Most critically ill patients ventilate on their own (spontaneously), however, there are situations where patients fail to maintain adequate oxygen and/or carbon dioxide levels to the point that they are at serious risk of arrest. These patients may require ventilatory assistance in the form of positive pressure ventilation (PPV). The objective of PPV is to maintain normal arterial oxygen pressure (PaO2), and carbon dioxide pressure (PaCO2) until an underlying disease can be identified and treated. It is therefore important to select cases that have a reversible underlying disease to improve the chances of a successful outcome.
Types Of PPV
There are two main forms of PPV: manual and mechanical. Manual PPV requires a person to manually deliver a breath to the patient and is only practical for short-term use (up to several hours of PPV). Mechanical PPV on the other hand, involves a machine that takes the place of the person to deliver a breath and is required for longer-term management (> 6–12 hours of PPV).
An Ambu bag attached to an oxygen supply is a relative inexpensive and readily available means of PPV. Ambu bags for small animals typically come in pediatric sizes for humans (typically 450–950 ml volume). The size of the Ambu bag should be at least 15 x BW (kg). They can be used without supplemental oxygen during initial intubation and ventilation, but should be attached to an oxygen supply as soon as possible. Set oxygen flow rates at 10–15 L/min (delivers 50–90% oxygen levels). If an oxygen reservoir bag is used higher (90–100%) inspired oxygen levels can be achieved. Ambu bags are excellent for short-term situations (i.e., during CPR, stabilization of patients prior to placement on long term PPV, or correction of some transient forms of apnea). It is possible to add positive end expiratory valves to the outflow arm of the Ambu bag, which creates positive end expiratory pressure (PEEP).
PEEP allows recruitment of collapsed alveoli and improves oxygen exchange (typically in 0–10 or 0–20 cm H2O sizes are available). Adjust PEEP by turning valve to the desired level (start with 5 cm H2O). It is hard to determine tidal volume and airway pressures delivered with Ambu bags (varies with size and degree of manual pressure applied to the bag), however, some have a removable safety valve that will open when a specific airway pressure is exceeded (typically 40 cm H2O) to prevent excessive pressures being delivered to the lungs. Note that patients can breathe spontaneously while connected to Ambu bags although airway resistance is higher.
Anesthesia Machine with Manual Ventilation
These are readily available which makes them convenient in most general practices. An advantage over Ambu bags is they allow the tidal volume delivered to be estimated based on the size of the reservoir bag that is used. Most anaesthesia machines can monitor airway pressure when an attached pressure gauge (decreases risk of lung injury) is available. It is also possible to apply adjustable PEEP valves to the expiratory limb of the circuit (typically range from 0–40 cm H2O) to help recruit alveoli and improve oxygenation. Delivery of PPV via this technique still requires someone to physically deliver every breath to the patient. Due to these limitations, indications for this form of PPV are similar to those listed above with the Ambu bag.
Anesthesia ventilators can provide an excellent means of mechanical ventilation, albeit typically short term. They allow many parameters such as tidal volume and respiratory rate to be adjusted. However, they are not used for longer term ventilation due to the limited choice of ventilator modes, the fact most only deliver 100% inspired oxygen concentrations (FiO2 cannot be adjusted) and they do not include humidifier systems, which is recommended to protect the airways with longer term ventilation. The inability to decrease the FiO2 setting puts the patient at increased risk of oxygen toxicity if ventilating > 12 hours, which is one of the biggest limitations of mechanical ventilators for long term PPV.
Mechanical ventilators allow the greatest control over type of breath delivered (pressure, volume, flow rate, FiO2, respiratory rate sensitivity to trigger a breath, and PEEP) and supply humidified oxygen to the patient making them ideal for long-term care (days to weeks). Unfortunately they are relatively expensive.
