Monthly Archives: July 2024

Chest Imaging in ICU – Part 2 – Evaluating the HARDWARE (ett, lines etc).

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One of the reasons that we perform portable AP chest x-rays (CXR) in the ICU is to confirm the correct positioning of hardware: endotracheal tubes, central lines, feeding tubes, pulmonary artery catheters, pacemaker wires and chest tubes. This tutorial discusses the correct position of each of these devices and looks at malplacement and complications.

The ideal location of the tip of the endotracheal tube is 3 to 5cm above the carina, below the clavicles and at the level of the T4 spinous process. If tube is too far in, there is a risk of endobronchial intubation and atelectasis of an entire lung (usually the left lung, but not infrequently the right upper lobe also).

The ideal location of a central line, placed in the SVC distribution (internal jugular, subclavian or PICC) is at the junction of the Superior Vena Cava and the Right Atrium. Although inadvertent arterial puncture is less likely, these days, due to ultrasound guided insertion, the tip of a central line can end up can end up in all kinds of places. The tip placement, for prolonged infusions in critical care (for example – pressors or TPN), needs to be confirmed by chest x ray. The major complication of central lines is pneumothorax due to inadvertent pleural puncture during placement.

The pulmonary artery catheter is floated through the right heart and lodged into a peripheral branch of the pulmonary artery, aided by a balloon. The ideal location of the tip is in the lower zone of the lung, and the appearance of the catheter may be a V – the tip is in the left pulmonary artery or a B – the tip is in the right pulmonary artery. It should not be curled up in the RV or, worse, in the inferior vena cava.

Intra-aortic balloon pumps are inserted in cardiology, to manage cardiogenic shock, and following cardiac surgery. The balloon inflates in diastole to increase diastolic pressure, increasing coronary artery perfusion pressure and improving cardiac performance. The tip of the IABP should be distal to the left subclavian artery as it comes off the thoracic aorta. If the tip is too proximal, there is a risk of ischemia to the left arm, if it is not high enough, then it doesn’t function as required and may injure the kidneys.

Chest drains are typically placed to drain air and fluid from the pleural cavity. The tip of the chest tube needs to be where the “stuff” that you wish to drain is located: in the lung apices for air (if the patient is erect or semi erect), in the bases for fluid. There are two “eyes” on each chest tube – both need to be located inside the pleura or air will leak into the subcutaneous tissues.

Finally you need to be able to identify single lead and dual lead pacemakers, implantable defibrillators (ICD) and loop recorders on chest x-ray.

Chest Imaging in ICU – 1. Anatomy, Lobar Collapse and Consolidation

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When patients arrive in the ICU, as soon as they are settled, an AP portable chest x-ray (CXR) is ordered. That x-ray will look different from one done in the radiology department, as the patient is likely semi-recumbent, may be in expiration and the projection is different than from an CXR taken from the back.

The lung has 5 lobes – three on the right and two on the left (the left lung is smaller to accommodate the heart). Each one of these lobes is connected to the trachea by one major airway, that may become plugged off, resulting in atelectasis or collapse of the lobe. As we often need to remove mucus plugs or other material causing these obstructions, it is imperative that you are able to identify the particular lobe that has collapsed. I sequentially go through each lobe of the lungs.

To identify a collapsed lung lobe I suggest that you follow the “Ds” listed in the image below.

the ds of lobar collapse

In addition, radiologists often report lung units as being “consolidated.” This is a catch all phrase that identifies the presence of liquid or semisolid material in airspaces – infectious exudate, blood, mucus, water-fluid, gastric contents etc. You should be able, with you anatomical knowledge, to identify which lung lobe is affected, in particular if you are planning on performing a broncho-alveolar lavage. @ccmtutorials

Assessing Mechanical Ventilation

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One of the most intimidating things about entering the ICU for the first time is the “life support machine” – the mechanical ventilator. Although I have posted an extensive series of tutorials on Mechanical Ventilation, covering most of the modes, oxygen therapy and applied respiratory physiology, I have attempted, in this tutorial, to distill everything to the “least you have to know” in 40 minutes. Keep in mind that modern machines look more like iPhones, and are far easier to use than the devices I grew up with that looked to me, on day 1, like something in the cartoon below.

I start with a discussion about the difference between normal breathing, CPAP and Positive Pressure Ventilation (PPV). PEEP is, effectively, CPAP during PPV. I then go on to discuss pressure limited modes of ventilation; worldwide this are the most widely used modes in ICU. I limit my discussion to Pressure Assist Control, Volume Guaranteed Pressure Control (VG_PC) and Pressure Support Ventilation (PSV). VG-PC is a popular and flexible option as an ICU’s default mode. However, as it is a pressure controlled mode, there is significant variability in tidal volume and airway pressure from minute to minute.

Several important rules are emphasized: the tidal volume should, in general be lower than 6ml/kg of ideal body weight, the plateau pressure lower than 30cmH2O and the driving pressure lower than 15cmH2O. I introduce the Spontaneous Breathing Index (SBI = RR/TV in L). The magic number is 100. We use the SBI to determine the success of weaning on PSV.

Volume Controlled Ventilation is the predominant mode use in the Operating Rooms (Theatres), and Volume Assist Control is a popular mode in North America. In ICU you must set a peak inspiratory flow and be aware that this may be insufficient during assisted breaths and lead to dys-synchrony. Volume Control is often used in ARDS to “lock in” the Tidal Volume (TV) but the operator must be aware that the TV that matters is not what is dialed up on the ventilator, but what the patient exhales.

I go on to discuss how to assess the patient on invasive mechanical ventilation, by looking at whether they are breathing spontaneously, in which case we determine whether they are suitable for a Pressure Support wean or not, or whether or not there is a problem with oxygenation (increase FiO2, PEEP, Mean Airway Pressure and seriously consider Prone Positioning) or Ventilation (increase Respiratory Rate, Tidal Volume or both, reduce PEEP).

The final part of the tutorial looks at Non Invasive Ventilation (NIV), and I explain how, in general we only use 2 modes on standalone devices – CPAP and Spontaneous Timed (S/T). The latter is similar to PSV with a backup rate, but I point out that instead of PEEP+PS the breath is EPAP + IPAP and IPAP is not built upon IPAP, as is the case with PSV. If one is delivering NIV on an ICU ventilator, then “leak” adjustment or “leak sync” should be used.

@ccmtutorials

Assessing the Patient’s Ventilation Status

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This tutorial looks at the assessment of PaCO2 on the blood gas and how it interfaces with the pH and the Bicarbonate (HCO3-). The control of PaCO2 is a major physiological mechanism for maintaining homeostasis. CO2 production by the body must be balanced by CO2 elimination. PaCO2 rises when there is hypoventilation, this results in a fall in pH and an rise in HCO3 and this is called “Acute Respiratory Acidosis.” If the patient hyperventilates, the PaCO2 and the HCO3 fall and the pH rises: this is “Acute Respiratory Alkalosis.” When there is chronic CO2 retention, the body adapts by wasting Chloride in the urine, the pH normalizes and the HCO3 rises substantially.

Any patient who is intubated, or who has a laryngeal mask in situ, must undergo end tidal (end of exhalation) CO2 monitoring. The capnography waveform is worth evaluating, particularly if airway obstruction or increased resistance is suspected.

Included in this tutorial are various rules of thumb that you can use to determine the Respiratory Acid Base Status of the Patient – including the “Rule of 40.”