular dysfunction as it has been shown to decrease cardiac output.⁽¹¹⁾

Continuous Positive Airway Pressure (CPAP):

In this mode Positive pressure is given throughout the cycle. It can also be delivered through a mask and is can be used in obstructive sleep apnea (esp. with a nasal mask), to postpone intubation, or to treat acute exacerbations of COPD.⁽¹¹⁾

Ventilator setting includes parameters like Respiratory rate, tidal volume, PEEP, oxygen concentration, inspiratory/expiratory ratio, pressure limit and flowrate. ⁽⁴⁾

The respiratory rate is the minimum number of breaths that the patient will be allowed to take. This rate is usually set between 12 and 18 breaths per minute.

Tidal Volume is the amount of air entering into the patient's lungs with each breath. This depends on the ideal body weight of the patient, and is most often calculated at 10 mL/kg. Some patients may require a lesser Tidal Volume due to poor lung compliance as the greater amount of tidal volume in such patients could stretch the lungs and can cause damage. Tidal Volume is usually set between 400 mL for a small person and up to 800 mL for a larger person.

PEEP is the minimum pressure that is required at the end of the expiratory phase to prevent the alveoli from shutting down when the patient exhales. PEEP is measured in centimetres of water and is often set at levels between 5 and 10 cm H2O.

FiO2 or fraction of inspired oxygen is the amount of oxygen that patient requires to maintain adequate blood oxygen levels.⁽⁵⁾

Inspiratory: Expiratory ratio refers to the ratio of inspiratory time and expiratory time. In a normal spontaneous respiration, the expiratory time is about twice as long as the inspiratory time. This gives an I:E ratio of 1:2 and it is read "one is to two".⁽¹²⁾

Pressure-controlled ventilation allows to control ventilatory pressure throughout the repiratory cycle in order to create the pressure necessary to expand the collapsed alveoli.⁽¹³⁾

Flow rate, or peak inspiratory flow rate, is the maximum flow at which a set tidal volume breath is delivered by the ventilator. Most modern ventilators can deliver flow rates between 60 and 120 L/min.⁽¹⁴⁾

Patient Positioning:

Typically a semi-upright/fowlers position is given to the patient who is on ventilator. This position helps in better ventilation as it helps to create more room for lung expansion in the thoracic cage.

Patient Comfort:

Endotracheal intubation is a very uncomfortable experience for the patient. Patients are usually very anxious and stressed after endotracheal intubation. Due to which they fight the artificial ventilation which is provided to them with the help of ventilators. To minimize this IV sedatives (e.g. propofol, lorazepam, midazolam) and analgesics (eg. Morphine, diclofenac) are administered to the patient.⁽¹⁾

Neuromuscular blocking agents are not used routinely in patients undergoing mechanical ventilation because of the risk of prolonged neuromuscular weakness and the need for continuous heavy sedation.⁽¹⁾

Complications of Artificial Respiration:

· Oxygen toxicity.

· Ventilator associated lung injury.

· Ventilator associated Pnuemonia.

· Venous thromboembolic disease.

· Skin breakdown, and atelectasis.⁽¹⁾

Nutritional needs:

Malnutrition is a very common problem faced by a patient on ventilator. It contributes to impaired respiratory functions and immune response. By recognising the malnutrition in early stages nurse can help to provide effective nutrition to the ventilator patient. It helps in the improvement of respiratory functions and greater ease in weaning from mechanical ventilation.⁽⁶⁾

Critical illness is usually followed by hyper metabolic state. This increases the risk of high energy expenditure, thus making the patient malnourished. Under feeding and overfeeding both have increased the hospital length of stay of patients and has also caused other organ problems.