Although settings vary patient to patient, the following are reasonable PPV settings to start with, keeping in mind several parameters may need to be adjusted to achieve the desired goal directed outcomes. Start with 100% FiO2 and decrease to the lowest FiO2 that maintains desired PaO2/SaO2 to avoid oxygen toxicity. Typical respiratory rates (RR) start at 8–20 breaths per minute (bpm), but may be increased if the PaCO2 is > 50 mm Hg. A new RR can be estimated based on the following formula: New RR = RR x PaCO2/Desired PaCO2. It is not uncommon to have a RR > 20 bpm if underlying pulmonary disease is present. Oxygen flow rates typically start at 0.5 to 1.0 L/kg/min, but often need to be increased, particularly with bad underlying pulmonary disease. Tidal volume of healthy spontaneously ventilating patients is 10–15 ml/kg, but values of 4–6 ml/kg are preferred to decrease the risk of ventilator induced lung injury. Tidal volumes in excess of 20 ml/kg are likely to cause barotrauma and volutrauma. However, it may be necessary to increase tidal volume if PaCO2 does not respond to increased RR. As a general rule of thumb it is best to use the lowest volume possible to achieve oxygen and carbon dioxide goals. Airway pressure should generally be 15–20 cm H2O and values > 30 cmH2O are likely to result in lung injury. The inspiratory time is normally between 1.5 to 2.5 seconds with an inspiratory to expiratory (I:E) ratio of 1:2–1:4. Try to keep the I:E ratio > 1:2 to prevent the next breath being delivered before exhalation is complete. Increasing the RR or tidal volume will decrease I:E ratio while increasing oxygen flow rates will increase I:E ratio. Changing one parameter may require a change in another to maintain desired I:E ratio. Adding PEEP to diseased lungs often improves oxygenation and 3 cm H2O is a common starting value. If 3 cmH2O does not achieved desired results (based on PaO2/SaO2) increase the PEEP by 2–4 cm H2O until desired values are achieved. Exceeding 15 cm H2O of PEEP increases the risk of lung injury and may decrease cardiac output.
General "rules of thumb" on when to provide PPV to the critically ill patient are provided below, however, it must be stressed that early intervention has the greatest chance of success. Keep in mind these are guidelines only and clinical judgement plays an important role - the saying "the indication for ventilation is thinking about it" should also be considered.
General Guidelines of When to Provide PPV to a Critically Ill Patient
Ventilatory failure - PaCO2 > 50 mm Hg and a pH < 7.3, although some patients will tolerate a higher PaCO2 (permissive hypercapnia). However, it is important to monitor pH levels (maintain above 7.2) in these patients as the more acidotic the patient the less tolerant they tend to be of hypercapnia.
Respiratory failure - PaO2 < 50 mm Hg or arterial oxygen saturation (SaO2) < 88% despite supplemental oxygen. These patients often require > 12 hours of PPV, carry a worse prognosis in general, and tend to be the patients that are most labour intensive.
Patients with sustained extreme respiratory distress - increased work of breathing are similar to those with respiratory failure and often require > 12 hours of PPV.
Patients that have an irreversible underlying disease should not be placed on PPV unless it is only for a short period of time to allow an owner to visit prior to euthanasia. Given the investment of time and labour intensity involved in placing a patient on PPV, it is imperative that proper equipment, facilities and properly trained staff be available. There should be a trained technician (or equivalent) with the animal and a veterinarian on the premises 24 hours. A lack of trained and/or available personal is a contraindication to start PPV. If > 6–12 hours of PPV is anticipated, initiating manual PPV is questionable if mechanical PPV is not available.
Although not a contraindication, pleural space disease (pneumothorax, pleural effusion) should be identified and treated prior to ventilation (it is very difficult to successfully ventilate patients with these diseases present). Perform thoracocentesis in suspected cases - untreated pleural space disease is associated with higher ventilatory complications.
The objective of PPV is to maintain normal arterial oxygen pressure (PaO2), and carbon dioxide pressure (PaCO2) until an underlying disease can be identified and treated. The following are general goal directed end points to try and establish with the patient on the ventilator. Again, they are general guidelines and individual case variation/outcomes must be considered.
PaO2 > 60 mm Hg (minimum, > 80 mm Hg preferred) or SaO2 > 90% (minimum, > 96% preferred).
PaCO2 < 50 mm Hg or end-tidal CO2 < 50 mm Hg (often settle for < 60 if the pH is > 7.2).