An observational study conducted by Faisy in 2009, demonstrated that the large negative energy balance is an independent determinant of intensive care unit (ICU) mortality in very sick medical patients requiring prolonged acute mechanical ventilation, especially when energy deficit exceeded 5,021 kJ per day.⁽⁷⁾

Oral and enteral feedings are sometimes perceived unsafe because of the risk of aspiration. Due to this adequate nutritional needs of the patent are not met adequately. (13)

A French study showed that nearly 60% of patients were starved during the first 2 days of treatment and only 2.6% received enteral nutrition. The Nutrition Day ICU audit of almost 10,000 patients worldwide, including 47% undergoing mechanical ventilation and 6.2% with NIV, found similar findings in that 40% of patients were starved during the first day of ventilation and 20% on the second day.⁽⁸⁾

Further studies are needed to develop the equations that are more accurate so as to provide optimal energy to patients. The SCCM and ASPEN guidelines recommend the caloric goal in critically ill adult patients as energy requirements calculated either through simplistic formulas (25–30 kcal/kg/day) or published predictive equations, as well as measured by IC (indirect calorimetry).⁽⁷⁾

Initial ventilator settings:

The points given below are an example of the steps involved in operating a mechanical ventilator. Before beginning mechanical ventilation, the nurse along with a respiratory therapist, should always review the manufacturer's instructions, which varies according to the equipment.

1. Set the machine to deliver the Tidal Volume required (10 mL/kg).

2. Set the ventilator to deliver the lowest concentration of oxygen to maintain normal PaO2 (80 to 100 mm Hg). This setting may be high initially but can be reduced gradually on the basis of PaO2 values in ABG results.

3. Record peak inspiratory flow.

4. Set mode (AC or SIMV) and respiratory rate of the ventilator according to the healthcare provider's order. Set PEEP and PS if ordered.

5. Adjust sensitivity so that the patient can trigger the ventilator with a minimal effort (usually 2mm Hg negative inspiratory force).

6. Record minute volume and obtain ABGs to measure partial pressure of carbon dioxide, pH, and Partial pressure of oxygen after 20 minutes of continuous mechanical ventilation.

7. Adjust setting (FiO2 and rate) according to results of ABG analysis to provide normal values or those set by the healthcare provider.

8 If the patient suddenly becomes confused or agitated or begins resisting the ventilator due to some unexplained reason, then assess the patient for hypoxia and manually ventilate on 100% oxygen with a resuscitation bag.⁽²⁾

REFERENCES:

1. Overview of Mechanical Ventilation, Bhakti K. Patel, MD, Assistant Professor, Department of Medicine, Sectionof Pulmonary/Critical Care, University of Chicago, MSD Manual, Professional Version

2. Smeltzer SC, Bare BG, Hinkle JL, Cheever KH. Brunner and Suddarth's Textbook of Medical-Surgical Nursing. 11th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2007.

3. Journal of association of physician of India, Chakor S Vora1, et al, Clinical Profile of Patients Requiring Prolonged Mechanical Ventilation and their Outcome in a Tertiary Care Medical ICU, Received: 19.03.2015; Revised: 15.05.2015; Accepted: 25.05.2015

4. Managing mechanical ventilation, Lippincott Williams & Wilkins, Inc.Nursing2005: December 2005 - Volume 35 - Issue 12 - p 41 Article

5. Set the stage for ventilator settings, Miller, Nichole BSN, Nursing Made Incredibly Easy!: May/June 2013 - Volume 11 - Issue 3 - p 44–52

6. Nutritional needs and support of mechanically ventilated patients, Keithley JK. Medsurg Nurs. 1997 Apr;6(2):74-5

7. Energy Requirements in Critically Ill Patients, Didace Ndahimana and Eun-Kyung Kim, Published online 2018 Apr 17, Clinical nutrition research.

8. To eat or to breathe? The answer is both! Nutritional management during noninvasive ventilation, Pierre Singer Email author and Sornwichate Rattanachaiwong, Critical Care2018Published: 6 February 2018

9. https://en.wikibooks.org/wiki/Human_Physiology/The_respiratory_system#Breathing_and_Lung_Mechanics

10. https://emedicine.medscape.com/article/304068-overview

11. https://www.openanesthesia.org/modes_of_mechanical_ventilation/

12. https://www.scottishintensivecare.org.uk/training-education/sics-induction-modules/ventilation-i-e-ratio/

13. https://www.sciencedirect.com/topics/medicine-and-dentistry/pressure-controlled-ventilation

14. https://www.sciencedirect.com/topics/medicine-and-dentistry/peak-inspiratory-flow

Received on 06.09.2019 Modified on 25.09.2019

Asian J. Nursing Education and Research. 2019; 9(4):588-590.

DOI: 10.5958/2349-2996.2019.00129.0