To achieve desired ventilatory end points the patient must be continually monitored. Minimum recommended monitoring should include vital signs, level of consciousness, arterial blood gasses or both end-tidal CO2 with capnometry (inserted between endotracheal tube and breathing device) and oxygen saturation (SaO2) with pulse oximetry (attached to the tongue), arterial blood pressure, continuous ECG, volume of fluids in and out (urinary catheter), hematocrit, total solids, glucose and urine specific gravity. It is also very important to monitor trends in the critically ill patient and ventilator setting should be recorded every hour.
The prognosis varies with underlying disease. Patients with ventilatory failure and normal lung function (i.e., opioid or barbiturate overdose, certain toxins, tick paralysis, botulism) have the best prognosis and the majority of these patients can be taken off the ventilator when the underlying cause is reversed. A reasonable goal is to discharge 25–40% of mechanically ventilated cases (it is obviously not a good thing to need ventilation and owner education is critical to ensure the cost, commitment and outcome is well understood).
Patients can be ventilated via inflated endotracheal or tracheostomy tubes (a cuff is required). Endotracheal intubation (compared to tracheostomy) is more common due to familiarity, ease of procedure and minimal risk of tissue damage. If PPV exceeds 24 hours, or when heavy sedation is not desired, a tracheostomy may be preferred (allows some dogs to eat and drink orally while being ventilated - maintains the gag reflex which may be important in cases requiring nutritional support). Regardless of the tube used, low pressure, high volume cuffs are preferred as they minimize tracheal injury.
Unless it is desirable to have the patients awake to eat or drink (tracheostomy tube) most patients will require anaesthesia to allow PPV to be initiated and maintained. Typical drugs used to maintain patients on PPV include: Pentobarbital: 2 mg/kg boluses to effect (up to 12 mg/kg) every 4–6 hours or as needed to maintain sedation, fentanyl: 5 µg/kg boluses to effect up to 50 µg/kg then 5–7 µg/kg/h CRI (can add diazepam), propofol: 2–8 mg/kg bolus, 0.1 to 0.3 mg/kg/min CRI. More recently, alfaxalone (2 mg/kg bolus, 0.1–0.12 mg/kg/min) has been used to maintain dogs on PPV.
Care of patients on the ventilator is labour intensive and there are some special considerations that must be attended to. Patients generally tend to do better in sternal recumbency. Ensure that suction devices are available. Oral and ocular ulcers are frequent unless good nursing care is provided. Lube eyes and cleanse mouth with 0.1% chlorhexidine solution soaked gauze sponges every 4 hours. The position of the tube within the mouth (i.e., have it exit the left side, centre then right side of the mouth) and the location of the pulse oximeter probe, as well as the endotracheal tube fixation material (usually gauze) should be changed every 4 hours to decrease the chance of pressure necrosis. Nutritional support should be available for long-term cases.
Supplemental oxygen is often required after weaning and oxygen supplementation should be set up prior to extubation (nasal oxygen is frequently used).
Cardiovascular effects: PPV increases intrathoracic pressures, which may impede venous return to the heart and subsequently decrease cardiac output - at the minimum, monitor heart rate and blood pressure closely.
Oxygen toxicity: Significant risk after 12 hours if FiO2 remains above 60% (decrease the FiO2 to the lowest level possible as soon as possible after starting to maintain desired goals of therapy).
Barotrauma: Avoid exceeding 30 cm H2O airway pressure.
Ventilator associated pneumonia: Monitor for deterioration of respiratory function or unexplained fever (this is a fairly common complication in longer term patients as endotracheal tubes decrease airway defenses - sterile technique during suctioning of tubes/changing of tubes is essential).
Pneumothorax: Can cause acute deterioration of the patient and is more common with high airway pressures (> 30 cm H2O). Auscultation usually reveals dull breath sounds dorsally and can be confirmed with lung ultrasound (absence of a glide sign) and/or thoracocentesis. Usually requires a thoracic tube be placed in order to continue with PPV.
Tension pneumothorax: Is associated with a severe decrease in cardiac output and hypotension - check for tension pneumothorax in ventilated patients that suddenly develop shock.